sys — System-specific Parameters and Functions

The sys module provides access to interpreter variables and functions.

import sys

Command-Line Arguments

import sys

# python script.py arg1 arg2
print(sys.argv)     # ['script.py', 'arg1', 'arg2']
print(sys.argv[0])  # 'script.py' — script name
print(sys.argv[1:]) # ['arg1', 'arg2'] — arguments

Module Search Paths

import sys

# List of paths where Python looks for modules
for p in sys.path:
    print(p)

# Add custom path
sys.path.append('/my/custom/modules')

# Loaded modules
print(list(sys.modules.keys())[:10])

Standard I/O Streams

import sys

# Standard output
sys.stdout.write("Hello\n")  # Like print()

# Standard error
sys.stderr.write("Error message\n")

# Redirect output to file
original_stdout = sys.stdout
sys.stdout = open('output.txt', 'w')
print("This goes to file")
sys.stdout = original_stdout  # Restore

System Information

import sys

sys.version         # '3.12.0 (main, Oct 2 2023, ...)'
sys.version_info    # sys.version_info(major=3, minor=12, ...)
sys.platform        # 'linux', 'win32', 'darwin'
sys.executable      # '/usr/bin/python3'
sys.prefix          # '/usr' — Python installation path
sys.byteorder       # 'little' or 'big'
sys.maxsize         # 9223372036854775807 (max int size)

Exiting the Program

import sys

sys.exit()       # Exit code 0 (success)
sys.exit(1)      # Exit code 1 (error)
sys.exit("Error: file not found")  # Prints message, exits with code 1

Recursion

import sys

print(sys.getrecursionlimit())  # 1000 (default)
sys.setrecursionlimit(5000)     # Change limit (careful!)

Object Sizes

import sys

sys.getsizeof(42)          # 28 bytes
sys.getsizeof("hello")    # 54 bytes
sys.getsizeof([1, 2, 3])  # 88 bytes
sys.getsizeof({})          # 64 bytes

Common Pitfalls

• sys.exit() raises SystemExit — it's an exception, can be caught with try/except
• Modifying sys.path is global — affects the entire process
• Increasing recursionlimit — can cause stack overflow

Official Documentation

sys — System-specific parameters and functions

API Reference

Important Attributes

Important Functions