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Nov 16, 2025
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Small docs fix #697

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20 changes: 15 additions & 5 deletions docs/customizing.md
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Original file line number Diff line number Diff line change
Expand Up @@ -191,6 +191,8 @@ Currently, the overrides only support generating dictionary un/structuring hooks
and support `omit_if_default`, `forbid_extra_keys`, `rename` and `omit`.

### `omit_if_default`
```{currentmodule} cattrs.gen
```

This override can be applied on a per-class or per-attribute basis.
The generated unstructuring hook will skip unstructuring values that are equal to their default or factory values.
Expand All @@ -205,7 +207,10 @@ The generated unstructuring hook will skip unstructuring values that are equal t
... b: dict = Factory(dict)

>>> c = cattrs.Converter()
>>> c.register_unstructure_hook(WithDefault, make_dict_unstructure_fn(WithDefault, c, b=override(omit_if_default=True)))
>>> c.register_unstructure_hook(
... WithDefault,
... make_dict_unstructure_fn(WithDefault, c, b=override(omit_if_default=True)),
... )
>>> c.unstructure(WithDefault(1))
{'a': 1}
```
Expand All @@ -216,7 +221,7 @@ For example, consider a class with a `dateTime` field and a factory for it: skip
So we apply the `omit_if_default` rule to the class, but not to the `dateTime` field.

```{note}
The parameter to `make_dict_unstructure_function` is named ``_cattrs_omit_if_default`` instead of just ``omit_if_default`` to avoid potential collisions with an override for a field named ``omit_if_default``.
The parameter to {meth}`make_dict_unstructure_fn` is named ``_cattrs_omit_if_default`` instead of just ``omit_if_default`` to avoid potential collisions with an override for a field named ``omit_if_default``.
```

```{doctest}
Expand All @@ -226,11 +231,16 @@ So we apply the `omit_if_default` rule to the class, but not to the `dateTime` f

>>> @define
... class TestClass:
... a: Optional[int] = None
... a: int | None = None
... b: datetime = Factory(datetime.utcnow)

>>> c = cattrs.Converter()
>>> hook = make_dict_unstructure_fn(TestClass, c, _cattrs_omit_if_default=True, b=override(omit_if_default=False))
>>> hook = make_dict_unstructure_fn(
... TestClass,
... c,
... _cattrs_omit_if_default=True,
... b=override(omit_if_default=False)
... )
>>> c.register_unstructure_hook(TestClass, hook)
>>> c.unstructure(TestClass())
{'b': ...}
Expand Down Expand Up @@ -283,7 +293,7 @@ This is useful if an attribute name is a reserved keyword in Python.

>>> @define
... class ExampleClass:
... klass: Optional[int]
... klass: int | None

>>> c = cattrs.Converter()
>>> unst_hook = make_dict_unstructure_fn(ExampleClass, c, klass=override(rename="class"))
Expand Down
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