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

import sys

# Use sys.argv to access command-line arguments.
# When run as: python script.py hello 42
print(sys.argv[0])  # Script name: prints 'script.py'
print(sys.argv[1])  # First argument: prints 'hello'
print(sys.argv[2])  # Second argument: prints '42'
print(len(sys.argv))  # Number of arguments: prints '3'

# Check for missing arguments
if len(sys.argv) < 2:
    print('Usage: python script.py <filename>', file=sys.stderr)
    sys.exit(1)  # Exit with an error code.

# Receive and process the argument.
filename = sys.argv[1]
print(f'Processing file: {filename}')

# Add a path to sys.path to make a module importable.
sys.path.append('/path/to/my/modules')

# Check the Python version with sys.version.
print(sys.version)  # e.g., prints '3.12.0 (main, ...)'

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

# Use sys.platform to detect the runtime environment.
if sys.platform == 'win32':
    print('Running on Windows.')
elif sys.platform == 'darwin':
    print('Running on macOS.')
else:
    print('Running on Linux.')

# Write a message to standard error.
print('An error occurred.', file=sys.stderr)

sys.argv is the fundamental mechanism for passing arguments to a script from the command line. All arguments are stored as strings, so convert them with int() or float() when numeric values are needed. For more complex argument parsing, the standard library's argparse module makes it easy to 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 should let it propagate to terminate the script normally. 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().