🐛 Fix nested dataclass comparison (#120)

connorbrinton · web-flow · commit f15a6dec066d · 2025-11-16T16:48:14.000-08:00
Fix how `IsDataclass` handles nested dataclasses so they can be compared successfully. Previously something like the following would not work: ```python from dataclasses import dataclass from dirty_equals import IsDataclass @DataClass class Address: street: str zip_code: str @DataClass class Person: name: str address: Address person = Person( name='Alice', address=Address(street='123 Main St', zip_code='12345'), ) assert person == IsDataclass( name='Alice', address=IsDataclass(street='123 Main St', zip_code='12345') ) ``` This is a result of `IsDataclass` converting dataclasses to a dictionary for comparison using `dataclasses.asdict`, which _recursively_ converts dataclasses to plain dictionaries. Thus the inner `IsDataclass` fails, because the other side of the comparison is now a dictionary, not a dataclass. These changes fix this by shallowly converting the dataclass to a dictionary using [the approach recommended in the dataclasses documentation](https://docs.python.org/3/library/dataclasses.html#dataclasses.asdict): ```python {field.name: getattr(obj, field.name) for field in fields(obj)} ``` These changes also include a test for nested dataclass comparisons to ensure this works as expected and continues to work in the future.
diff --git a/dirty_equals/_other.py b/dirty_equals/_other.py
@@ -2,7 +2,7 @@
 
 import json
 import re
-from dataclasses import asdict, is_dataclass
+from dataclasses import fields, is_dataclass
 from enum import Enum
 from functools import lru_cache
 from ipaddress import IPv4Address, IPv4Network, IPv6Address, IPv6Network, ip_network
@@ -486,10 +486,11 @@ def _fields_check(self, other: Any) -> bool:
         """
         Checks exactness of fields using [`IsDict`][dirty_equals.IsDict] with given settings.
 
-        Remark that if this method is called, then `other` is an instance of a dataclass, therefore we can call
-        `dataclasses.asdict` to convert to a dict.
+        Remark that if this method is called, then `other` is an instance of a dataclass, therefore we can use
+        `dataclasses.fields` to get its fields. We use a shallow conversion to preserve nested dataclass instances.
         """
-        return asdict(other) == IsDict(self._repr_kwargs).settings(strict=self.strict, partial=self.partial)
+        other_dict = {field.name: getattr(other, field.name) for field in fields(other)}
+        return other_dict == IsDict(self._repr_kwargs).settings(strict=self.strict, partial=self.partial)
 
 
 class IsPartialDataclass(IsDataclass):
diff --git a/tests/test_other.py b/tests/test_other.py
@@ -39,6 +39,21 @@ class Foo:
 foo = Foo(1, 2, 'c')
 
 
+@dataclass
+class Address:
+    street: str
+    zip_code: str
+
+
+@dataclass
+class Person:
+    name: str
+    address: Address
+
+
+person = Person(name='Alice', address=Address(street='123 Main St', zip_code='12345'))
+
+
 @pytest.mark.parametrize(
     'other,dirty',
     [
@@ -373,6 +388,10 @@ def test_is_dataclass_false(other, dirty):
     assert other != dirty
 
 
+def test_is_dataclass_nested():
+    assert person == IsDataclass(name='Alice', address=IsDataclass(street='123 Main St', zip_code='12345'))
+
+
 @pytest.mark.parametrize(
     'other,dirty',
     [
