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gh-141510: Fix frozendict.fromkeys() for subclasses by vstinner · Pull Request #144947 · python/cpython · GitHub

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gh-141510: Fix frozendict.fromkeys() for subclasses - #144947

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gh-141510: Fix frozendict.fromkeys() for subclasses#144947
vstinner wants to merge 1 commit into
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vstinner:frozendict_fromkeys

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vstinner commented Feb 18, 2026 •
edited by bedevere-app Bot
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Don't call the constructor of the subclass, but frozendict constructor instead.

pythongh-141510: Fix frozendict.fromkeys() for subclasses
7833d7d 
Don't call the constructor of the subclass, but frozendict
constructor instead.
vstinner requested a review from methane as a code owner February 18, 2026 11:14
bedevere-app Bot mentioned this pull request Feb 18, 2026
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vstinner mentioned this pull request Feb 18, 2026
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corona10 reviewed Feb 18, 2026
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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

I think we should discuss this before taking any action here.
Do we really need to handle cases where users shoot themselves in the foot? It doesn’t cause a crash at the moment, right?

vstinner commented Feb 18, 2026

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Currently, frozendict.fromkeys() allows modifying an immutable frozendict using a special subclass.

Example:

class SpecialDict(frozendict):
    def __new__(self):
        return frozendict(x=None)

print(SpecialDict.fromkeys("y"))

Current output:

frozendict({'x': None, 'y': None})

That's bad: the immutable frozendict is mutated!

corona10 commented Feb 18, 2026 •
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How about copy whole object at

cpython/Objects/dictobject.c

Line 3319 in dd64e42

else if (PyFrozenDict_CheckExact(d)) {
not to mutate original frozendict simply?

It follows the original intention of immutable data structures: copy instead of mutating.

vstinner marked this pull request as draft February 18, 2026 13:03

vstinner commented Feb 18, 2026

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How about copy whole object

Ok, good idea: I wrote #144952 to copy the frozendict.

vstinner commented Feb 18, 2026

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I close this PR in favor of #144952 fix.

vstinner closed this Feb 18, 2026
vstinner deleted the frozendict_fromkeys branch February 18, 2026 15:37
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