HTTP/2 302 date: Tue, 22 Jul 2025 23:51:32 GMT content-type: text/html; charset=utf-8 content-length: 0 vary: X-PJAX, X-PJAX-Container, Turbo-Visit, Turbo-Frame, X-Requested-With,Accept-Encoding, Accept, X-Requested-With location: https://objects.githubusercontent.com/github-production-repository-file-5c1aeb/842037/11702385?X-Amz-Algorithm=AWS4-HMAC-SHA256&X-Amz-Credential=AKIAVCODYLSA53PQK4ZA%2F20250722%2Fus-east-1%2Fs3%2Faws4_request&X-Amz-Date=20250722T235132Z&X-Amz-Expires=300&X-Amz-Signature=f762af4548f07b6994738c6709fe5f168dc5a95083cd610e35929cd285653f33&X-Amz-SignedHeaders=host&response-content-disposition=attachment%3Bfilename%3DNim-for-Python-Programmers.md&response-content-type=text%2Fmarkdown cache-control: no-cache strict-transport-security: max-age=31536000; includeSubdomains; preload x-frame-options: deny x-content-type-options: nosniff x-xss-protection: 0 referrer-policy: no-referrer-when-downgrade content-security-policy: default-src 'none'; base-uri 'self'; child-src github.githubassets.com github.com/assets-cdn/worker/ github.com/assets/ gist.github.com/assets-cdn/worker/; connect-src 'self' uploads.github.com www.githubstatus.com collector.github.com raw.githubusercontent.com api.github.com github-cloud.s3.amazonaws.com github-production-repository-file-5c1aeb.s3.amazonaws.com github-production-upload-manifest-file-7fdce7.s3.amazonaws.com github-production-user-asset-6210df.s3.amazonaws.com *.rel.tunnels.api.visualstudio.com wss://*.rel.tunnels.api.visualstudio.com objects-origin.githubusercontent.com copilot-proxy.githubusercontent.com proxy.individual.githubcopilot.com proxy.business.githubcopilot.com proxy.enterprise.githubcopilot.com *.actions.githubusercontent.com wss://*.actions.githubusercontent.com productionresultssa0.blob.core.windows.net/ productionresultssa1.blob.core.windows.net/ productionresultssa2.blob.core.windows.net/ productionresultssa3.blob.core.windows.net/ productionresultssa4.blob.core.windows.net/ productionresultssa5.blob.core.windows.net/ productionresultssa6.blob.core.windows.net/ productionresultssa7.blob.core.windows.net/ productionresultssa8.blob.core.windows.net/ productionresultssa9.blob.core.windows.net/ productionresultssa10.blob.core.windows.net/ productionresultssa11.blob.core.windows.net/ productionresultssa12.blob.core.windows.net/ productionresultssa13.blob.core.windows.net/ productionresultssa14.blob.core.windows.net/ productionresultssa15.blob.core.windows.net/ productionresultssa16.blob.core.windows.net/ productionresultssa17.blob.core.windows.net/ productionresultssa18.blob.core.windows.net/ productionresultssa19.blob.core.windows.net/ github-production-repository-image-32fea6.s3.amazonaws.com github-production-release-asset-2e65be.s3.amazonaws.com insights.github.com wss://alive.github.com api.githubcopilot.com api.individual.githubcopilot.com api.business.githubcopilot.com api.enterprise.githubcopilot.com; font-src github.githubassets.com; form-action 'self' github.com gist.github.com copilot-workspace.githubnext.com objects-origin.githubusercontent.com; frame-ancestors 'none'; frame-src viewscreen.githubusercontent.com notebooks.githubusercontent.com; img-src 'self' data: blob: github.githubassets.com media.githubusercontent.com camo.githubusercontent.com identicons.github.com avatars.githubusercontent.com private-avatars.githubusercontent.com github-cloud.s3.amazonaws.com objects.githubusercontent.com release-assets.githubusercontent.com secured-user-images.githubusercontent.com/ user-images.githubusercontent.com/ private-user-images.githubusercontent.com opengraph.githubassets.com copilotprodattachments.blob.core.windows.net/github-production-copilot-attachments/ github-production-user-asset-6210df.s3.amazonaws.com customer-stories-feed.github.com spotlights-feed.github.com objects-origin.githubusercontent.com *.githubusercontent.com; manifest-src 'self'; media-src github.com user-images.githubusercontent.com/ secured-user-images.githubusercontent.com/ private-user-images.githubusercontent.com github-production-user-asset-6210df.s3.amazonaws.com gist.github.com; script-src github.githubassets.com; style-src 'unsafe-inline' github.githubassets.com; upgrade-insecure-requests; worker-src github.githubassets.com github.com/assets-cdn/worker/ github.com/assets/ gist.github.com/assets-cdn/worker/ server: github.com set-cookie: _gh_sess=rnhzuv9S2zo9m8TSpVzPN7e14vHY%2B%2BCLR0u8NlZ0xEf1rfbA9n2Vg4awsFAmLrdvLpqDedZahyTb%2B75VvOdhWJfhfQKbKaa%2BRhMOMCX8EGJpke6UwjeUH1Ry1hwreq6pRI7Mk5e%2FAj3pgwDDwciEPiTvsV2B%2BnwN8nqGOgGa2XX%2Bidad%2Fo5YbgyE9XvY1EV89xbzbrxWLtlhpyQaLjROJDvVp%2B8B4x4npCKn45UrRYUJJO6SCeOLLIiymE8QPIgKff78hVHf7eR3esW%2FsJIkxQ%3D%3D--wECLFB2fx3zK2PUJ--rxIHDc8YGDkRXll52lZzNw%3D%3D; Path=/; HttpOnly; Secure; SameSite=Lax set-cookie: _octo=GH1.1.1091539166.1753228292; Path=/; Domain=github.com; Expires=Wed, 22 Jul 2026 23:51:32 GMT; Secure; SameSite=Lax set-cookie: logged_in=no; Path=/; Domain=github.com; Expires=Wed, 22 Jul 2026 23:51:32 GMT; HttpOnly; Secure; SameSite=Lax x-github-request-id: E554:B5FA6:1D126F:284C08:68802404 HTTP/2 200 content-type: text/markdown last-modified: Wed, 05 Mar 2025 01:47:21 GMT etag: "0x8DD5B87AB9892F5" server: Windows-Azure-Blob/1.0 Microsoft-HTTPAPI/2.0 x-ms-request-id: dbf2e719-e01e-0003-42f1-f97695000000 x-ms-version: 2025-05-05 x-ms-creation-time: Wed, 05 Mar 2025 01:47:21 GMT x-ms-blob-content-md5: L4KAwpm4YFH5mYDUwWq1tA== x-ms-lease-status: unlocked x-ms-lease-state: available x-ms-blob-type: BlockBlob content-disposition: attachment;filename=Nim-for-Python-Programmers.md x-ms-server-encrypted: true via: 1.1 varnish, 1.1 varnish fastly-restarts: 1 accept-ranges: bytes age: 0 date: Tue, 22 Jul 2025 23:51:33 GMT x-served-by: cache-iad-kiad7000031-IAD, cache-bom-vanm7210045-BOM x-cache: HIT, MISS x-cache-hits: 1, 0 x-timer: S1753228293.556156,VS0,VE282 content-length: 91284 # Table Of Contents | | | | | |:-----------------------------------------:|:-------------------------------------------------------:|:--------------------------------:|:---------------------------------------------:| | [Comparison](#Comparison) | [Objects](#Objects) | [Do I have to know C?](#do-i-have-to-know-c) | [Named Tuples](#named-tuple-1) | | [Variables](#Variables) | [`self.__init__()`](#self__init__) | [Strings, F-Strings](#Strings) | [Lists](#Lists) | | [Variable Naming](#variable-naming) | [Consistent Spacing](#consistent-spacing) | [Scoping](#scoping) | [Mutable arguments](#mutable-arguments) | | [Imports](#Imports) | [Ranges](#Ranges) | [String Operations](#string-ops) | [List Comprehensions](#List-Comprehensions) | | [try/import/except](#tryimportexcept) | [Static Bounds Checks](#static-bounds-check) | [Null Coalescing](#null-coalescing) | [`with` Context Manager](#with-context-manager) | | [Arrays](#Arrays) | [Slices](#Slices) | [Tuples](#Tuples) | [Dict Comprehensions](#Dict-Comprehensions) | | [Set Comprehensions](#Set-Comprehensions) | [Reading and writing files](#Reading-and-writing-files) | [Decorators](#Decorators) | [Lambdas](#Lambdas) | | [Sets](#Sets) | [JSON](#JSON) | [Map & Filter](#map--filter) | [Optional Indentation](#Optional-Indentation) | | [Dictionaries](#Dictionaries) | [CamelCase](#CamelCase) | [DocStrings](#Docstrings) | [Import Nim files in Python](#Import-Nim-files-on-Python) | | [Ternary operators](#Ternary-operators) | [Unittests](#Unittests) | [def Vs proc / func](#def-vs-procfunc) | [Self-Execution of Main Module](#Self-Execution-of-Main-Module) | | [Python Syntax for Nim](#Python-Syntax-for-Nim) | [Publish to PYPI](#Publish-to-PYPI) | [Silent Compilation](#Silent-Compilation) | [Compiler Help](#Compiler-Help) | | [Build Modes](#Build-Modes) | [Abstract Base Classes](#ABC) | [Decorators](#Decorators) | [WebAssembly](https://github.com/nim-lang/Nim/wiki/Nim-for-TypeScript-Programmers#WebAssembly) | | [Templates](#Templates) | [Nim running Interpreted](https://nim-lang.org/docs/nims.html) | [Nim on the browser](https://github.com/nim-lang/Nim/wiki/Nim-for-TypeScript-Programmers#table-of-contents) | [Standard Library Equivalents](#Standard-Library-Equivalents) | | [Async](#Async) | [Install Nim from PIP](https://github.com/juancarlospaco/choosenim_install#use) | [Cheatsheet PDF](https://www.overleaf.com/read/svnxffsjvscy) | [Arrow Functions](https://github.com/nim-lang/Nim/wiki/Nim-for-TypeScript-Programmers#Arrow-Functions) | | [How to share variables between functions?](#How-to-share-variables-between-functions) | [Change File Permissions](#Change-File-Permissions) | [Temporarily Change Folder](#Temporarily-Change-Folder) | [Pattern Matching](https://github.com/nim-lang/fusion/blob/master/src/fusion/matching.rst) | | [Best Practices](https://github.com/nim-lang/Nim/wiki/best-practices#best-practices) | [In-Place Vs Out-Place](#in-Place-vs-out-place) | [Run in NodeJS](https://github.com/nim-lang/Nim/wiki/Nim-for-TypeScript-Programmers#nodejs-compatibility) | [Code Expansion](#run-time-code-expansion) | | [Arduino, MicroPython, ESP32, FreeRTOS](#MicroPython) | [LiveCoding, FoxDot, SuperCollider](#SuperCollider) | [PIP-installable Nim embedded inside Python](https://github.com/juancarlospaco/nim4py#nim4python) | [Install Nim packages from PIP](https://github.com/juancarlospaco/nimble_install#use) | * [English Version](https://github.com/nim-lang/Nim/wiki/Nim-for-Python-Programmers#table-of-contents) * [Spanish Version](https://github.com/nim-lang/Nim/wiki/Nim-for-Python-Programmers-ES#tabla-de-contenidos) * [Brazilian Portuguese Version](https://github.com/nim-lang/Nim/wiki/Nim-for-Python-Programmers-PT_BR#índice) ## Comparison Feature | :snake: Python | :crown: Nim ---------------------|--------------------------------------|----------------------------------------- Execution model | Virtual Machine (Interpreter) | Machine code via C/C++ (Compiler) Written using | C (CPython) | Nim License | Python Software Foundation License | [MIT](https://tldrlegal.com/license/mit-license) Version (Major) | `3.x` | `1.x` Metaprogramming | :heavy_check_mark: metaclass, exec, eval, [ast](https://docs.python.org/3/library/ast.html) (Run-time code expansion) | :heavy_check_mark: [template, macros](https://nim-lang.github.io/Nim/manual.html#macros) (Compile-time code expansion) Memory management | Garbage collector | [Multi-paradigm memory management](https://nim-lang.github.io/Nim/gc.html) (garbage collectors, [ARC/ORC](https://www.youtube.com/watch?v=aUJcYTnPWCg), manual) Typing | Dynamic, Duck Typing | Static Dependent types | :negative_squared_cross_mark: | :heavy_check_mark: Partial support Generics | Duck Typing | :heavy_check_mark: int8/16/32/64 types | :negative_squared_cross_mark: | :heavy_check_mark: uint8/16/32/64 types | :negative_squared_cross_mark: | :heavy_check_mark: float32/float64 types | :negative_squared_cross_mark: | :heavy_check_mark: Char types | :negative_squared_cross_mark: | :heavy_check_mark: Subrange types | :heavy_check_mark: | :heavy_check_mark: Enum types | :heavy_check_mark: | :heavy_check_mark: Bigints (arbitrary size) | :heavy_check_mark: | :heavy_check_mark: [jsbigints](https://nim-lang.github.io/Nim/jsbigints.html), [#14696](https://github.com/nim-lang/Nim/issues/14696#issue-640117621) Biggest built-in integer | Unknown, limited by free memory | `18_446_744_073_709_551_615` for `uint64` type Arrays | :heavy_check_mark: | :heavy_check_mark: Type inference | Duck typing | :heavy_check_mark: Closures | :heavy_check_mark: | :heavy_check_mark: Operator overloading | :heavy_check_mark: | :heavy_check_mark: on any type Custom operators | :negative_squared_cross_mark: | :heavy_check_mark: Object-Oriented | :heavy_check_mark: | :heavy_check_mark: Methods | :heavy_check_mark: | :heavy_check_mark: Exceptions | :heavy_check_mark: | :heavy_check_mark: Anonymous functions | :heavy_check_mark: multi-line, single-expression | :heavy_check_mark: multi-line, multi-expression List comprehensions | :heavy_check_mark: | :heavy_check_mark: Dict comprehensions | :heavy_check_mark: | :heavy_check_mark: Set comprehensions | :heavy_check_mark: | :heavy_check_mark: Custom object comprehensions | :heavy_check_mark: generator expression | :heavy_check_mark: Pattern Matching builtin | :heavy_check_mark: As of Python 3.10 | :heavy_check_mark: Immutability of types | Basic types (number, string, bool), tuple, frozenset | :heavy_check_mark: Immutability of variables | :negative_squared_cross_mark: | :heavy_check_mark: Function arguments immutability | Depending on type | Immutable Formatted string literals | :heavy_check_mark: f-strings | :heavy_check_mark: [strformat](https://nim-lang.github.io/Nim/strformat.html) FFI | :heavy_check_mark: ctypes, C extension API (Cython via pip) | :heavy_check_mark: C, C++, Objective C, JS (depending on used backend) Async | :heavy_check_mark: | :heavy_check_mark: Threads | :heavy_check_mark: Global Interpreter Lock | :heavy_check_mark: Regex | :heavy_check_mark: Perl-compatible | :heavy_check_mark: Perl-compatible Documentation comments | :heavy_check_mark: plain-text multi-line strings (reStructuredText via Sphinx) | :heavy_check_mark: [ReStructuredText/Markdown](https://nim-lang.github.io/Nim/rst.html) Package publishing | :heavy_check_mark: not built-in, requires `twine` | :heavy_check_mark: built-in, `nimble` Package manager | :heavy_check_mark: `pip` | :heavy_check_mark: `nimble` Code autoformatter | :heavy_check_mark: `black` and others via pip | :heavy_check_mark: `nimpretty` built-in, [nimlint](https://github.com/nim-dev/nimlint) File extensions | .py, .pyw, .pyc, .pyd, .so | .nim, .nims Temporary intermediate representation (IR) format | .pyc (CPython VM bytecode) | C, C++, Objective C (LLVM IR via nlvm) Uses #!shebang on files | :heavy_check_mark: | :heavy_check_mark: `nimr`, `nimcr` REPL | :heavy_check_mark: | [inim](https://github.com/inim-repl/INim#inim-interactive-nim-shell--), [Nim4Colab](https://github.com/demotomohiro/nim4colab), [JupyterNim](https://github.com/stisa/jupyternim) Indentation | Tabs and spaces, uniform per code block, 4 spaces by convention | Spaces only, uniform per code block, 2 spaces by convention Notes: - Python anonymous function (lambdas) are known to be slow compared to normal functions. - Python Regex claims to be PCRE compatible, but in practice PCRE Regexes may not work. - Python "multi-line" anonymous functions may require using `;` and Linters/IDE may complain about it. ## Variables Creating a new variable uses `var` or `let` or `const`. Nim has immutability and compile-time function execution. You can assign functions to variables. | Feature | `const` | `let` | `var` | |---------------------|------------------------|------------------------|------------------------| | Run-Time | NO | :heavy_check_mark: YES | :heavy_check_mark: YES | | Compile-Time | :heavy_check_mark: YES | NO | NO | | Immutable | :heavy_check_mark: YES | :heavy_check_mark: YES | NO | | AutoInitialized | :heavy_check_mark: YES | :heavy_check_mark: YES | :heavy_check_mark: YES | | Reassignable | NO | NO | :heavy_check_mark: YES | | Requires Assignment | :heavy_check_mark: YES | :heavy_check_mark: YES | NO | | Can be Global | :heavy_check_mark: YES | :heavy_check_mark: YES | :heavy_check_mark: YES | [For advanced users, it is possible to skip variable Auto-Initialization.](https://nim-lang.github.io/Nim/manual.html#statements-and-expressions-var-statement) ### Variable Naming Variables can be multi-line without "escaping" them or using parentheses. This is useful for long lines and long ternary operators. Minimal example: ```python variable = 666 + \ 420 * \ 42 - \ 9 assert variable == 18297 ``` :arrow_up: Python :arrow_up:          :arrow_down: Nim :arrow_down: ```nim var variable = 666 + 420 * 42 - 9 assert variable == 18297 ``` This works with function calls too: ```nim import std/strutils var variable = " 12345 " .strip .parseInt assert variable == 12345 ``` You can use underscores, new lines, and whitespace in variable names: ```nim let `this must be positive`: Positive = 42 assert this_must_be_positive == 42 const `this is my nice named variable` = 42 ``` [You can use reserved keywords as variable names.](https://nim-lang.github.io/Nim/manual.html#lexical-analysis-stropping) It's okay to use `var` while learning Nim or for quick prototyping, although it's much better to learn the difference between different variable declarations. ## Consistent Spacing Spaces must be consistent on your code, mainly around operators: ```nim echo 2 - 1 # OK echo 2-1 # OK ``` Bad inconsistent spaces: ```nim echo 2 -1 # Error # ^ parses as "-1" ``` Omitting spaces on your code has no effect on performance. All operators are functions in Nim. ## Scoping - Scope "leaks", "bugs", "glitches", etc. ```python for x in range(0, 9): if x == 6: print(x) print(x) ``` Output: ``` 6 8 # Leak! ``` :arrow_up: Python :arrow_up:          :arrow_down: Nim :arrow_down: ```nim for x in 0..9: if x == 6: echo x echo x ``` Output: ``` Error: undeclared identifier: 'x' ``` Note that in the example above we use a simple `int`, so the problem may not seem severe. But if `x` were a few gigabytes of RAM in size, it would "leak" out of the `for` loop to the rest of the outer or main scope instead of being reclaimed. Nim avoids this problem. Another example: ```python x = 0 y = 0 def example(): x = 1 y = 1 class C: nonlocal x, y assert x == 1 and y == 1 x = 2 example() ``` :arrow_up: Python :arrow_up:          :arrow_down: Nim :arrow_down: ```nim var x = 0 var y = 0 proc example() = var x = 1 var y = 1 type C = object assert x == 1 and y == 1 x = 2 example() ``` Another example: ```python x = 0 y = 0 def example(): x = 1 y = 1 class C: nonlocal x, y assert x == 1 and y == 1 x = 2 try: raise except Exception as _: pass example() ``` :arrow_up: Python :arrow_up:          :arrow_down: Nim :arrow_down: ```nim var x = 0 var y = 0 proc example() = var x = 1 var y = 1 type C = object assert x == 1 and y == 1 x = 2 try: raise except Exception as y: discard example() ``` # Boolean conditionals - Boolean comparisons "bugs", "glitches", etc. ```python assert True == not False ``` Fails with the error: ``` SyntaxError: invalid syntax. ``` :arrow_up: Python :arrow_up:          :arrow_down: Nim :arrow_down: The Nim example compiles and runs without incident; the operator precedence is resolved correctly: ```nim assert true == not false ``` Another example: ```python assert False + 1 assert not True - 1 ``` This runs because `bool` is a subtype of `int` in Python, so it supports the same mathematical operations. In Nim this is not the case: :arrow_up: Python :arrow_up:          :arrow_down: Nim :arrow_down: ```nim assert false + 1 assert not true - 1 ``` Does not Compile: ``` Error: type mismatch: got ``` # block `block` explicitly creates a new scope, without the overhead of a function. It can have a "name" without the name polluting the local namespace, and can be interrupted anywhere without requiring `return`. `block` can be used with `var`, `let` and `const` too. Imagine that you need to get out of a nested `if`, without executing any other code from other `if` and `else` blocks. You can do: ```python print("Before") # this is a function, has overhead, pollutes namespace, must return to interrupt, etc. def example(): if True: print("Inside if true") if 42 > 0: print("Inside if 42 > 0") if 'z' > 'a': print("Inside if z > a") return # Must return to interrupt if 3.14 > 0.0: print("Inside if 3.14 > 0.0") else: print("else of z > a") else: print("else of 42 > 0") else: print("else of true") example() # example in namespace print("After") ``` :arrow_up: Python :arrow_up:          :arrow_down: Nim :arrow_down: ```nim echo "Before" block example: # Creates a new explicit named scope. This is not a function; there is no overhead. if true: echo "Inside if true" if 42 > 0: echo "Inside if 42 > 0" if 'z' > 'a': echo "Inside if z > a" break example # Gets out of block example. if 3.14 > 0.0: echo "Inside if 3.14 > 0.0" else: echo "else of z > a" else: echo "else of 42 > 0" else: echo "else of true" # No function call. "example" is not polluting the local namespace. echo "After" ``` - https://nim-lang.github.io/Nim/manual.html#statements-and-expressions-block-statement - https://nim-lang.github.io/Nim/manual.html#statements-and-expressions-block-expression ## Mutable arguments ```python def example(argument = [0]): argument.append(42) return argument print(example()) print(example()) print(example()) ``` Output: ```python [0, 42] [0, 42, 42] [0, 42, 42, 42] ``` :arrow_up: Python :arrow_up:          :arrow_down: Nim :arrow_down: ```nim func example(argument = @[0]): auto = argument.add 42 return argument echo example() echo example() echo example() ``` Output: ```nim Error: type mismatch: got but expected one of: proc add[T](x: var seq[T]; y: sink T) first type mismatch at position: 1 required type for x: var seq[T] but expression 'argument' is immutable, not 'var' ``` ## Imports Import | :snake: Python | :crown: Nim --------------------------------------------|---------------------------------|----------------------------- Only one symbol, use unqualified | `from math import sin` | `from std/math import sin` All symbols, use unqualified | `from math import *` | **`import std/math` (recommended)** All symbols, use fully qualified | **`import math` (recommended)** | `from std/math import nil` "import as" another name | `import math as potato` | `import std/math as potato` Both of the above at the same time | :negative_squared_cross_mark: | `from std/math as m import nil` All symbols except one, use unqualified | :negative_squared_cross_mark: | `import std/math except sin` All symbols except several, use unqualified | :negative_squared_cross_mark: | `import std/math except sin, tan, PI` Include another module in this module | :negative_squared_cross_mark: | `include somemodule` **Your modules and types are not going to collide!, even if you have types named like modules, just chill and keep coding...** In Nim, ``import std/math`` imports all of the symbols from the ``math`` module (``sin``, ``cos``, etc) so that they can be used unqualified. The Python equivalent is ``from math import *``. If you prefer to not import all the symbols, and always use qualified names instead, the Nim code is ``from std/math import nil``. Then you can call ``math.sin()``, ``math.cos()``, etc. The Python equivalent is ``import math``. It is generally safe to import all names in Nim because the compiler will not actually compile any unused functions (so there's no overhead). Furthermore, since Nim is statically typed, it can usually distinguish between the two imported functions with the same names based on the types of the arguments they are called with. In the rare cases where the types are the same, you can still fully qualify the name to disambiguate. The prefix `std/` enforces that the module is imported from the standard library. If a Nimble package has a module with the same name, the compiler can resolve the ambiguity and it is explicit in the code. Local modules and Nimble modules do not need `std/` prefix. Python and Nim share these import statements: ``` # Python and Nim import foo, bar, baz import foo import bar import baz ``` Alternative syntaxes: ```python # Python import foo, \ bar, \ baz ``` ```nim # Nim import foo, bar, baz # Useful for small diffs when adding/removing imports import foo, bar, baz import foo, bar, baz, more, imports ``` The variant with one `import` statement per line is common in Python and Nim, but in Nim the form `import foo, bar, baz` is also seen often. More examples: ```nim ## This is documentation for the module. # This is a comment. include prelude import std/sugar as stevia from std/math import nil from std/with as what import nil ``` ### Programatically ```python __import__("math") ``` :arrow_up: Python :arrow_up:          :arrow_down: Nim :arrow_down: ```nim template imports(s) = import s imports math ``` ### Code without imports Sometimes in the wild you may see code samples or files **without** the imports but they somehow work anyway. The reason is that Nim can use `import` from the compile command, or from a `.nims` file: - `nim c --import:sugar file.nim` - `nim c --import:folder/mymodule file.nim` - `nim js --import:strutils --include:mymodule file.nim` Sometimes projects or quick code examples use this to save some typing. Thanks to Dead Code Elimination, if the imported symbols are not used they will not exist on the compiled output. See also: - [Nim Command line switches](https://nim-lang.github.io/Nim/nimc.html#compiler-usage-command-line-switches) ### Prelude Sometimes you may feel that Python has more symbols available by default without any `import` compared to Nim. To get a similar experience of having the basic data structures and most common imports ready so you can get coding right away, you can use [prelude](https://nim-lang.github.io/Nim/prelude.html): ```nim include prelude echo now() echo getCurrentDir() echo "Hello $1".format("World") ``` [prelude](https://nim-lang.github.io/Nim/prelude.html) is an [`include` file](https://nim-lang.github.io/Nim/manual.html#modules-include-statement) that simply imports common modules for your convenience, to save some typing. [prelude](https://nim-lang.github.io/Nim/prelude.html) works for JavaScript target too. ### Where Symbols come from? - If symbols are unqualified, how do you know where symbols come from? Given `foo()` is a symbol: - Nim: you typically have `foo()`, [with UFCS support](https://en.wikipedia.org/wiki/Uniform_Function_Call_Syntax#Nim_programming_language). - Python: you typically have `object.foo()` rather than `module.foo()`, no UFCS. Typically the [Editor/IDE](https://marketplace.visualstudio.com/items?itemName=kosz78.nim) should hint where the symbols come from, like in any other programming language:  Nim comes built-in with [NimSuggest](https://nim-lang.org/docs/nimsuggest.html) for Editor/IDE integrations. Contrary to Python, Nim's type system has all the information about all of the symbols: ```nim import std/macros macro findSym(thing: typed) = echo thing.getType.lineInfo findSym: echo # Where echo comes from?. ``` `echo` comes from: ``` lib/system.nim(1929, 12) ``` When learning Nim, or for quick prototyping, it is okay to use the symbols fully qualified. Doing so produces no errors, but idiomatic Nim avoids this. ### Exports In Python all symbols in the module are visible and mutable from modules that import it, including symbols that should not be used or mutated outside the module. In Nim everything is private by default and therefore is not visible from other modules. To make symbols public and visible in other modules, you have to use the asterisk `*`: ```nim let variable* = 42 const constant* = 0.0 proc someFunction*() = discard template someTemplate*() = discard type Platypus* = object fluffyness*: int ``` The asterisk not only makes the symbol visible to the outside world, the symbol will also appear in the generated documentation (`nim doc`). When you import the module, the symbol will be automatically added to the namespace, but private (not exported) symbols without `*` will not be visible. The asterisk is like a __visual cue__ for humans. You can immediately understand what symbols are a part of the "the public API" just by looking at the module's source code. The asterisk `*` is pronounced as "star". For more information, read: https://narimiran.github.io/2019/07/01/nim-import.html ## try/import/except In Python, imports are a runtime operation and can fail. It is a fairly common pattern that platform-dependent imports are placed inside a try block, and an alternative or fallback inside the except: ```python try: import module except ImportError: import othermodule as module try: from module import some_func except ImportError: # Fallback implementation def somefunc(): return some_value ``` Nim resolves all imports at compile-time, so something like an `ImportError` does not exist. There's no need to handle import errors at runtime. ## Arrays Arrays in Nim are fixed size, start at index `0`, and must contain elements of the same type. When passing an array to a function in Nim, the argument is an immutable reference. Nim will include run-time checks on the bounds of the arrays. You can use an [`openarray`](https://nim-lang.github.io/Nim/manual.html#types-open-arrays) to accept an array of any size on the function arguments, and you can use `low(your_array)` and `high(your_array)` to query the bounds of the array. Nim `string` is compatible with `openArray[char]` to avoid unneeded copies for optimization, and `char` is compatible with `int`. Therefore `string` manipulation can be done with math in-place transparently. A function that takes `openArray[char]` accepts `"abcd"` *and* `['a', 'b', 'c', 'd']`. Array contents are always contiguous in memory, as are arrays of arrays. See also: - [Views](https://nim-lang.github.io/Nim/manual_experimental.html#view-types) ## Data Type Sizes - Whats the size of the different data types?. ```nim import std/json type Foo = object type Bar = enum true, false # (Weird spacing intended) assert sizeOf( Foo ) == 1 assert sizeOf( Bar ) == 1 assert sizeOf( bool ) == 1 assert sizeOf( {true} ) == 1 assert sizeOf( [true] ) == 1 assert sizeOf( (true) ) == 1 assert sizeOf( int8 ) == 1 assert sizeOf( {'k': 'v'} ) == 2 assert sizeOf( int16 ) == 2 assert sizeOf( int32 ) == 4 assert sizeOf( float32 ) == 4 assert sizeOf( int ) == 8 assert sizeOf( float ) == 8 assert sizeOf( @[true] ) == 8 assert sizeOf( %*{} ) == 8 assert sizeOf( pointer ) == 8 ``` This is only an approximation for the empty primitives on 64-bit. ## Objects Objects in Nim behave quite differently from classes in Python. Objects support inheritance and OOP. Classes are named types in Nim. Functions (procs) are free floating functions, not bound to objects (however, you can use them in a very similar way to Python). You can call a function on objects with the `object.function()` syntax as well as `function(object)`; [these are entirely equivalent](https://en.wikipedia.org/wiki/Uniform_Function_Call_Syntax). Nim does not have an implicit `self` nor `this`. It is best practice to put all your types near the top of the file, but this is not mandatory. A way to imagine this is that **procs get "glued" to the types of their arguments at compile-time**, and then you can use them at runtime as if they were Python classes and methods. From Python to Nim, *as minimal as possible* example: ```python class Kitten: """ Documentation Here """ def purr(self): print("Purr Purr") Kitten().purr() ``` :arrow_up: Python :arrow_up:          :arrow_down: Nim :arrow_down: ```nim type Kitten = object ## Documentation Here proc purr(self: Kitten) = echo "Purr Purr" Kitten().purr() ``` Minimal inheritance example: ```nim type Animal = object of RootObj type Kitten = object of Animal assert Kitten is Animal ``` Python-like object orientation examples: ``` Nim type Animal = ref object of RootObj ## Animal base object. age: int name: string ## Attributes of base object. type Cat = ref object of Animal ## Cat inherited object. playfulness: float ## Attributes of inherited object. func increase_age(self: Cat) = self.age.inc() # Cat object function, access and *modify* object. var kitten = Cat(name: "Tom") # Cat object instance. kitten.increase_age() # Cat object function used. assert kitten.name == "Tom" # Assert on Cat object. assert kitten.age == 1 ``` Inheritance example: ```nim type LUCA = ref object of RootObj Archea = ref object of LUCA Prokaryota = ref object of Archea Eukaryota = ref object of Prokaryota Animalia = ref object of Eukaryota Chordata = ref object of Animalia Mammalia = ref object of Chordata Primates = ref object of Mammalia Haplorhini = ref object of Primates Simiiformes = ref object of Haplorhini Hominidae = ref object of Simiiformes Homininae = ref object of Hominidae Hominini = ref object of Homininae Homo = ref object of Hominini Homosapiens = ref object of Homo assert Homosapiens() is LUCA assert LUCA() isnot Homosapiens assert sizeOf(Homosapiens) == sizeOf(LUCA) let human = Homosapiens() assert human is Homosapiens ``` See also: - [Dynamic Typing via metaprogramming](https://github.com/Carpall/nobject#here-a-simple-object-for-allowing-dynamic-typing-in-nim) ## `self.__init__()` After the Cat example you are probably wondering how to do `def __init__(self, arg):`. Python `__init__()` is Nim `newObject()` or `initObject()`. Lets make an `__init__()` for the Cat: ```nim type Cat = object # Cat object. age: int name: string # Attributes of Cat object. func initCat(age = 2): Cat = # Cat.__init__(self, age=2) result.age = age # self.age = age result.name = "adopt_me" # self.name = "adopt_me" var kitten = initCat() # Cat object instance. assert kitten.name == "adopt_me" # Assert on Cat object. assert kitten.age == 2 ``` Naming is a convention and best practice. When you want init for `Foo` just make `newFoo()` or `initFoo()`. As you may notice `initCat` is just a function that returns a `Cat`. - `initFoo()` for `object`. - `newFoo()` for `ref object`. [Read the documentation for Naming things following conventions and best practices.](https://nim-lang.org/docs/apis.html) #### Object Attribute Default Values The object constructor is also the way to set custom default values to the attributes of your objects: ```nim type Cat = object age: int # AutoInitialized to 0 name: string # AutoInitialized to "" playfulness: float # AutoInitialized to 0.0 sleeping: bool # AutoInitialized to false func initCat(): Cat = result.age = 1 # Set default value to 1 result.name = "Bastet" # Set default value to "Bastet" result.playfulness = 9.0 # Set default value to 9.0 result.sleeping = true # Set default value to true ``` A more complete structure for a basic program can be something like: ```nim ## Simple application to do Foo with the Bar. type Animal = ref object of RootObj age: int name: string Cat = ref object of Animal playfulness: float func initCat(age = 2): Cat = result = new Cat # must initialize a ref object result.age = age result.name = "adopt_me" func increase_age(self: Cat) = self.age.inc() proc main() = var kitten = Cat(name: "Tom") kitten.increase_age() assert kitten.name == "Tom" assert kitten.age == 1 var kitten2 = initCat() # new kitten assert kitten2.age == 2 assert kitten2.name == "adopt_me" when isMainModule: main() runnableExamples: echo "Optionally some documentation code examples here" assert 42 == 42 ``` ## Run-time Code Expansion Python objects that *internally* use code generation are very very slow, scaling with size. The more you use it the slower it runs. `dataclass`, `metaclass`, decorators, etc can be more than 25 ~ 50x slower than a normal `class`. [`pathlib.Path` and its methods can be more than 25 ~ 50x slower than a normal `str`](https://youtu.be/tFrh9hKMS6Y), and defeats any optimization, including a `.pyc` file. Cython does not have CTFE, so it does not help with this specifically. - Nim code expansion is done at compile-time, making its code generation zero cost at run-time. For example, you can see the result of ARC code expansion during compilation using `--expandArc`. This is how Nim does compile-time memory management (approximation):  See also: - [`macros.expandMacros` shows you the code expansion on the terminal during compilation](https://nim-lang.github.io/Nim/macros.html#expandMacros.m%2Ctyped) # Unsafe Type Hints Python "type hints" can be almost anything and are implicitly executed at run-time. Needless to say, this can be very unsafe: ```console $ cat example.py class X: _: "print('PWNED')" # os.system("rm -rf /folder ") __import__("typing").get_type_hints(X) $ python3 example.py 'PWNED' $ ``` Nim types must be a valid Nim type, types are type checked at compile-time: ```console $ cat example.nim type X = object a: "echo('PWNED')" echo X() $ nim r example.nim # Will not compile. Error: type expected, but got: "echo('PWNED')" $ ``` Another example ```console $ cat example.nim var example: "echo('PWNED')" echo example $ nim r example.nim # Will not compile. Error: type expected, but got: "echo('PWNED')" $ ``` # By-Value vs By-Reference - Does Nim passes data around "by value" or "by reference"? It depends ... The Nim compiler automatically determines whether a parameter is passed by-value or by-reference based on the parameter type's size. If a parameter must be passed by-value or by-reference (such as when interfacing with a C library), then use the `{.bycopy.}` or `{.byref.}` pragmas. **Nim passes objects larger than `3 * sizeOf(int)` by reference for performance**, but this is architecture and implementation defined. So the following information is just an *approximation* for `x86_64`: Declaration | Value or Reference? | Implicit or Explicit? | Managed or Unmanaged? | Observations | ----------------------|---------------------|-----------------------|-----------------------|-----------------| `symbol: int` | By value | Implicit | Managed | Frequent use | `symbol: var int` | By reference | Implicit | Managed | Frequent use | `symbol: ref int` | By reference | Explicit | Managed | Rare | `symbol: ptr int` | By reference | Explicit | Unmanaged | C/C++ FFI | `symbol: var ref int` | By reference | Implicit | Managed | Rare | `symbol: var ptr int` | By reference | Implicit | Unmanaged | Rare | `symbol: pointer` | By reference | Explicit | Unmanaged Pointer | C/C++ FFI | - Iff a "by value" symbol is big, then it is passed "by reference" automatically. - [You can disable this optimization using `{.bycopy.}` pragma on the symbol.](https://nim-lang.github.io/Nim/manual.html#foreign-function-interface-bycopy-pragma) - [The `{.byref.}` forces passing "by reference", the reverse of `{.bycopy.}`.](https://nim-lang.github.io/Nim/manual.html#foreign-function-interface-byref-pragma) - Nim `seq` is passed around "by reference" by default. - Nim `string` is [Copy-On-Write](https://en.wikipedia.org/wiki/Copy-on-write) COW (ARC/ORC). - [Pointer Arithmetic can be performed with pointer.](https://en.wikipedia.org/wiki/Pointer%20arithmetic) ## Ranges In Python, simple integer for loops use the `range` generator function. For the 1- and 2- argument forms of this function, nim's [`..` iterator](https://nim-lang.org/docs/system.html#...i%2CT%2CT) works almost the same way: ``` Nim for i in 0 .. 10: echo i # Prints 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 for i in 5 .. 10: echo i # Prints 5, 6, 7, 8, 9, 10 ``` Note that the `..` operator includes the end of the range, whereas Python's `range(a, b)` does not include `b`. If you prefer this behavior, use the [`..<` iterator](https://nim-lang.org/docs/system.html#..%3C.i%2CT%2CT) instead: ``` Nim for i in 0 ..< 10: echo i # Prints 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 ``` Python `range()` also has an optional third parameter, which is the value to increment by each step. This can be positive or negative. If you need this behavior, use the [`countup`](https://nim-lang.org/docs/system.html#countup.i%2CT%2CT%2CPositive) or [`countdown`](https://nim-lang.org/docs/system.html#countdown.i%2CT%2CT%2CPositive) iterators: ``` Nim for i in countup(1, 10, 2): echo i # Prints 1, 3, 5, 7, 9 for i in countdown(9, 0, 2): echo i # Prints 9, 7, 5, 3, 1 ``` Convert from `range` to `seq`: ```nim import sequtils const subrange = 0..9 const seqrange = toSeq(subrange) assert seqrange is seq[int] ``` See also: - [enumerate](https://nim-lang.github.io/Nim/enumerate.html#enumerate.m%2CForLoopStmt) ## Slices The syntax for slice ranges is different. Python's `a[x:y]` is Nim's `a[x ..< y]`. ``` Nim let variable = [1, 2, 3, 4] assert variable[0 .. 0] == @[1] assert variable[0 .. 1] == @[1, 2] assert variable[0 ..< 2] == @[1, 2] assert variable[0 .. 3] == @[1, 2, 3, 4] ``` #### Reverse Index Slices In Nim a reverse index or backwards index uses `^` with the number, like `^1`. Backwards indexes have a specific type `BackwardsIndex`, and they can also be "prepared" at compile-time as a `const`: ```nim const lastOne = ^1 # Compile-time assert lastOne is BackwardsIndex assert [1, 2, 3, 4, 5][2 .. lastOne] == @[3, 4, 5] assert [1, 2, 3, 4, 5][2 .. ^1] == @[3, 4, 5] var another = ^3 # Run-time assert [1, 2, 3, 4, 5][0 .. another] == @[1, 2, 3] assert [1, 2, 3, 4, 5][^3 .. ^1] == @[3, 4, 5] # 2 Reverse index ``` ## Static Bounds-Check - Nim has [static bounds-checking at compile-time.](https://en.wikipedia.org/wiki/Static_program_analysis) Lets compare very simplified examples: ```python [0, 1, 2][9] # No Index 9 ``` This crashes at run-time because there is no index 9: ```console $ python3 example.py Traceback (most recent call last): File "example.py", line 1, in [0, 1, 2][9] IndexError: list index out of range $ ``` Let's see Nim: ```nim discard [0, 1, 2][9] # No Index 9 ``` Compile and run: ```console $ nim compile --run example.nim example.nim(1, 19) Warning: can prove: 9 > 2 [IndexCheck] example.nim(1, 18) Error: index 9 not in 0..2 [0, 1, 2][9] $ ``` Nim checks at compile-time that `[0, 1, 2]` has no index `9`, because `9 > 2`. Therefore, it won't compile nor run. This also works with Subrange. Let's say you have a integer variable that **must be positive**: ```nim let must_be_positive: Positive = -9 ``` Compile and run: ```console $ nim compile --run example.nim example.nim(1, 34) Warning: can prove: 1 > -9 [IndexCheck] example.nim(1, 34) Error: conversion from int literal -9 to Positive is invalid. $ ``` Nim checks at compile-time that `must_be_positive` is not `Positive` because `1 > -9`. It won't compile nor run. Another example: ```nim var variable0: 5..8 = 5 # int range type, value must be between '5' and '8'. variable0 = 8 variable0 = 7 assert not compiles(variable0 = 4) assert not compiles(variable0 = 9) assert not compiles(variable0 = 0) assert not compiles(variable0 = -1) assert not compiles(variable0 = -9) var variable1: 3.3..7.5 = 3.3 # float range type, value must be between '3.3' and '7.5'. variable1 = 7.5 variable1 = 5.5 assert not compiles(variable1 = 3.2) assert not compiles(variable1 = 7.6) assert not compiles(variable1 = 0.0) assert not compiles(variable1 = -1.0) assert not compiles(variable1 = -9.0) var variable2: 'b'..'f' = 'b' # char range type, value must be between 'b' and 'f'. variable2 = 'f' variable2 = 'c' assert not compiles(variable2 = 'g') assert not compiles(variable2 = 'a') assert not compiles(variable2 = 'z') assert not compiles(variable2 = '0') assert not compiles(variable2 = '9') var variable3: Positive = 1 # Positive type, value must be > 0. variable3 = 1 variable3 = 999 assert not compiles(variable3 = 0) assert not compiles(variable2 = -1) assert not compiles(variable2 = -9) var variable4: Natural = 0 # Natural type, value must be >= 0. variable4 = 1 variable4 = 999 assert not compiles(variable4 = -1) assert not compiles(variable4 = -9) ``` You can control this with `--staticBoundChecks:on` or `--staticBoundChecks:off`. With `--staticBoundChecks:off` it may raise an error at run-time like Python does. - For better documentation see: https://nim-lang.github.io/Nim/drnim.html # Null Coalescing Python does not have a [null-coalescing operator](https://en.wikipedia.org/wiki/Null_coalescing_operator) (at the time of writing). Python programmers use the ternary conditional operator instead: ```python other = bar if bar is not None else "default value" # "bar" may be null?, or not ?. ``` Nim has a [wrapnils module](https://nim-lang.github.io/Nim/wrapnils.html) with a `?.` null-coalescing operator, which simplifies code by reducing the need for `if..elif...else` branches around **intermediate values that may be null**. ```nim assert ?.foo.bar.baz == "" # "bar" may be Null?, or not ?. ``` Null is `None` in Python. Null is `nil` in Nim. See: https://nim-lang.github.io/Nim/wrapnils.html ## With Context Manager There is no direct built-in equivalent to Python's `with` construct. In Nim there are the following options: - [`template`](#templates) - [`macros`](https://nim-lang.github.io/Nim/manual.html#macros) - Named [`block`](https://nim-lang.github.io/Nim/manual.html#statements-and-expressions-block-statement) [See the Templates section for examples.](#templates) ## Strings | Lang | String | Multi-line string | Raw String | Multi-line Raw string | Formatted Literals | Quote | |---------------------------|---------|-------------------|-------------|-----------------------|--------------------|---------| | :snake: Python | `"foo"` | `"""foo"""` | `r"foo"` | `r"""foo"""` | `f"""{1 + 2}"""` | `"` `'` | | :crown: Nim | `"foo"` | `"""foo"""` | `r"foo"` | `r"""foo"""` | `fmt"""{1 + 2}"""` | `"` | #### String Ops | Ops | :snake: Python | :crown: Nim | |-------|------------------|-------------------------| | Lower | `"ABCD".lower()` | `"ABCD".toLowerAscii()` | | Strip | `" ab ".strip()` | `" ab ".strip()` | | Split | `"a,b,c".split(",")` | `"a,b,c".split(",")` | | Concatenation | `"a" + "b"` | `"a" & "b"` | | Find | `"abcd".find("c")` | `"abcd".find("c")` | | Starts With | `"abc".startswith("ab")` | `"abc".startswith("ab")` | | Ends With | `"abc".endswith("ab")` | `"abc".endswith("ab")` | | Split Lines | `"1\n2\n3".splitlines()` | `"1\n2\n3".splitlines()` | | Slicing | `"abcd"[0:2]` | `"abcd"[0 ..< 2]` | | Slicing 1 char | `"abcd"[2]` | `"abcd"[2]` | | Reverse Slicing | `"abcd"[-1]` | `"abcd"[^1]` | | Normalize | [`unicodedata.normalize("NFC", "Foo")`](https://docs.python.org/3/library/unicodedata.html#unicodedata.normalize "str().lower() is not the recommended way") | `"Foo".normalize()` | Count Lines | `len("1\n2\n3".splitlines())` | `"1\n2\n3".countLines()` | | Repeat | `"foo" * 9` | `"foo".repeat(9)` | | Indent | `textwrap.indent("foo", " " * 9)` | `"foo".indent(9)` | | Unindent | [`textwrap.dedent("foo")`](https://docs.python.org/3/library/textwrap.html#textwrap.dedent "textwrap.dedent() Removes ALL indentation!") | `"foo".unindent(9)` | | Parse Bool | [`bool(distutils.util.strtobool("fALse"))`](https://stackoverflow.com/q/715417 "bool('fALse') == True") :question: | `parseBool("fALse")` | | Parse Int | `int("42")` | `parseInt("42")` | | Parse Float | `float("3.14")` | `parseFloat("3.14")` | | Formatted String Literals | `f"foo {1 + 2} bar {variable}"` | [`fmt"foo {1 + 2} bar {variable}"`](https://nim-lang.org/docs/strformat.html) | | Levenshtein distance | :negative_squared_cross_mark: | [`editDistance("Kitten", "Bitten")`](https://nim-lang.org/docs/editdistance.html) | - **Nim string operations require [`import std/strutils`.](https://nim-lang.org/docs/strutils.html)** - A [very detailed comparison.](https://scripter.co/notes/string-functions-nim-vs-python/) #### String Efficiency Single memory allocation strings can be done with `newStringOfCap(capacity = 42)`, that returns a new empty string `""` but with allocated `capacity` of `42`. If you pass beyond the `capacity` it will not crash nor buffer overflow: ```python variable = "" assert variable == "" # length is 0, capacity is 0, 1 allocations, 0 copies variable += "a" # length is 1, capacity is 1, 2 allocations, 1 copies variable += "b" # length is 2, capacity is 2, 3 allocations, 2 copies variable += "c" # length is 3, capacity is 3, 4 allocations, 3 copies variable += "d" # length is 4, capacity is 4, 5 allocations, 4 copies assert variable == "abcd" # TOTAL: 5 allocations, 4 copies ``` :arrow_up: Python :arrow_up:          :arrow_down: Nim :arrow_down: ```nim var variable = newStringOfCap(2) assert variable == "" # length is 0, capacity is 2, 1 allocations, 0 copies variable.add "a" # length is 1, capacity is 2, 1 allocations, 0 copies variable.add "b" # length is 2, capacity is 2, 1 allocations, 0 copies variable.add "c" # length is 3, capacity is 3, 2 allocations, 0 copies variable.add "d" # length is 4, capacity is 4, 3 allocations, 0 copies assert variable == "abcd" # TOTAL: 3 allocations, 0 copies ``` This difference may get bigger for strings inside for loops or while loops. ### F-Strings Nim `strformat` implements formatted string literals inspired by Python f-strings. `strformat` is implemented using metaprogramming and the code expansion is done at compile-time. It also works for the JavaScript target. Similar to Python f-strings, you can [debug the key-value inside the string using an equal sign](https://nim-lang.github.io/Nim/strformat.html#debugging-strings). `fmt"{key=}"` expands to `fmt"key={value}"`: ```nim let x = "hello" assert fmt"{x=}" == "x=hello" assert fmt"{x = }" == "x = hello" ``` Nim `strformat` supports backslash, while Python's f-strings do not: ```python >>> print( f"""{ "yep\nope" }""" ) # Run-time error. Error: f-string expression part cannot include a backslash. ``` :arrow_up: Python :arrow_up:          :arrow_down: Nim :arrow_down: ```nim echo fmt"""{ "yep\nope" }""" # Nim works. yep ope ``` You can choose a custom character pair to open and close the formatting inside the string by passing the `char` as argument: ```nim import std/strformat let variable = 42 assert fmt("( variable ) { variable }", '(', ')') == "42 { variable }" assert fmt("< variable > { variable }", '<', '>') == "42 { variable }" ``` Using characters like backtick and space `' '` works: ```nim import std/strformat let variable = 42 assert fmt(" variable`{variable}", ' ', '`') == "42{variable}" ``` - [It is recommended to read the full documentation of `strformat`.](https://nim-lang.github.io/Nim/strformat.html) ## Raw String Python raw string can not end in `"\"`, but Nim raw string works just fine: ```python >>> print(r"\") # Run-time error. SyntaxError: EOL while scanning string literal. ``` :arrow_up: Python :arrow_up:          :arrow_down: Nim :arrow_down: ```nim nim> echo r"\" "\" ``` This may be relevant when working with strings that use `"\"`, like filesystems paths ` r"C:\mypath\" `, etc. ## Standard Library Equivalents - [Python Standard Library for Nim.](https://github.com/juancarlospaco/cpython#alternative-stdlib-for-nim-for-python-targets) Use | :snake: Python | :crown: Nim -----------------------------|------------------|------------------------------ Operating System | os | [os](https://nim-lang.org/docs/os.html) String operations | string | [strutils](https://nim-lang.org/docs/strutils.html) Date & time | datetime | [times](https://nim-lang.org/docs/times.html) Random | random | [random](https://nim-lang.org/docs/random.html) Regular expressions (Backend) | re | [re](https://nim-lang.org/docs/re.html) Regular expressions (Frontend) | :negative_squared_cross_mark: | [jsre](https://nim-lang.github.io/Nim/jsre.html) HTTP | urllib | [httpclient](https://nim-lang.org/docs/httpclient.html) Logging | logging | [logging](https://nim-lang.org/docs/logging.html) Run external commands | subprocess | [osproc](https://nim-lang.org/docs/osproc.html) Path manipulation | pathlib, os.path | [os](https://nim-lang.org/docs/os.html) Mathematic | math, cmath | [math](https://nim-lang.org/docs/math.html) MIME Types | mimetypes | [mimetypes](https://nim-lang.org/docs/mimetypes.html) SQLite SQL | sqlite3 | [db_sqlite](https://nim-lang.org/docs/db_sqlite.html) Postgres SQL | :negative_squared_cross_mark: | [db_postgres](https://nim-lang.org/docs/db_postgres.html) Levenshtein Distance | :negative_squared_cross_mark: | [editdistance](https://nim-lang.github.io/Nim/editdistance.html) Serialization | pickle | [json](https://nim-lang.org/docs/json.html), [jsonutils](https://nim-lang.github.io/Nim/jsonutils.html), [marshal](https://nim-lang.org/docs/marshal.html) Base64 | base64 | [base64](https://nim-lang.org/docs/base64.html) Open web browser URL | webbrowser | [browsers](https://nim-lang.org/docs/browsers.html) Async | asyncio | [asyncdispatch](https://nim-lang.org/docs/asyncdispatch.html), [asyncfile](https://nim-lang.org/docs/asyncfile.html), [asyncnet](https://nim-lang.org/docs/asyncnet.html), [asyncstreams](https://nim-lang.org/docs/asyncstreams.html) Unittests | unittest | [unittest](https://nim-lang.org/docs/unittest.html) Diff | difflib | [diff](https://nim-lang.org/docs/diff.html) Colors | colorsys | [colors](https://nim-lang.org/docs/colors.html) MD5 | hashlib.md5 | [md5](https://nim-lang.org/docs/md5.html) SHA1 | hashlib.sha1 | [sha1](https://nim-lang.org/docs/sha1.html) HTTP Server | http.server | [asynchttpserver](https://nim-lang.org/docs/asynchttpserver.html) Lexer | shlex | [lexbase](https://nim-lang.org/docs/lexbase.html) Multi-Threading | threading | [threadpool](https://nim-lang.org/docs/threadpool.html) URL & URI | urllib.parse | [uri](https://nim-lang.org/docs/uri.html) CSV | csv | [parsecsv](https://nim-lang.org/docs/parsecsv.html) Parse command line arguments | argparse | [parseopt](https://nim-lang.org/docs/parseopt.html) SMTP | smtplib | [smtp](https://nim-lang.org/docs/smtp.html) HTTP Cookies | http.cookies | [cookies](https://nim-lang.org/docs/cookies.html) Statistics | statistics | [stats](https://nim-lang.org/docs/stats.html) Text wrapping | textwrap | [wordwrap](https://nim-lang.org/docs/wordwrap.html) Windows Registry | winreg | [registry](https://nim-lang.org/docs/registry.html) POSIX | posix | [posix](https://nim-lang.org/docs/posix.html), [posix_utils](https://nim-lang.org/docs/posix_utils.html) SSL | ssl | [openssl](https://nim-lang.org/docs/openssl.html) CGI | cgi | [cgi](https://nim-lang.org/docs/cgi.html) Profiler | cprofile, profile | [nimprof](https://nim-lang.github.io/Nim/nimprof.html) Monotonic time | time.monotonic | [monotimes](https://nim-lang.github.io/Nim/monotimes.html) Run functions at exit | atexit | [exitprocs](https://nim-lang.github.io/Nim/exitprocs.html) Set file permissions | os, stat | [os](https://nim-lang.github.io/Nim/os.html), [filepermissions](https://nim-lang.github.io/Nim/filepermissions.html) Recursive walk of filesystem | os.walk | [os.walkDirRec](https://nim-lang.github.io/Nim/os.html#walkDirRec.i%2Cstring), [globs.walkDirRecFilter](https://nim-lang.github.io/Nim/globs.html#walkDirRecFilter.i%2Cstring%2Cproc%28PathEntry%29) Templating engine | string.Template | [Source Code Filters](https://nim-lang.github.io/Nim/filters.html) Deques | collections.deque | [deques](https://nim-lang.github.io/Nim/deques.html) B-Tree based ordered Dictionary | :negative_squared_cross_mark: | [btreetables](https://nim-lang.github.io/Nim/btreetables.html) Critical Bit Tree Dict/Set | :negative_squared_cross_mark: | [critbits](https://nim-lang.github.io/Nim/critbits.html) Pooled Memory Allocation | :negative_squared_cross_mark: | [pools](https://nim-lang.github.io/Nim/pools.html) Parse JSON | json | [parsejson](https://nim-lang.org/docs/parsejson.html), [json](https://nim-lang.org/docs/json.html) Parse INI | configparser | [parsecfg](https://nim-lang.org/docs/parsecfg.html) Parse XML | xml | [parsexml](https://nim-lang.org/docs/parsexml.html), [xmltree](https://nim-lang.org/docs/xmltree.html) Parse HTML | html.parser | [htmlparser](https://nim-lang.org/docs/htmlparser.html) Parse SQL | :negative_squared_cross_mark: | [parsesql](https://nim-lang.org/docs/parsesql.html) Colors on the Terminal | :negative_squared_cross_mark: | [terminal](https://nim-lang.org/docs/terminal.html) Linux Distro Detection | :negative_squared_cross_mark: | [distros](https://nim-lang.org/docs/distros.html) HTML Generator | :negative_squared_cross_mark: | [htmlgen](https://nim-lang.org/docs/htmlgen.html) Arrow Functions | :negative_squared_cross_mark: | [sugar](https://nim-lang.org/docs/sugar.html#%3D%3E.m%2Cuntyped%2Cuntyped) In-Place to Work-on-Copy | :negative_squared_cross_mark: | [sugar.dup](https://nim-lang.org/docs/sugar.html#dup.m%2CT%2Cvarargs%5Buntyped%5D) Syntax Sugar | :negative_squared_cross_mark: | [sugar](https://nim-lang.org/docs/sugar.html) JavaScript & Frontend | :negative_squared_cross_mark: | [dom](https://nim-lang.org/docs/dom.html), [asyncjs](https://nim-lang.org/docs/asyncjs.html), [jscore](https://nim-lang.org/docs/jscore.html), [jsffi](https://nim-lang.org/docs/jsffi.html), [dom_extensions](https://nim-lang.github.io/Nim/dom_extensions.html), [jsre](https://nim-lang.github.io/Nim/jsre.html) - This is not complete, but just a quick overview. For more information see https://nim-lang.org/docs/lib.html ## Tuples Tuples are fixed size, start at index `0`, can contain mixed types, and can be anonymous or named. Named tuples have no extra overhead over anonymous tuples. #### Anonymous Tuple ```python (1, 2, 3) ``` :arrow_up: Python :arrow_up:          :arrow_down: Nim :arrow_down: ```nim (1, 2, 3) ``` #### Named Tuple - Nim allows the fields to be named, without requiring the tuple itself to be named. Python NamedTuple requires `import collections`, and we need to give it a dummy underscore name: ```python collections.namedtuple("_", "key0 key1")("foo", 42) ``` :arrow_up: Python :arrow_up:          :arrow_down: Nim :arrow_down: ```nim (key0: "foo", key1: 42) ``` #### Named Tuple - We can also name both the fields and the tuple: ```python collections.namedtuple("NameHere", "key0 key1")("foo", 42) ``` :arrow_up: Python :arrow_up:          :arrow_down: Nim :arrow_down: ```nim type NameHere = tuple[key0: string, key1: int] var variable: NameHere = (key0: "foo", key1: 42) ``` `tuple[key0: string, key1: int]` is the `typedesc` for Declarations. `(key0: "foo", key1: 42)` is the Literal for Assigns. Nim tuples are a lot like Python NamedTuple in that the tuple members have names. Do NOT use named tuples to "mimic" proper objects (the compiler re-uses generic instantiations for "identical" tuples). See [manual](https://nim-lang.org/docs/manual.html#types-tuples-and-object-types) for a more in depth look at tuples. ## Lists [Nim sequences](https://nim-lang.org/docs/tut1.html#advanced-types-sequences) are *not* fixed size. They can grow and shrink, start at index `0`, and must contain elements all of the same type. ```python ["foo", "bar", "baz"] ``` :arrow_up: Python :arrow_up:          :arrow_down: Nim :arrow_down: ```nim @["foo", "bar", "baz"] ``` [@](https://nim-lang.github.io/Nim/system.html#%40%2CopenArray%5BT%5D) is a function that converts from `array` to `seq`. ## List Comprehensions ```python variable = [item for item in (-9, 1, 42, 0, -1, 9)] ``` :arrow_up: Python :arrow_up:          :arrow_down: Nim :arrow_down: ```nim let variable = collect(newSeq): for item in @[-9, 1, 42, 0, -1, 9]: item ``` A comprehension can be assigned to `const` too, and it will run at compile-time. The comprehension is implemented as a `macro` that is expanded at compile-time, you can see the expanded code using the `--expandMacro` compiler option: ```nim let variable = var collectResult = newSeq(Natural(0)) for item in items(@[-9, 1, 42, 0, -1, 9]): add(collectResult, item) collectResult ``` The comprehensions can be nested, multi-line, multi-expression, all with zero overhead: ```nim import std/sugar let values = collect(newSeq): for val in [1, 2]: collect(newSeq): for val2 in [3, 4]: if (val, val2) != (1, 2): (val, val2) assert values == @[@[(1, 3), (1, 4)], @[(2, 3), (2, 4)]] ``` Single-line example: ```python print([i for i in range(0, 9)]) ``` :arrow_up: Python :arrow_up:          :arrow_down: Nim :arrow_down: ```nim echo(block: collect newSeq: (for i in 0..9: i)) ``` Python comprehensions convert the code to a generator, but Nim comprehensions do not convert the code to an iterator: ```nim import std/sugar func example() = discard collect(newSeq): for item in @[-9, 1, 42, 0, -1, 9]: if item == 0: return item example() ``` :arrow_up: Nim :arrow_up:          :arrow_down: Python :arrow_down: ```python def example(): [item for item in [-9, 1, 42, 0, -1, 9] if item == 0: return] example() ``` Python complains: ``` SyntaxError: invalid syntax. ``` Some things that in Python are syntactically disallowed inside comprehensions (like `return`) are allowed in Nim. This is because Nim comprehensions are just macros that expand to normal code. - Whats `collect()`?. [collect](https://nim-lang.github.io/Nim/sugar.html#collect.m%2Cuntyped%2Cuntyped) takes as argument whatever your returning type uses as the constructor. ## Dict Comprehensions ```python variable = {key: value for key, value in enumerate((-9, 1, 42, 0, -1, 9))} ``` :arrow_up: Python :arrow_up:          :arrow_down: Nim :arrow_down: ```nim let variable = collect(initTable(4)): for key, value in @[-9, 1, 42, 0, -1, 9]: {key: value} ``` - `collect()` requires [`import std/sugar`.](https://nim-lang.github.io/Nim/sugar.html#collect.m%2Cuntyped%2Cuntyped) ## Set Comprehensions ```python variable = {item for item in (-9, 1, 42, 0, -1, 9)} ``` :arrow_up: Python :arrow_up:          :arrow_down: Nim :arrow_down: ```nim let variable = collect(initHashSet): for item in @[-9, 1, 42, 0, -1, 9]: {item} ``` - `collect()` requires [`import std/sugar`.](https://nim-lang.github.io/Nim/sugar.html#collect.m%2Cuntyped%2Cuntyped) ## Sets | Lang | Set | Ordered Set | Bitset | [Bit Fields](https://en.wikipedia.org/wiki/Bit_field) | Imports | |---------------------------|------------|-------------------------------|-------------------------------|---------------|---------------| :snake: Python | `set()` | :negative_squared_cross_mark: | :negative_squared_cross_mark: | :negative_squared_cross_mark: | | :crown: Nim | [`HashSet()`](https://nim-lang.github.io/Nim/sets.html#HashSet) | [`OrderedSet()`](https://nim-lang.github.io/Nim/sets.html#OrderedSet) | [`set`](https://nim-lang.org/docs/manual.html#types-set-type) | [Bit Fields](https://nim-lang.org/docs/manual.html#set-type-bit-fields) | `import std/sets` | - **Python Set can be replaced with [HashSet](https://nim-lang.org/docs/sets.html).** Python sets are not like the [Nim set type](https://nim-lang.org/docs/manual.html#types-set-type). The "default" set is a [bitset](https://en.wikipedia.org/wiki/Bitset). For every possible value of the contained type, it stores 1 bit indicating whether it is present in the set. This means you should use it if the type has a finite, limited range of possible values. If all of the possible values are also known at compile-time, you can create an `Enum` for them. The biggest integer you can fit on a set normally is `65535` equals to `high(uint16)`. You can fit bigger integers using an integer Subrange, if you don't need small integers. An example really stressing set to fit `2_147_483_647` equals to `high(int32)` on a set at compile-time: ```nim const x = {range[2147483640..2147483647](2147483647)} assert x is set # Equals to {2147483647} ``` The basic Nim set type is fast and memory-efficient, compared to the [HashSet](https://nim-lang.org/docs/sets.html) which is implemented as a dictionary. For simple flag types and small mathematical sets, use set. For larger collections, or if you are just learning, use HashSet. ## Dictionaries Use [tables](https://nim-lang.org/docs/tables.html) for Python-like dicts. | Lang | Dictionary | Ordered Dictionary | Counter | Imports | |---------------------------|------------|-----------------------------|----------------|----------------------| :snake: Python | `dict()` | `OrderedDict()` | `Counter()` | `import std/collections` | :crown: Nim | [`Table()`](https://nim-lang.org/docs/tables.html#basic-usage-table) | [`OrderedTable()`](https://nim-lang.org/docs/tables.html#basic-usage-orderedtable) | [`CountTable()`](https://nim-lang.org/docs/tables.html#basic-usage-counttable) | `import std/tables` | #### Table Constructors ```python dict(key="value", other="things") ``` :arrow_up: Python :arrow_up:          :arrow_down: Nim :arrow_down: ```nim to_table({"key": "value", "other": "things"}) ``` #### Ordered Dictionary ```python collections.OrderedDict([(8, "hp"), (4, "laser"), (9, "engine")]) ``` :arrow_up: Python :arrow_up:          :arrow_down: Nim :arrow_down: ```nim to_ordered_table({8: "hp", 4: "laser", 9: "engine"}) ``` #### Counters ```python collections.Counter(["a", "b", "c", "a", "b", "b"]) ``` :arrow_up: Python :arrow_up:          :arrow_down: Nim :arrow_down: ```nim to_count_table("abcabb") ``` **Examples:** ``` Nim import std/tables var dictionary = to_table({"hi": 1, "there": 2}) assert dictionary["hi"] == 1 dictionary["hi"] = 42 assert dictionary["hi"] == 42 assert len(dictionary) == 2 assert dictionary.has_key("hi") for key, value in dictionary: echo key, value ``` Tables are just syntax sugar for an array of tuples: ```nim assert {"key": "value", "k": "v"} == [("key", "value"), ("k", "v")] assert {"key": true, "k": false} == @[("key", true), ("k", false)] ``` #### B-Tree Tables [B-Tree based generic sorted Tables](https://nim-lang.github.io/Nim/btreetables.html) using the same API. See also: - [Whats B-Tree ? (Wikipedia).](https://en.wikipedia.org/wiki/B-tree) - [Whats B-Tree ? (1 Minute Animated Video).](https://youtu.be/coRJrcIYbF4) ### Ternary operators ```python "result0" if conditional else "result1" ``` :arrow_up: Python :arrow_up:          :arrow_down: Nim :arrow_down: ```nim if conditional: "result0" else: "result1" ``` In Nim the "ternary operator" is simply an `if..else` inline. Unlike Python, the ordinary `if..else` is an expression, so it can be assigned to a variable. These snippets are equivalent: ```nim var foo = if 3 < 10: 50 else: 100 ``` ```nim var foo = if 3 < 10: 50 else: 100 ``` ## Reading and writing files **Reading files line by line** ```python with open("yourfile.txt", "r") as f: for line in f: print(line) ``` :arrow_up: Python :arrow_up:          :arrow_down: Nim :arrow_down: ```nim for line in lines("yourfile.txt"): echo line ``` - `lines()` Documentation https://nim-lang.org/docs/io.html#lines.i%2Cstring **Reading and writing files:** ```nim write_file("yourfile.txt", "this string simulates data") assert read_file("yourfile.txt") == "this string simulates data" ``` **Reading files at compile-time:** ```nim const constant = static_read("yourfile.txt") # Returns a string at compile-time ``` ## Change File Permissions ```python import os os.chmod("file.txt", 0o777) ``` :arrow_up: Python :arrow_up:          :arrow_down: Nim :arrow_down: ```nim import fusion/filepermissions chmod "file.txt", 0o777 ``` These examples assume a file `"file.txt"` exists. [Both use the octal Unix file permissions.](https://en.wikipedia.org/wiki/File-system_permissions#Notation_of_traditional_Unix_permissions) [Also a lower level API is available on `os` module.](https://nim-lang.github.io/Nim/os.html#setFilePermissions%2Cstring%2Cset%5BFilePermission%5D) See https://nim-lang.github.io/fusion/src/fusion/filepermissions.html ## Temporarily Change Folder ```python import os class withDir: # Unsafe without a __del__() def __init__(self, newPath): self.newPath = os.path.expanduser(newPath) def __enter__(self): self.savedPath = os.getcwd() os.chdir(self.newPath) def __exit__(self, etype, value, traceback): os.chdir(self.savedPath) with withDir("subfolder"): print("Inside subfolder") print("Go back outside subfolder") ``` :arrow_up: Python :arrow_up:          :arrow_down: Nim :arrow_down: ```nim import fusion/scripting withDir "subfolder": echo "Inside subfolder" echo "Go back outside subfolder" ``` These examples assume a folder `"subfolder"` exists. Python optionally has third-party dependencies which do the same thing; the examples use the standard library. Some Python third-party dependencies may convert the code inside `withDir` to a generator, forcing you to change the code (like `return` to `yield` etc), examples use standard library. See https://nim-lang.github.io/fusion/src/fusion/scripting.html ## Map & Filter ```python def isPositive(arg: int) -> bool: return arg > 0 map(isPositive, [1, 2,-3, 5, -9]) filter(isPositive, [1, 2,-3, 5, -9]) ``` :arrow_up: Python :arrow_up:          :arrow_down: Nim :arrow_down: ```nim proc isPositive(arg: int): bool = return arg > 0 echo map([1, 2,-3, 5, -9], isPositive) echo filter([1, 2,-3, 5, -9], isPositive) ``` - map and filter operations require [`import std/sequtils`.](https://nim-lang.org/docs/sequtils.html) ## Lambdas ```python variable: typing.Callable[[int, int], int] = lambda var1, var2: var1 + var2 ``` :arrow_up: Python :arrow_up:          :arrow_down: Nim :arrow_down: ```nim var variable = proc (var1, var2: int): int = var1 + var2 ``` Multi-line example: ```nim var anon = func (x: int): bool = if x > 0: result = true else: result = false assert anon(9) ``` Python anonymous functions can not use `return`, but it just works in Nim: ```python example = lambda: return 42 assert example() == 42 ``` Complains `SyntaxError: invalid syntax`. :arrow_up: Python :arrow_up:          :arrow_down: Nim :arrow_down: ```nim let example = func: int = return 42 assert example() == 42 ``` Python anonymous functions can not use `yield`, but it just works in Nim: ```python example = lambda: for i in range(0, 9): yield i for _ in example(): pass ``` Complains `SyntaxError: invalid syntax`. :arrow_up: Python :arrow_up:          :arrow_down: Nim :arrow_down: ```nim let example = iterator: int = for i in 0..9: yield i for _ in example(): discard ``` [Anonymous procs in Nim](https://nim-lang.org/docs/manual.html#procedures-anonymous-procs) are basically functions without a name. ## Decorators - Templates and macros can be used similarly to Python's decorators. ```python def decorator(argument): print("This is a Decorator") return argument @decorator def function_with_decorator() -> int: return 42 print(function_with_decorator()) ``` :arrow_up: Python :arrow_up:          :arrow_down: Nim :arrow_down: ```nim template decorator(argument: untyped) = echo "This mimics a Decorator" argument func function_with_decorator(): int {.decorator.} = return 42 echo function_with_decorator() ``` - Why doesn't Nim use `@decorator` syntax?. Nim uses `{.` and `.}` because it can have several decorators together. Also in Nim one works on variables and types: ```nim func function_with_decorator(): int {.discardable, inline, compiletime.} = return 42 let variable {.compiletime.} = 1000 / 2 type Colors {.pure.} = enum Red, Green, Blue ``` [@](https://nim-lang.github.io/Nim/system.html#%40%2CopenArray%5BT%5D) is a function that converts from `array` to `seq`. ## JSON Python uses multi-line strings with JSON inside, Nim uses literal JSON directly in the code. ```python import json variable = """{ "key": "value", "other": true }""" variable = json.loads(variable) print(variable) ``` :arrow_up: Python :arrow_up:          :arrow_down: Nim :arrow_down: ```nim import std/json var variable = %*{ "key": "value", "other": true } echo variable ``` - `%*` converts everything inside the braces to JSON, JSON has a type `JsonNode`. - `%*` can have variables and literals inside the braces. - JSON can have Nim comments inside the braces of `%*`. - If the JSON is not valid JSON, the code will not compile. - `JsonNode` can be useful in Nim because is a type that can have [mixed types](https://nim-lang.github.io/Nim/manual.html#types-object-variants) and can grow/shrink. - You can read JSON at compile-time, and store it in a constant as a string. - To parse JSON from a string you can use `parseJson("{}")`. - To parse JSON from a file use `parseFile("file.json")`. - [JSON documentation](https://nim-lang.org/docs/json.html) ## Self-Execution of Main Module ```python if __name__ == "__main__": main() ``` :arrow_up: Python :arrow_up:          :arrow_down: Nim :arrow_down: ```nim when isMainModule: main() ``` ## Unittests ```python import unittest def setUpModule(): """Setup: Run once before all tests in this module.""" pass def tearDownModule(): """Teardown: Run once after all tests in this module.""" pass class TestName(unittest.TestCase): """Test case description""" def setUp(self): """Setup: Run once before each tests.""" pass def tearDown(self): """Teardown: Run once after each test.""" pass def test_example(self): self.assertEqual(42, 42) if __name__ == "__main__": unittest.main() ``` :arrow_up: Python :arrow_up:          :arrow_down: Nim :arrow_down: ```nim import std/unittest suite "Test Name": echo "Setup: Run once before all tests in this suite." setup: echo "Setup: Run once before each test." teardown: echo "Teardown: Run once after each test." test "example": assert 42 == 42 echo "Teardown: Run once after all tests in this suite." ``` - [Unittest documentation.](https://nim-lang.org/docs/unittest.html) - [The Nimble package manager can also run Unittests.](https://github.com/nim-lang/nimble#nimble-tasks) - [NimScript can also run Unittests.](https://nim-lang.org/docs/nims.html) - [You can run documentation as Unittests with `runnableExamples`](https://nim-lang.org/docs/system.html#runnableExamples%2Cuntyped). #### Assert with customized messages - `assert` can take a `block`. You can customize the message for better user experience: ```nim let a = 42 let b = 666 doAssert a == b, block: ("\nCustom Error Message!:" & "\n a equals to " & $a & "\n b equals to " & $b) ``` ### Testament An alternative to `unittest`. It is prepared for big projects and has more features. - https://nim-lang.github.io/Nim/testament.html **(Recommended)** ## Docstrings Docstrings in Nim are ReSTructuredText *and* MarkDown comments starting with `##`. ReSTructuredText and MarkDown can be mixed together if you want. Generate HTML, Latex (PDF) and JSON documentation from source code with `nim doc file.nim`. Nim can generate a dependency graph DOT `.dot` file with `nim genDepend file.nim`. [You can run documentation as Unittests with `runnableExamples`](https://nim-lang.org/docs/system.html#runnableExamples%2Cuntyped). ```python """Documentation of module""" class Kitten(object): """Documentation of class""" def purr(self): """Documentation of method""" print("Purr Purr") ``` :arrow_up: Python :arrow_up:          :arrow_down: Nim :arrow_down: ```nim ## Documentation of Module *ReSTructuredText* and **MarkDown** type Kitten = object ## Documentation of type age: int ## Documentation of field proc purr(self: Kitten) = ## Documentation of function echo "Purr Purr" ``` ## Optional Indentation You can write constructs like `if..then` and `try..except..finally` on a single line without errors or warnings; indentation is optional. Obviously, this is only a good idea if the snippet is short and simple. ```nim let a = try: 1 + 2 except: 42 finally: echo "Inline try" let b = if true: 2 / 4 elif false: 4 * 2 else: 0 for i in 0 .. 9: echo i proc foo() = echo "Function" (proc () = echo "Anonymous function")() template bar() = echo "Template" macro baz() = echo "Macro" var i = 0 while i < 9: i += 1 when isMainModule: echo 42 ``` ## CamelCase - Why is Nim CamelCase instead of snake_case?. It really isn't, Nim is style-agnostic! ```nim let camelCase = 42 # Declaring as camelCase assert camel_case == 42 # Using as snake_case let snake_case = 1 # Declaring as snake_case assert snakeCase == 1 # Using as camelCase let `free style` = 9000 assert free_style == 9000 ``` **This feature allows Nim to seamlessly interoperate with a lot of programming languages with different casing styles.** For more homogeneous code you can enforce a default casing style using the compiler command `--styleCheck:hint`. Nim will *style check* your code before compilation, similar to `pycodestyle` in Python. If you want even more strict styling you can use `--styleCheck:error`. Nim comes with a builtin code auto-formatter named Nimpretty. A lot of programming languages have some kind of case-insensitivity, such as: PowerShell, SQL, PHP, Lisp, Assembly, Batch, ABAP, Ada, Visual Basic, VB.NET, Fortran, Pascal, Forth, Cobol, Scheme, Red, Rebol. If you are just starting from scratch, you can use Python-like names while learning. It will not produce an error unless you tell the compiler you want that. ## def vs proc/func - Why doesn't Nim use `def` instead of `proc`?. Nim uses `proc` for normal functions, which is short for "procedure". Use `func` for when your routine cannot and should not access global or thread-local variables (see also: [pure functions](https://en.wikipedia.org/wiki/Pure_function)). [Nim has side-effects tracking.](https://nim-lang.github.io/Nim/manual.html#procedures-func) You can not use `echo` inside `func`, because `echo` mutates `stdout`, which is a side-effect. Use `debugEcho` instead. See also: - [Strict Func](https://nim-lang.github.io/Nim/manual_experimental.html#strict-funcs) - [Write Tracking for Nim](https://nim-lang.org/araq/writetracking_2.html) If you are just starting from scratch, you can use `proc` for all the functions while learning. It will not produce an error for doing so. # Async Nim has had async built-in for a long time. It works as you may expect with `async`, `await`, `Future`, etc. [asyncdispatch](https://nim-lang.org/docs/asyncdispatch.html) is a module to write concurrent code using the `async`/`await` syntax. `Future` is a type (like a Future in Python, or a Promise in JavaScript). `{.async.}` is a pragma that converts functions to async (like `async def` in Python). Let's convert the official Python asyncio *Hello World* to Nim: ```python async def main(): print("Hello ...") await asyncio.sleep(1) print("... World!") asyncio.run(main()) ``` :arrow_up: Python :arrow_up:          :arrow_down: Nim :arrow_down: ```nim proc main() {.async.} = echo("Hello ...") await sleep_async(1) echo("... World!") wait_for main() ``` Internally async is implemented using metaprogramming (macros, templates, pragmas, etc). | Description | [asyncCheck](https://nim-lang.org/docs/asyncfutures.html#asyncCheck%2CFuture%5BT%5D) | [waitFor](https://nim-lang.org/docs/asyncdispatch.html#waitFor%2CFuture%5BT%5D) | [await](https://nim-lang.org/docs/asyncdispatch.html#await%2CT) | |----------------------------------|------------|---------|-------| | Waits for the Future to complete | :negative_squared_cross_mark: | :heavy_check_mark: | :heavy_check_mark: | | Ignores the Future | :heavy_check_mark: | :negative_squared_cross_mark: | :negative_squared_cross_mark: | | Returns result inside Future | :negative_squared_cross_mark: | :heavy_check_mark: | :heavy_check_mark: | | Only available inside async | :negative_squared_cross_mark: | :negative_squared_cross_mark: | :heavy_check_mark: | - Why doesn't Nim use `async def`?. Async is just a `macro` in Nim, no need to change the syntax of the language. It is like a decorator in Python, only more powerful. Also in Nim the same function can be asynchronous *and* synchronous at the same time, with the same code, with the same name. In Python when you have a library *"foo"*, you may need both `foo` (sync) and `aiofoo` (async), which are usually completely different projects, repos, developers and APIs. This is not needed in Nim, or rarely seen, thanks to said feature. [Because async is just a `macro` in Nim you can create your own async your way too.](https://github.com/disruptek/cps#why) See also [asyncfile](https://nim-lang.org/docs/asyncfile.html), [asyncnet](https://nim-lang.org/docs/asyncnet.html), [asyncstreams](https://nim-lang.org/docs/asyncstreams.html), [asyncftpclient](https://nim-lang.org/docs/asyncftpclient.html), [asyncfutures](https://nim-lang.org/docs/asyncfutures.html). # Do I have to know C? You never have to actually manually edit C, the same way in Python you never manually edit the .pyc files. In Nim you code by writing Nim, the same way in Python you code by writing Python. ## Templates A template replaces its invocation with the template body at compile-time. Essentially, **the compiler will copy and paste a chunk of code for you**. A template allows us to have a function-like constructs without any overhead, or to split huge functions into smaller parts. Too many function and variable names may pollute the local namespace. Variables inside templates do not exist outside of their template. Templates do not exist in the namespace at run-time (if you do not export them), Templates may optimize certain values if they are known at compile-time. Templates *cannot* do `import` nor `export` of libraries automatically implicitly. Templates do *not* "auto-import" symbols used inside itself. If you use any imported library on the body of a template, you must import that library when invoking that template. Inside templates you can not use `return` because a template is not a function. Templates allow you to implement very high-level, beautiful APIs for everyday usage, while keeping the low-level optimized details out of your head and [DRY](https://en.wikipedia.org/wiki/Don%27t_repeat_yourself). Python `with open("file.txt", mode = "r") as file:` implemented using a template: [](https://github.com/juancarlospaco/nim-presentation-slides/blob/master/templates.xcf) The GIF is not perfect, but a lazy simplified approximation! This is not the way to read files in Nim, just an exercise. This template is not perfect, but a lazy approximation. It is an exercise for the reader to try to improve it ;P ```nim template withOpen(name: string, mode: char, body: untyped) = let flag = if mode == 'w': fmWrite else: fmRead # "flag" doen't exist outside of this template let file {.inject.} = open(name, flag) # Create and inject `file` variable, `file` exists outside of this template because of {.inject.} try: body # `body` is the code passed as argument finally: file.close() # Code after the code passed as argument withOpen("testing.nim", 'r'): # Mimic Python with `open("file", mode='r') as file` echo "Hello Templates" # Code inside the template, this 2 lines are "body" argument on the template echo file.read_all() # This line uses "file" variable ``` If you are just starting from scratch, do not worry, you can use functions for everything while learning. ## How to share variables between functions? Sharing variables between functions is similar to Python. **Global variable:** ```python global_variable = "" def function0(): global global_variable global_variable = "cat" def function1(): global global_variable global_variable = "dog" function0() assert global_variable == "cat" function1() assert global_variable == "dog" function0() assert global_variable == "cat" ``` :arrow_up: Python :arrow_up:          :arrow_down: Nim :arrow_down: ```nim var global_variable = "" proc function0() = global_variable = "cat" proc function1() = global_variable = "dog" function0() assert global_variable == "cat" function1() assert global_variable == "dog" function0() assert global_variable == "cat" ``` **Object Attribute:** ```python class IceCream: def __init__(self): self.object_attribute = None def function_a(food): food.object_attribute = 9 def function_b(food): food.object_attribute = 5 food = IceCream() function_a(food) assert food.object_attribute == 9 function_b(food) assert food.object_attribute == 5 function_a(food) assert food.object_attribute == 9 ``` :arrow_up: Python :arrow_up:          :arrow_down: Nim :arrow_down: ```nim type IceCream = object object_attribute: int proc functiona(food: var IceCream) = food.object_attribute = 9 proc functionb(food: var IceCream) = food.object_attribute = 5 var food = IceCream() functiona(food) assert food.object_attribute == 9 functionb(food) assert food.object_attribute == 5 functiona(food) assert food.object_attribute == 9 ``` You can pass functions as arguments to other functions like in Python. Functions (procs) are first class objects. ### In-Place vs Out-Place If you are migrating from an interpreted language, like Python or JavaScript, you may find strange mentions of "In-Place" and "Out-Place" somewhere in Nim. If you don't know what it means then Nim looks like it has duplicated functions. Python allocates a new string or object when something in it changes somehow. Let's say you have a huge string in a variable and you want to change a single character. Since Python strings are immutable, the only solution is to duplicate the string in memory but with the new copy having that character changed. Returning a new copy is an "Out-Place" operation. Most of Python works like this. On the other hand, Nim's strings are mutable. In Nim you can change only the character you want to change, rather than copying the string in memory. Some functions work in-place, some functions work on a new copy. The documentation will (usually) tell which. [using `macro` Nim can turn from an in-place function to out-place one.](https://nim-lang.github.io/Nim/sugar.html#dup.m%2CT%2Cvarargs%5Buntyped%5D) Nim stdlib modules designed for the JavaScript target usually work on a new copy. This is just how the JavaScript target is; there is no in-place API nor benefits from using it. Some Nim stdlib modules that work on a new copy may or may not be changed to work in-place in the future. Examples: ```nim import std/sugar # sugar.dup func inplace_function(s: var string) = # Does not use "string" but "var string" s = "CHANGED" # In-Place algo. var bar = "in-place" inplace_function(bar) ## Variable mutated in-place. assert bar == "CHANGED" # Out-Place algo. assert "out-place".dup(inplace_function) == "CHANGED" ## Variable mutated on a new copy. ``` ## Import Nim files in Python - https://github.com/Pebaz/nimporter#nimporter ## Python Syntax for Nim - https://github.com/Yardanico/nimpylib#nimpylib ## Publish to PYPI - https://github.com/yglukhov/nimpy/wiki#publish-to-pypi - https://github.com/sstadick/ponim/blob/master/README.md#nim--python--poetry-- - https://github.com/sstadick/nython#nython ## Silent Compilation If you want the compilation to be completely silent (you may miss important warnings and hints), you can add to the compile command ` --hints:off --verbosity:0 `. ## Compiler Help The compiler help is long. To make it more user friendly only the most frequent commands are shown with `--help`. If you want to see the full help you can use `--fullhelp`. ## Build Modes When your code is ready for production you should use a Release build, you can add to the compile command ` -d:release `. Feature | Release Build | Debug Build | -----------|-------------------------------|-------------------------------| Speed | Fast | Slow File Size | Small | Big Optimized | :heavy_check_mark: | :negative_squared_cross_mark: Tracebacks | :negative_squared_cross_mark: | :heavy_check_mark: Run-time checks | :heavy_check_mark: | :heavy_check_mark: Compile-time checks | :heavy_check_mark: | :heavy_check_mark: `assert` | :negative_squared_cross_mark: | :heavy_check_mark: [`doAssert`](https://nim-lang.org/docs/assertions.html#doAssert.t%2Cuntyped%2Cstring) | :heavy_check_mark: | :heavy_check_mark: # MicroPython Nim compiles to C, so it can run on Arduino and similar hardware. Has several memory management strategies to fit your needs, including full manual memory management. Nim binaries are small when built for Release and it can fit the hardware tiny storage. - https://github.com/zevv/nim-arduino - https://github.com/elcritch/nesper#example-code - https://gitlab.com/endes123321/nimcdl/tree/master#nimcdl-nim-circuit-design-language - https://github.com/cfvescovo/Arduino-Nim#arduino-nim - https://gitlab.com/nimbed/nimbed#nimbed - https://gitlab.com/endes123321/led-controller-frontend#led-controller-frontend - https://gitlab.com/jalexander8717/msp430f5510-nim - https://github.com/mwbrown/nim_stm32f3 - https://github.com/gokr/ardunimo - https://gitlab.com/NetaLabTek/Arduimesp - https://ftp.heanet.ie/mirrors/fosdem-video/2020/AW1.125/nimoneverything.webm # SuperCollider SuperCollider is C++ so it can be re-utilized using Nim. Theoretically, Nim SuperCollider plugins should be just as fast as C code. Nim's metaprogramming allows us to build LiveCoding friendly DSLs. Some projects for Nim LiveCoding: - https://github.com/vitreo12/omni#omni - https://github.com/capocasa/scnim#scnim---writing-supercollider-ugens-using-nim #### ABC See [this](https://rosettacode.org/wiki/Abstract_type#Nim) ## Philosophy The key to understanding Nim is that Nim was designed to be as fast as C, but to be much safer. Many of the design decisions are based on making it harder to shoot yourself in the foot. In Python, there are no pointers (everything is treated as a reference). While Nim does give you pointers, Nim gives you other, safer tools for your everyday needs, while pointers are mostly reserved for interfacing with C and doing low-level system programming. Contrarily to Python, most Nim code can be executed at compile time to perform meta-programming. You can do a lot of the DSLs possible with Python decorators/metaprogramming with Nim macros and pragmas. (And some stuff that you can't!). Of course, this requires some different patterns and more type safety. [ :arrow_up: :arrow_up: :arrow_up: :arrow_up: ](#table-of-contents "Go to top")