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gh-133653: Fix subclassing `HelpFormatter` by hugovk · Pull Request #133668 · python/cpython · GitHub

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gh-133653: Fix subclassing HelpFormatter - #133668

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hugovk wants to merge 3 commits into
python:mainfrom
hugovk:3.14-fix-subclassing-helpformatter
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gh-133653: Fix subclassing HelpFormatter#133668
hugovk wants to merge 3 commits into
python:mainfrom
hugovk:3.14-fix-subclassing-helpformatter

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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.

hugovk requested a review from savannahostrowski as a code owner May 8, 2025 13:32
hugovk added the needs backport to 3.14 bugs and security fixes label May 8, 2025
bedevere-app Bot mentioned this pull request May 8, 2025
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iritkatriel reviewed May 8, 2025
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Comment thread Lib/argparse.py Outdated
color=self.color,
)
else:
except TypeError:

iritkatriel May 8, 2025

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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

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

hugovk May 8, 2025

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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

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

hugovk May 8, 2025

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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

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

serhiy-storchaka reviewed May 8, 2025
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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

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.

hugovk commented May 8, 2025

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Would you prefer the try/except or another approach?

serhiy-storchaka commented May 8, 2025

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

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.

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.

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.

iritkatriel commented May 8, 2025

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

serhiy-storchaka commented May 10, 2025

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See alternative PR #133813.

hugovk closed this May 10, 2025
hugovk deleted the 3.14-fix-subclassing-helpformatter branch May 13, 2025 21:10
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 left review comments
iritkatriel iritkatriel approved these changes
savannahostrowski Awaiting requested review from savannahostrowski savannahostrowski is a code owner

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