sys.argv / sys.exit() / sys.path

import sys

# List of command-line arguments (index 0 is the script name)
sys.argv

# Exits the script (you can specify an exit code)
sys.exit(0) # Normal exit
sys.exit(1) # Error exit

# List of directories searched for modules
sys.path

# Python version string
sys.version

# Platform identifier of the runtime environment
sys.platform

sys_argv_basic.py

import sys

print(sys.argv[0])
print(sys.argv[1])
print(sys.argv[2])
print(len(sys.argv))

Running the code produces the following output:

python3 sys_argv_basic.py hello 42
sys_argv_basic.py
hello
42
3

sys_argv_validate.py

import sys

if len(sys.argv) < 3:
    print('Usage: python3 sys_argv_validate.py <name> <age>', file=sys.stderr)
    sys.exit(1)

name = sys.argv[1]
age = int(sys.argv[2])
print(f'Name: {name}, Age: {age}')

Running the code produces the following output:

python3 sys_argv_validate.py
Usage: python3 sys_argv_validate.py <name> <age>

Running the code produces the following output:

python3 sys_argv_validate.py "Light Yagami" 17
Name: Light Yagami, Age: 17

sys_info.py

import sys

print(sys.version)

if sys.version_info >= (3, 10):
    print('Python 3.10 or later: match statements are available.')

if sys.platform == 'win32':
    print('Running on Windows.')
elif sys.platform == 'darwin':
    print('Running on macOS.')
else:
    print('Running on Linux.')

print('An error occurred.', file=sys.stderr)

Running the code produces the following output:

python3 sys_info.py
3.12.4 (main, Jun  6 2024, 18:26:44)
Python 3.10 or later: match statements are available.
Running on macOS.

Common Mistake 1: Accessing arguments without checking the count

Accessing sys.argv[1] without checking whether any arguments were passed will raise an IndexError when the script is run without arguments. Check the argument count first or use try/except.

import sys

filename = sys.argv[1]
print(f'File: {filename}')

The same logic can also be written as:

import sys

if len(sys.argv) < 2:
    print('Usage: python3 mistake1_ok.py <filename>', file=sys.stderr)
    sys.exit(1)

filename = sys.argv[1]
print(f'File: {filename}')

Common Mistake 2: Using arguments as numbers without converting

All command-line arguments are stored as strings. You can convert them with int() or float() when a numeric value is needed. Using a string argument directly in arithmetic raises a TypeError.

import sys

score = sys.argv[1]
print(score + 10)

The same logic can also be written as:

import sys

score = int(sys.argv[1])
print(score + 10)

sys.argv is the fundamental mechanism for passing arguments to a script from the command line. All arguments are stored as strings, so you can convert them with int() or float() when numeric values are needed. For more complex argument parsing, the standard library's argparse module lets you implement automatic help messages, type conversion, and optional arguments.

sys.exit() internally raises a SystemExit exception, which means it can be caught with try/except. In most cases, however, you can write logic to let it propagate and terminate the script. Exit codes are used by shell scripts and CI/CD systems to determine success or failure. Always exit with a non-zero code when an error occurs.

For reading environment variables, see os.environ / os.getenv().