Undocumented exception: argparse add_argument(action='store_true', metavar='...') raises TypeError

[ipilcher]

Documentation

Not 100% sure whether this is a bug or a documentation error, but the behavior is reasonable, so filing this as a documentation issue for now.

>>> from argparse import ArgumentParser
>>> parser = ArgumentParser()
>>> parser.add_argument('-v', metavar='VERBOSE', dest='verbose', action='store_true')
Traceback (most recent call last):
  File "<python-input-3>", line 1, in <module>
    parser.add_argument('-v', metavar='VERBOSE', dest='verbose', action='store_true')
    ~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib64/python3.14/argparse.py", line 1539, in add_argument
    action = action_class(**kwargs)
TypeError: _StoreTrueAction.__init__() got an unexpected keyword argument 'metavar'

There is no indication in the documentation that this combination will raise an exception.

(FWIW, this behavior is inconsistent with action='store_const', which silently swallows metavar.)

AI Usage

Claude Code, with Opus 4.8, was used to identify and analyze this issue.

[terryjreedy]

Sorry for the noise. We do not generally doc TypeErrors for specific functions, but incompatibilities between parameters should be. An argparse expert should decide if this is really intentional and if there are bad pairs.