What does metavar and action mean in argparse in Python?

metavar is used in help messages in a place of an expected argument. See FOO is a default metavar here:

>>> parser = argparse.ArgumentParser()
>>> parser.add_argument('--foo')
>>> parser.add_argument('bar')
>>> parser.parse_args('X --foo Y'.split())
Namespace(bar='X', foo='Y')
>>> parser.print_help()
usage:  [-h] [--foo FOO] bar
...

action defines how to handle command-line arguments: store it as a constant, append into a list, store a boolean value etc. There are several built-in actions available, plus it's easy to write a custom one.

Metavar: It provides a different name for optional argument in help messages. Provide a value for the metavar keyword argument within add_argument().

>>> parser = argparse.ArgumentParser()
>>> parser.add_argument('--foo', metavar='YYY')
>>> parser.add_argument('bar', metavar='XXX')
>>> parser.parse_args('X --foo Y'.split())
Namespace(bar='X', foo='Y')
>>> parser.print_help()
usage:  [-h] [--foo YYY] XXX

positional arguments:
  XXX

optional arguments:
  -h, --help  show this help message and exit
  --foo YYY

Reference: http://www.usatlas.bnl.gov/~caballer/files/argparse/add_argument.html

Action: Arguments can trigger different actions, specified by the action argument to add_argument(). There are six built-in actions that can be triggered when an argument is encountered:

1. store: Save the value, after optionally converting it to a different type. This is the default action taken if none is specified explicitly.

2. store_true/store_false: Save the appropriate boolean value.

3. store_const: Save a value defined as part of the argument specification, rather than a value that comes from the arguments being parsed. This is typically used to implement command line flags that aren’t booleans.

4. append: Save the value to a list. Multiple values are saved if the argument is repeated.

5. append_const: Save a value defined in the argument specification to a list.

6. version: Prints version details about the program and then exits.

Reference: http://bioportal.weizmann.ac.il/course/python/PyMOTW/PyMOTW/docs/argparse/index.html