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Title: Argparse always stores True for positional arguments
Type: behavior Stage: resolved
Components: Library (Lib) Versions: Python 3.4
Status: closed Resolution: not a bug
Dependencies: Superseder:
Assigned To: Nosy List: Tristan.Fisher, paul.j3
Priority: normal Keywords:

Created on 2014-09-26 02:13 by Tristan.Fisher, last changed 2022-04-11 14:58 by admin. This issue is now closed.

File name Uploaded Description Edit Tristan.Fisher, 2014-09-26 02:13
Messages (2)
msg227584 - (view) Author: Tristan Fisher (Tristan.Fisher) * Date: 2014-09-26 02:13
It's my understanding that giving the action="store_true" to an argument in argparse defaults to False.  When using non-double-dashed/positional arguments, the argument resorts to True (even if explicitly marked default=False).

I've attached a minimal example, but, for clarity, the relevant line is as such:

    parser.add_argument("meow", action="store_true", default=False)

I realize that this might strike some as an odd usage, and I always have the option of using "--meow," but I found it odd that a positional argument is always True, even if not specified in sys.argv.
msg227585 - (view) Author: paul j3 (paul.j3) * (Python triager) Date: 2014-09-26 03:40
A 'store_true' action takes 0 arguments.  In effect `nargs=0`.

With an `optional` (flagged) argument, the default `False` is used if the flag is absent, and set to `True` when the flag is encountered (its Action `__call__` function is run).

A `positional` is 'encountered' whenever there are enough values to meet its `nargs`.  With `nargs=0`, an empty list of strings, i.e. none, is enough.  Thus such a `positional` is always found, and its `__call__` is run, setting the value to `True`.

As a result, action types like 'store_true', 'store_false', 'store_const' only make sense with `optionals`.

And I can't think of simple way of using a 'positional' to set an Namespace 'dest' to boolean values.  It could be done with a custom Action, but not with the predefined ones.  Or you could translate the values after parsing.
Date User Action Args
2022-04-11 14:58:08adminsetgithub: 66690
2015-03-29 18:44:51berker.peksagsetresolution: not a bug
stage: resolved
2015-03-27 20:54:26paul.j3setstatus: open -> closed
2014-09-26 03:40:53paul.j3setnosy: + paul.j3
messages: + msg227585
2014-09-26 02:13:11Tristan.Fishercreate