Decorators with optional arguments

sqlite-utils provides a decorator for registering custom Python functions that looks like this:

from sqlite_utils import Database
db = Database(memory=True)
@db.register_function
def reverse_string(s):
    return "".join(reversed(list(s)))

I wanted to add an optional deterministic=True parameter to the decorator.

Problem: the decorator currently does not take any arguments. But I wanted this to work as well:

@db.register_function(deterministic=True)
def reverse_string(s):
    return "".join(reversed(list(s)))

I don't want to break backwards compatibility with existing code.

First lesson: any time you are designing a decorator that might accept arguments in the future, design it to work like this!

@db.register_function()
def reverse_string(s):
    return "".join(reversed(list(s)))

Since I hadn't done that, I needed an alternative pattern that could differentiate between the two ways in which the decorator might be called. I ended up going with this:

    def register_function(self, fn=None, deterministic=None):
        def register(fn):
            name = fn.__name__
            arity = len(inspect.signature(fn).parameters)
            kwargs = {}
            if deterministic and sys.version_info >= (3, 8):
                kwargs["deterministic"] = True
            self.conn.create_function(name, arity, fn, **kwargs)
            return fn
        if fn is None:
            return register
        else:
            register(fn)

Related

• spatialite Creating a minimal SpatiaLite database with Python - 2021-12-17
• python Introspecting Python function parameters - 2020-05-27
• sphinx Adding Sphinx autodoc to a project, and configuring Read The Docs to build it - 2021-08-10
• sqlite Saving an in-memory SQLite database to a file in Python - 2023-04-08
• sqlite Loading SQLite extensions in Python on macOS - 2023-01-07
• sqlite Using LD_PRELOAD to run any version of SQLite with Python - 2020-06-17
• pytest Registering temporary pluggy plugins inside tests - 2020-07-21
• python Protocols in Python - 2023-07-26
• python Defining setup.py dependencies using a URL - 2022-08-13
• django Adding a Datasette ASGI app to Django - 2022-10-20

Created 2020-10-28T14:54:48-07:00, updated 2020-12-29T13:55:23-08:00 · History · Edit