Document that omit_defaults ignores custom default_factory - #1076
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This was referenced Jun 16, 2026
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| still type checks. Specifying ``default=[]`` works too: ``msgspec`` doesn't | ||
| share mutable default values between instances. |
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This isn't true and can be misleading. It is true for builtin mutable collections, but it is not true for any mutable object.
with a list:
>>> import msgspec
>>> DEFAULT = []
>>> class A(msgspec.Struct):
... field: list[int] = msgspec.field(default=DEFAULT)
...
>>> print(A().field is A().field)
Falsewith a custom class:
>>> import msgspec
>>> from collections import UserDict
>>> class MyDict(UserDict):
... pass
...
>>> DEFAULT = MyDict()
>>> class A(msgspec.Struct):
... field: dict[str, int] = msgspec.field(default=DEFAULT)
...
>>> print(A().field is A().field)
True
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omit_defaultsonly recognizes empty-collection defaults when the default is given by value or via a builtin collection constructor (list/dict/set/tuple/frozenset). A customdefault_factory(a user function,lambda, or aStruct/dataclass/attrstype) is never called during detection, so its field is always encoded, even when the produced value is empty.This is intentional: detection runs in the encode hot path and stays O(1) without invoking user code. The behavior wasn't documented though, so the
matches_defaultsection now spells out thedefault_factorylimitation and points atdefault_factory=list(which type checks fine via the field annotation) as the supported way to omit an empty collection default.Closes #1032
Closes #645