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Siyet merged 1 commit into from
Jun 16, 2026
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Document that omit_defaults ignores custom default_factory #1076

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17 changes: 17 additions & 0 deletions docs/structs.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -677,6 +677,23 @@ detection logic is as follows:
... return True
... return False

This detection never calls a ``default_factory``. A field configured with a
custom ``default_factory`` is only omitted when the factory is one of the
builtin collection constructors (``list``, ``dict``, ``set``, ``tuple``, or
``frozenset``). Any other callable (a user-defined function, a ``lambda``, or a
``Struct``/``dataclass``/``attrs`` type) is treated as opaque, so the field is
always encoded, even when the value it produces is empty. To omit an empty
collection default, configure the builtin constructor directly:

.. code-block:: python

>>> class Basket(msgspec.Struct, omit_defaults=True):
... items: list[int] = msgspec.field(default_factory=list)

The field annotation supplies the element type, so ``default_factory=list``
still type checks. Specifying ``default=[]`` works too: ``msgspec`` doesn't
share mutable default values between instances.
Comment on lines +694 to +695

@provinzkraut provinzkraut Jun 16, 2026

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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)
False

with 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



.. _forbid-unknown-fields:

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