Python sys Module

The sys module provides access to variables and functions that interact closely with Python interpreter and runtime environment. It allows developers to manipulate various aspects of program execution and interpreter itself.

Example:

import sys
print(sys.version)

Output

3.6.9 (default, Oct  8 2020, 12:12:24)
[GCC 8.4.0]

Explanation: This code prints the version of the Python interpreter currently in use, which helps in identifying compatibility and environment details.

Why do we need sys module?

1. Gives access to system-specific parameters and functions like command-line arguments.
2. Allows interaction with Python runtime environment (e.g., path, version, exit).
3. Useful for reading input and writing output using sys.stdin and sys.stdout.
4. Enables control over the interpreter with functions like sys.exit() and sys.getsizeof().
5. Helps in debugging and managing system-level behavior in Python scripts.

Input and Output using sys

The sys module controls program input, output and error streams, enabling precise data handling beyond standard input and print functions.

1. sys.stdin:

Reads input directly from the standard input stream and supports reading multiple lines or redirected input.

import sys
for line in sys.stdin:
    if 'q' == line.rstrip():
        break
    print(f'Input : {line}')
print("Exit")

Output

2. sys.stdout:

Writes output to the standard output stream and allows low-level control over printed output.

import sys
sys.stdout.write('Geeks')

Output

Geeks

3. sys.stderr:

Writes messages to the standard error stream and separates error messages from regular output

import sys
def fun(*args):
    print(*args, file=sys.stderr)
fun("Hello World")

Output

Command-Line Arguments

Command-line arguments are those which are passed during the calling of the program along with the calling statement. To achieve this using the sys module, the sys module provides a variable called sys.argv. It's main purpose are:

• It is a list of command-line arguments.
• len(sys.argv) provides the number of command-line arguments.

Example:

import sys
n = len(sys.argv)
print("Total arguments passed:", n)
print("Name of Python script:", sys.argv[0])
print("Arguments passed:", end=" ")
for i in range(1, n):
    print(sys.argv[i], end=" ")
Sum = 0
for i in range(1, n):
    Sum += int(sys.argv[i])
print(Sum)

Output

Explanation: This code sums the command-line arguments passed to the script by converting each to an integer and adding them up using the sys module.

Exiting the Program

sys.exit([arg]) can be used to exit the program. The optional argument arg can be an integer giving the exit or another type of object. If it is an integer, zero is considered "successful termination".

Note: A string can also be passed to the sys.exit() method.

import sys
age = 17
if age < 18:
    sys.exit("Age less than 18")
else:
    print("Age is not less than 18")

Output

An exception has occurred, use %tb to see the full traceback.
SystemExit: Age less than 18

Explanation: This code uses sys.exit() to terminate the program if age is less than 18, displaying a message otherwise, it prints that the age is not less than 18.

Working with Modules

sys.path is a list in the sys module that defines directories Python searches for modules after checking built-ins. As a regular list, it can be modified at runtime to add, remove or reorder paths.

Note: sys.path is an ordinary list and can be manipulated.

Example 1: Listing all paths

import sys
print(sys.path)

Output

Explanation: This code will print the system paths that Python uses to search for modules. The sys.path list contains the directories that Python will search for modules when it imports them.

Example 2: Truncating sys.path

import sys
sys.path = []
import pandas

Output

ModuleNotFoundError: No module named 'pandas'

Explanation: This code will raise an error because the pandas module cannot be found if sys.path is emptied. By setting sys.path to an empty list, Python is prevented from locating any modules outside built-ins.

sys.modules()

sys.modules is a dictionary that contains all the modules currently imported in the Python interpreter. The keys are module names, and the values are the corresponding module objects. Example:

import sys
print(sys.modules)

Output

Explanation: This code will print a dictionary of all the modules that have been imported by the current Python interpreter. The dictionary keys are the module names, and the dictionary values are the module objects.

Reference Count

sys.getrefcount() method is used to get the reference count for any given object. This value is used by Python as when this value becomes 0, the memory for that particular value is deleted. Example:

import sys
a = 'Geeks'
print(sys.getrefcount(a))

4

Explanation:This code prints the reference count of the object a, which indicates how many times it is referenced. When the count reaches 0, the object is no longer used and is garbage collected.

More Functions in Python sys