[issue42501] Improve error messages for argparse choices using enum

[Raymond Hettinger]

Change by Raymond Hettinger :


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components: +Documentation -Library (Lib)
resolution:  -> fixed
stage: patch review -> resolved
status: open -> closed

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[issue42501] Improve error messages for argparse choices using enum

[Raymond Hettinger]

Raymond Hettinger  added the comment:


New changeset aab93903347ec6d7f23dda2994dd26f6111d19d2 by Miss Islington (bot) 
in branch '3.9':
bpo-42501:  Revise the usage note for Enums with the choices (GH-23563) 
(GH-23573)
https://github.com/python/cpython/commit/aab93903347ec6d7f23dda2994dd26f6111d19d2


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[issue42501] Improve error messages for argparse choices using enum

[Raymond Hettinger]

Raymond Hettinger  added the comment:


New changeset 7f82f22eba1312617e1aa19cb916da1aae1609a4 by Raymond Hettinger in 
branch 'master':
bpo-42501:  Revise the usage note for Enums with the choices (GH-23563)
https://github.com/python/cpython/commit/7f82f22eba1312617e1aa19cb916da1aae1609a4


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[issue42501] Improve error messages for argparse choices using enum

[miss-islington]

Change by miss-islington :


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nosy: +miss-islington
nosy_count: 4.0 -> 5.0
pull_requests: +22454
pull_request: https://github.com/python/cpython/pull/23573

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[issue42501] Improve error messages for argparse choices using enum

[Raymond Hettinger]

Change by Raymond Hettinger :


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assignee:  -> rhettinger

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[issue42501] Improve error messages for argparse choices using enum

[Raymond Hettinger]

Raymond Hettinger  added the comment:

How does PR 23563 look to you all?

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[issue42501] Improve error messages for argparse choices using enum

[Raymond Hettinger]

Change by Raymond Hettinger :


--
keywords: +patch
pull_requests: +22444
stage:  -> patch review
pull_request: https://github.com/python/cpython/pull/23563

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[issue42501] Improve error messages for argparse choices using enum

[paul j3]

paul j3  added the comment:

choices is fine for a few strings, but quickly becomes awkward with other types 
and large numbers.  The testing isn't an issue, since it just does a simple 
`in/contains` test.  But display, whether in usage, help or error, is 
problematic if you try anything too fancy.  metavar gets around some of those 
issues, but doesn't change the error messages.

Custom type or action is the best alternative.

I'm in favor omitting the enums mention in the docs, since it seems to be more 
confusing than helpful.

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[issue42501] Improve error messages for argparse choices using enum

[shangxiao]

shangxiao  added the comment:

Oh apologies, I had "open" selected when I searched for prior issues.

Upon reading that issue I agree with Mr Hettinger's points about enum values 
not being a concern of the parser.

The solution for my needs was simple enough: I made my own action which simply 
set choices based on member values (I'm using user-friendly strings) and 
converted to the correct type upon being called [1].

Instead of removing any mention of enums from the docs - would a small example 
showing how to deal with them be worthwhile?


[1]
def enum_action_factory(enum_class):

class EnumAction(argparse.Action):
def __init__(self, option_strings, dest, **kwargs):
kwargs["choices"] = [member.value for member in enum_class]
super().__init__(option_strings, dest, **kwargs)

def __call__(self, parser, namespace, values, option_string):
if isinstance(values, str):
converted_values = enum_class(values)
else:
converted_values = [enum_class(value) for value in values]
setattr(namespace, self.dest, converted_values)

return EnumAction

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[issue42501] Improve error messages for argparse choices using enum

[Karthikeyan Singaravelan]

Karthikeyan Singaravelan  added the comment:

https://bugs.python.org/issue25061 also had some discussion over error message 
display for enums

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nosy: +xtreak

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[issue42501] Improve error messages for argparse choices using enum

[Raymond Hettinger]

Raymond Hettinger  added the comment:

I'm inclined to state in the docs that choices is designed to work with 
collections of strings and that there is no special support for enums.

Paul, what do you think?

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[issue42501] Improve error messages for argparse choices using enum

[Karthikeyan Singaravelan]

Change by Karthikeyan Singaravelan :


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nosy: +paul.j3, rhettinger

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[issue42501] Improve error messages for argparse choices using enum

[shangxiao]

New submission from shangxiao :

Summary
---

The argparse module mentions that it will happily accept an enum for the 
choices argument option [1].  There are currently 2 issues with this:

1. The usage displays non user-friendly values for the enum;
2. The error message for invalid values is not consistent with those provided 
by iterable-based choices in that it doesn't show the list of available 
choices. This becomes a problem when you specify the metavar as no choices are 
displayed at all.


Example of 1


For example using the GameMove example in the argparse documentation:

class GameMove(Enum):
 ROCK = 'rock'
 PAPER = 'paper'
 SCISSORS = 'scissors'

parser = argparse.ArgumentParser(prog='game.py')
parser.add_argument('move', type=GameMove, choices=GameMove)
parser.print_help()


Gives the usage:

usage: game.py [-h] {GameMove.ROCK,GameMove.PAPER,GameMove.SCISSORS}

positional arguments:
  {GameMove.ROCK,GameMove.PAPER,GameMove.SCISSORS}

optional arguments:
  -h, --helpshow this help message and exit


As you can see, the string representations of the valid enum members is used 
instead of the values.



Example of 2


Below is an example of the error message shown for invalid enum values, with 
metavar specified:

parser = argparse.ArgumentParser(prog='game.py')
parser.add_argument('move', metavar='your-move', type=GameMove, 
choices=GameMove)
parser.parse_args(['asdf'])

Gives the following output:

usage: game.py [-h] your-move
game.py: error: argument your-move: invalid GameMove value: 'asdf'


This is in contrast with using standard iterable-based choices:

parser = argparse.ArgumentParser(prog='game.py')
parser.add_argument('move', metavar='your-move', choices=[x.value for x in 
GameMove])
parser.parse_args(['asdf'])

Gives the following output:

usage: game.py [-h] your-move
game.py: error: argument your-move: invalid choice: 'asdf' (choose from 
'rock', 'paper', 'scissors')


Analysis of 2
--

The reason for this behaviour is because argparse attempts to convert to the 
correct type before checking the choices membership and an invalid enum value 
results in the first error message.

It would be helpful (& consistent) if the choices are also displayed along with 
the invalid value message.


Possible Solution
-

I believe that both issues could be solved by using the enum member values in 
the choices and then checking for choice membership _before_ the value is 
converted to an enum (ie for enums only).



[1] https://docs.python.org/3/library/argparse.html#choices

--
components: Library (Lib)
messages: 382051
nosy: shangxiao
priority: normal
severity: normal
status: open
title: Improve error messages for argparse choices using enum
type: behavior
versions: Python 3.10

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