Apply attrs' converter to default before omit_if_default check (#696)

* Apply attrs' converter to default before omit_if_default check

* Fix ruff

* Add a test

* Add docs

* Add more attrs to test

Author: antony-frolov

HISTORY.md

@@ -17,6 +17,8 @@ Our backwards-compatibility policy can be found [here](https://github.com/python
   ([#688](https://github.com/python-attrs/cattrs/pull/688))
 - Python 3.9 is no longer supported, as it is end-of-life. Use previous versions on this Python version.
   ([#698](https://github.com/python-attrs/cattrs/pull/698))
+- Apply the attrs converter to the default value before checking if it is equal to the attribute's value, when `omit_if_default` is true and an attrs converter is specified.
+  ([#696](https://github.com/python-attrs/cattrs/pull/696))
 
 ## 25.3.0 (2025-10-07)
 

src/cattrs/gen/__init__.py

@@ -4,7 +4,7 @@
 from collections.abc import Callable, Iterable, Mapping
 from typing import TYPE_CHECKING, Any, Final, Literal, TypeVar
 
-from attrs import NOTHING, Attribute, Factory
+from attrs import NOTHING, Attribute, Converter, Factory
 from typing_extensions import NoDefault
 
 from .._compat import (
@@ -99,6 +99,10 @@ def make_dict_unstructure_fn_from_attrs(
     ..  versionchanged:: 25.2.0
         The `_cattrs_use_alias` parameter takes its value from the given converter
         by default.
+    .. versionchanged:: NEXT
+        When `_cattrs_omit_if_default` is true and the attribute has an attrs converter
+        specified, the converter is applied to the default value before checking if it
+        is equal to the attribute's value.
     """
 
     fn_name = "unstructure_" + cl.__name__
@@ -177,16 +181,32 @@ def make_dict_unstructure_fn_from_attrs(
             if isinstance(d, Factory):
                 globs[def_name] = d.factory
                 internal_arg_parts[def_name] = d.factory
-                if d.takes_self:
-                    lines.append(f"  if instance.{attr_name} != {def_name}(instance):")
-                else:
-                    lines.append(f"  if instance.{attr_name} != {def_name}():")
-                lines.append(f"    res['{kn}'] = {invoke}")
+                def_str = f"{def_name}(instance)" if d.takes_self else f"{def_name}()"
             else:
                 globs[def_name] = d
                 internal_arg_parts[def_name] = d
-                lines.append(f"  if instance.{attr_name} != {def_name}:")
-                lines.append(f"    res['{kn}'] = {invoke}")
+                def_str = def_name
+
+            c = a.converter
+            if c is not None:
+                conv_name = f"__c_conv_{attr_name}"
+                if isinstance(c, Converter):
+                    globs[conv_name] = c
+                    internal_arg_parts[conv_name] = c
+                    field_name = f"__c_field_{attr_name}"
+                    globs[field_name] = a
+                    internal_arg_parts[field_name] = a
+                    def_str = f"{conv_name}({def_str}, instance, {field_name})"
+                elif isinstance(d, Factory):
+                    globs[conv_name] = c
+                    internal_arg_parts[conv_name] = c
+                    def_str = f"{conv_name}({def_str})"
+                else:
+                    globs[def_name] = c(d)
+                    internal_arg_parts[def_name] = c(d)
+
+            lines.append(f"  if instance.{attr_name} != {def_str}:")
+            lines.append(f"    res['{kn}'] = {invoke}")
 
         else:
             # No default or no override.

tests/test_converter.py

@@ -16,6 +16,7 @@
     Union,
 )
 
+import attrs
 import pytest
 from attrs import Factory, define, field, fields, has, make_class
 from hypothesis import assume, given
@@ -339,6 +340,64 @@ class C:
     assert inst == converter.structure(unstructured, C)
 
 
+@given(simple_typed_classes(defaults="always", allow_nan=False))
+def test_omit_default_with_attrs_converter_roundtrip(cl_and_vals):
+    """
+    Omit default with an attrs converter works.
+    """
+    converter = Converter(omit_if_default=True)
+    cl, vals, kwargs = cl_and_vals
+
+    @define
+    class C:
+        a1: int = field(default="1", converter=int)
+        a2: int = field(default="1", converter=attrs.Converter(int))
+        a3: int = field(factory=lambda: "1", converter=int)
+        a4: int = field(factory=lambda: "1", converter=attrs.Converter(int))
+        a5: int = field(
+            factory=lambda: "2",
+            converter=attrs.Converter(
+                lambda obj, self: int(obj) + self.a4, takes_self=True
+            ),
+        )
+        a6: int = field(
+            factory=lambda: "2",
+            converter=attrs.Converter(
+                lambda obj, field: int(obj) + int(field.default.factory()) - 2,
+                takes_field=True,
+            ),
+        )
+        a7: int = field(
+            factory=lambda: "3",
+            converter=attrs.Converter(
+                lambda obj, self, field: (
+                    int(obj) + self.a6 + int(field.default.factory()) - 3
+                ),
+                takes_self=True,
+                takes_field=True,
+            ),
+        )
+        c: cl = Factory(lambda: cl(*vals, **kwargs))
+
+    inst = C()
+    unstructured = converter.unstructure(inst)
+    assert unstructured == {}
+    assert inst == converter.structure(unstructured, C)
+
+    inst = C(0, 0, 0, 0, 0, 0, 0)
+    unstructured = converter.unstructure(inst)
+    assert unstructured == {
+        "a1": 0,
+        "a2": 0,
+        "a3": 0,
+        "a4": 0,
+        "a5": 0,
+        "a6": 0,
+        "a7": 0,
+    }
+    assert inst == converter.structure(unstructured, C)
+
+
 def test_dict_roundtrip_with_alias():
     """
     A class with an aliased attribute can be unstructured and structured.