Background

CPython's defaultdict.__missing__ (Modules/_collectionsmodule.c::defdict_missing) calls default_factory() first; if the factory's recursion populated self[key] while running, the existing value is preserved instead of being overwritten.

RustPython ships a Python fallback at Lib/collections/_defaultdict.py (the C-implemented _collections.defaultdict is not available — see the comment at Lib/collections/__init__.py:60). That fallback unconditionally executed self[key] = val after the factory returned, overwriting any value the recursive call had already stored.

Repro

from collections import defaultdict
key = 'k'
count = 0
def fac():
    global count
    count += 1
    local = count
    if count == 1:
        d[key]   # recursive lookup — inner factory stores d[key] = 2
    return local
d = defaultdict(fac)

d[key]
# CPython 3.14: 2  (outer factory's local=1 does NOT overwrite)
# RustPython:   1  (overwrote)   (before this PR)

Fix

Add if key in self: return self[key] after the factory call, before the assignment. dict.__contains__ does not invoke __missing__, so there's no recursion risk; in the common (non-reentrant) case the check is False and behavior is unchanged.

Tests unmasked

• test_defaultdict.TestDefaultDict.test_factory_conflict_with_set_value

Verification

• CPython 3.14.4 byte-identical for the test's repro (result=2, count=2)
• Normal cases unchanged: defaultdict(int) increment, defaultdict() with no factory still raises KeyError
• No regressions across test_defaultdict, test_collections, test_dict, test_typing, test_descr (~1,116 tests)