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GeezerVison is showing github.com — Fix str subclass map keys in the Python fallback by SakayanagiYuna · Pull Request #771 · msgpack/msgpack-python

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SakayanagiYuna commented Nov 7, 2026

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msgpack.fallback.unpackb() raises TypeError: can't intern Key when
ext_hook returns a str subclass as a map key, even with
strict_map_key=False. The C extension accepts the same key.

The fallback passes the subclass to sys.intern(), which only accepts exact
str objects on CPython. The fix skips interning for subclasses, preserving
the key's type and identity.

Adds one regression test: it fails before the fix and passes afterward. It
also checks that strict_map_key=True still rejects subclass keys. The full
suites pass on Windows with CPython 3.14.7: 179 C-extension tests and 176 pure
Python tests.

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          fix: preserve str subclass map keys in fallback unpacker

6d28910

Only intern exact str keys so decoding hooks can return str subclasses
when strict_map_key=False, matching the C extension.

Add a regression test for key identity and strict_map_key validation.

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