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GeezerVison is showing github.com — gh-133653: Fix subclassing `HelpFormatter` by hugovk · Pull Request #133668 · python/cpython

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@hugovk

@hugovk hugovk commented May 8, 2025 •
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The problem was with this code:

        if isinstance(self.formatter_class, type) and issubclass(
            self.formatter_class, HelpFormatter
        ):
            return self.formatter_class(
                prog=self.prog,
                prefix_chars=self.prefix_chars,
                color=self.color,
            )
        else:
            return self.formatter_class(prog=self.prog)

We want to set the extra arguments for HelpFormatter and argparse's own subclasses, which don't have their own __init__s.

But many third-party subclasses do have an __init__ and don't pass on kwargs. So we can't just check for a subclass of HelpFormatter, so let's try/except instead.

  • Issue: argparse.HelpFormatter subclasses may be broken by 3.14 changes #133653

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@hugovk
Fix subclassing HelpFormatter
b1c0c60
@hugovk
hugovk requested a review from savannahostrowski as a code owner May 8, 2025 13:32
@hugovk hugovk added the needs backport to 3.14 bugs and security fixes label May 8, 2025
@bedevere-app bedevere-app Bot mentioned this pull request May 8, 2025
argparse.HelpFormatter subclasses may be broken by 3.14 changes #133653
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@bedevere-app bedevere-app Bot added the awaiting core review label May 8, 2025
@hugovk
Fix Sphinx warning
fa4d044
iritkatriel
iritkatriel reviewed May 8, 2025
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Comment thread Lib/argparse.py Outdated
color=self.color,
)
else:
except TypeError:

@iritkatriel iritkatriel May 8, 2025

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TypeError seems a bit generic (you're calling user code).

What do you think about this?

len([v.kind for (k, v) in inspect.signature(self.formatter_class).parameters.items() if k in ('y', 'z') and v.kind in (inspect._ParameterKind.POSITIONAL_OR_KEYWORD, inspect._ParameterKind.KEYWORD_ONLY)]) == 2

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@hugovk hugovk May 8, 2025

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That doesn't work: FAILED (failures=2, errors=156)

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@hugovk hugovk May 8, 2025

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Wait, it does, obviously need to replace y and z :)

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@hugovk
Big if
91c1f4f
iritkatriel
iritkatriel approved these changes May 8, 2025
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@bedevere-app bedevere-app Bot added awaiting merge and removed awaiting core review labels May 8, 2025
serhiy-storchaka
serhiy-storchaka reviewed May 8, 2025
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@serhiy-storchaka serhiy-storchaka left a comment

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I would not like to use inspect.signature() here. It is not reliable. If it fails (or return incorrect result), you cannot do anything with this. It can only be used in interactive introspection, when errors can be ignored.

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@hugovk

hugovk commented May 8, 2025

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Would you prefer the try/except or another approach?
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@serhiy-storchaka

serhiy-storchaka commented May 8, 2025

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No, I think the user code should be changed. They use undocumented feature.
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@iritkatriel

iritkatriel commented May 8, 2025

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We can do my original suggestion - make __init__ accept **kwargs and extract the new args with this.

Already two external libraries were broken by this in the first alpha testing. Leaving this as it is will break user code for sure.
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@serhiy-storchaka

serhiy-storchaka commented May 8, 2025

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This would not help. The problem is that the user code does not accept any arguments besides prog. And they do this because they use undocumented feature.
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@iritkatriel

iritkatriel commented May 8, 2025

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What's the concern with inspect? That it won't work for some weird function? In that case we can just use the default values for these arguments.
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@iritkatriel

iritkatriel commented May 8, 2025

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Another option is to set these fields through a setter method and not via __init__.
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@serhiy-storchaka

serhiy-storchaka commented May 10, 2025

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See alternative PR #133813.
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@hugovk hugovk closed this May 10, 2025
@hugovk
hugovk deleted the 3.14-fix-subclassing-helpformatter branch May 13, 2025 21:10
@AA-Turner AA-Turner removed the needs backport to 3.14 bugs and security fixes label Aug 14, 2025
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@serhiy-storchaka serhiy-storchaka serhiy-storchaka left review comments
@iritkatriel iritkatriel iritkatriel approved these changes
@savannahostrowski savannahostrowski Awaiting requested review from savannahostrowski savannahostrowski is a code owner

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@hugovk @serhiy-storchaka @iritkatriel @AA-Turner
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