Unpack the single type arg for Counter keys when unstructuring

[uttam12331]

Summary

mapping_unstructure_factory's single-type-arg branch (comment: "Probably a Counter") assigns the whole args tuple to the singular key_arg:

if len(args) == 2:
    key_arg, val_arg = args
else:
    # Probably a Counter
    key_arg, val_arg = args, Any          # <-- args, not args[0]
# ...
kh = key_handler or converter.get_unstructure_hook(key_arg, cache_result=False)

get_unstructure_hook expects a single type, but receives the 1-tuple (KeyType,), which resolves to the identity hook. So a Counter[KeyType] whose keys need real unstructuring has its keys left unchanged.

The parallel structure factory handles the identical case correctly by unpacking:

else:
    # Probably a Counter
    (key_type,) = args

Reproduction

from collections import Counter
from attrs import define
from cattrs import Converter

@define(frozen=True)
class Key:
    v: int

c = Converter()
c.register_unstructure_hook(Key, lambda k: k.v)
c.unstructure(Counter({Key(1): 5, Key(2): 3}), unstructure_as=Counter[Key])
# before: {Key(v=1): 5, Key(v=2): 3}   (keys not unstructured)
# after:  {1: 5, 2: 3}

Fix

-        key_arg, val_arg = args, Any
+        key_arg, val_arg = args[0], Any

Tests

Added test_counter_unstructure_applies_key_hook, which fails on the previous behavior (keys left un-unstructured) and passes with the fix. The existing Counter[int] tests are unaffected (integer keys use the identity hook either way).

[codspeed-hq]

Merging this PR will not alter performance

✅ 64 untouched benchmarks

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_{Comparing uttam12331:fix/counter-unstructure-key-arg (cca9472) with main (a6c0900)}

[Tinche]

Ooph, good catch!

[Tinche]

Thanks!