The sys module provides access to functions and values concerning the program's runtime environment, such as the command line parameters in
sys.argv or the function
sys.exit() to end the current process from any point in the program flow.
While cleanly separated into a module, it's actually built-in and as such will always be available under normal circumstances.
Import the sys module and make it available in the current namespace:
Import a specific function from the sys module directly into the current namespace:
from sys import exit
For details on all sys module members, refer to the official documentation.
if len(sys.argv) != 4: # The script name needs to be accounted for as well. raise RuntimeError("expected 3 command line arguments") f = open(sys.argv, 'rb') # Use first command line argument. start_line = int(sys.argv) # All arguments come as strings, so need to be end_line = int(sys.argv) # converted explicitly if other types are required.
Note that in larger and more polished programs you would use modules such as click to handle command line arguments instead of doing it yourself.
# The name of the executed script is at the beginning of the argv list. print('usage:', sys.argv, '<filename> <start> <end>') # You can use it to generate the path prefix of the executed program # (as opposed to the current module) to access files relative to that, # which would be good for assets of a game, for instance. program_file = sys.argv import pathlib program_path = pathlib.Path(program_file).resolve().parent
# Error messages should not go to standard output, if possible. print('ERROR: We have no cheese at all.', file=sys.stderr) try: f = open('nonexistent-file.xyz', 'rb') except OSError as e: print(e, file=sys.stderr)
def main(): if len(sys.argv) != 4 or '--help' in sys.argv[1:]: print('usage: my_program <arg1> <arg2> <arg3>', file=sys.stderr) sys.exit(1) # use an exit code to signal the program was unsuccessful process_data()