argparse vs optparse

Originally, the argparse module had attempted to maintain compatibility with optparse. However, optparse was difficult to extend transparently, particularly with the changes required to support the new nargs=specifiers and better usage messages. When most everything in optparse had either been copy-pasted over or monkey-patched, it no longer seemed practical to try to maintain the backwards compatibility.

The argparse module improves on the standard library optparse module in a number of ways including:

• Handling positional arguments.
• Supporting sub-commands.
• Allowing alternative option prefixes like + and /.
• Handling zero-or-more and one-or-more style arguments.
• Producing more informative usage messages.
• Providing a much simpler interface for custom type and action.

A partial upgrade path from optparse to argparse:

• Replace all optparse.OptionParser.add_option() calls with ArgumentParser.add_argument() calls.
• Replace (options, args) = parser.parse_args() with args = parser.parse_args() and add additional ArgumentParser.add_argument() calls for the positional arguments. Keep in mind that what was previously called options, now in argparse context is called args.
• Replace callback actions and the callback_* keyword arguments with type or action arguments.
• Replace string names for type keyword arguments with the corresponding type objects (e.g. int, float, complex, etc).
• Replace optparse.Values with Namespace and optparse.OptionError and optparse.OptionValueError with ArgumentError.
• Replace strings with implicit arguments such as %default or %prog with the standard Python syntax to use dictionaries to format strings, that is, %(default)s and %(prog)s.
• Replace the OptionParser constructor version argument with a call to parser.add_argument('--version', action='version', version=' version>').