What does metavar and action mean in argparse in Python?

python action argparse
eagertoLearn picture eagertoLearn · Oct 1, 2013 · Viewed 63.5k times · Source

I am reading through argparse module. I got stuck as what to metavar and action means

>>> parser.add_argument('integers', metavar='N', type=int, nargs='+',
...                     help='an integer for the accumulator')
>>> parser.add_argument('--sum', dest='accumulate', action='store_const',
...                     const=sum, default=max,
...                     help='sum the integers (default: find the max)')

I might have missed but from what I read, I could not find definitions for metavar and action (action="store_const", etc). what do they actually mean?

Answer

subhadarshi samal picture subhadarshi samal · Apr 27, 2017

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

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