sys.argv is part of the interpreter. The other three are abstractions over it. optparse is deprecated (kept around for obvious backward-compat reasons) getopt

sys.argv is part of the interpreter. The other three are abstractions over it. optparse is deprecated (kept around for obvious backward-compat reasons) getopt is just a different API for people who feel more comfortable with C's getopt API and don't want to learn something else argparse is the CLI option parser for the future. tl;dr: There is one way to parse command line arguments: sys.argv. But there are a few abstractions of doing that available in the stdlib.