gh-124176: Add special support for dataclasses to create_autospec

Lib/test/test_unittest/testmock/testhelpers.py

@@ -1034,6 +1034,75 @@ def f(a): pass
         self.assertEqual(mock.mock_calls, [])
         self.assertEqual(rv.mock_calls, [])
 
+    def test_dataclass(self):
+        from dataclasses import dataclass, field, InitVar
+        from typing import ClassVar
+
+        @dataclass
+        class WithPostInit:
+            a: int = field(init=False)
+            b: int = field(init=False)
+            def __post_init__(self):
+                self.a = 1
+                self.b = 2
+
+        for mock in [
+            create_autospec(WithPostInit, instance=True),
+            create_autospec(WithPostInit()),
+        ]:
+            with self.subTest(mock=mock):
+                self.assertIsInstance(mock.a, int)
+                self.assertIsInstance(mock.b, int)
+
+        # Classes do not have these fields:
+        mock = create_autospec(WithPostInit)
+        msg = "Mock object has no attribute"
+        with self.assertRaisesRegex(AttributeError, msg):
+            mock.a
+        with self.assertRaisesRegex(AttributeError, msg):
+            mock.b
+
+        @dataclass
+        class WithDefault:
+            a: int
+            b: int = 0
+
+        for mock in [
+            create_autospec(WithDefault, instance=True),
+            create_autospec(WithDefault(1)),
+        ]:
+            with self.subTest(mock=mock):
+                self.assertIsInstance(mock.a, int)
+                self.assertIsInstance(mock.b, int)
+
+        @dataclass
+        class WithMethod:
+            a: int
+            def b(self) -> int:
+                return 1
+
+        for mock in [
+            create_autospec(WithMethod, instance=True),
+            create_autospec(WithMethod(1)),
+        ]:
+            with self.subTest(mock=mock):
+                self.assertIsInstance(mock.a, int)
+                mock.b.assert_not_called()
+
+        @dataclass
+        class WithNonFields:
+            a: ClassVar[int]
+            b: InitVar[int]
+
+        for mock in [
+            create_autospec(WithNonFields, instance=True),
+            create_autospec(WithNonFields(1)),
+        ]:
+            with self.subTest(mock=mock):
+                with self.assertRaisesRegex(AttributeError, msg):
+                    mock.a
+                with self.assertRaisesRegex(AttributeError, msg):
+                    mock.b
 
 class TestCallList(unittest.TestCase):
 

Lib/unittest/mock.py

@@ -2754,7 +2754,19 @@ def create_autospec(spec, spec_set=False, instance=False, _parent=None,
         raise InvalidSpecError(f'Cannot autospec a Mock object. '
                                f'[object={spec!r}]')
     is_async_func = _is_async_func(spec)
-    _kwargs = {'spec': spec}
+
+    placeholder = object()
+    entries = [(entry, placeholder) for entry in dir(spec)]
+    # Not using `is_dataclass` to avoid an import of dataclasses module
+    # for types that don't need that.
+    if is_type and instance and hasattr(spec, '__dataclass_fields__'):
+        from dataclasses import fields
+        dataclass_fields = fields(spec)
+        entries.extend((f.name, f.type) for f in dataclass_fields)
+        _kwargs = {'spec': [f.name for f in dataclass_fields]}
+    else:
+        _kwargs = {'spec': spec}
+
     if spec_set:
         _kwargs = {'spec_set': spec}
     elif spec is None:
@@ -2811,7 +2823,7 @@ def create_autospec(spec, spec_set=False, instance=False, _parent=None,
                                             _name='()', _parent=mock,
                                             wraps=wrapped)
 
-    for entry in dir(spec):
+    for entry, original in entries:
         if _is_magic(entry):
             # MagicMock already does the useful magic methods for us
             continue
@@ -2825,10 +2837,11 @@ def create_autospec(spec, spec_set=False, instance=False, _parent=None,
         # AttributeError on being fetched?
         # we could be resilient against it, or catch and propagate the
         # exception when the attribute is fetched from the mock
-        try:
-            original = getattr(spec, entry)
-        except AttributeError:
-            continue
+        if original is placeholder:
+            try:
+                original = getattr(spec, entry)
+            except AttributeError:
+                continue
 
         child_kwargs = {'spec': original}
         # Wrap child attributes also.

Misc/NEWS.d/next/Library/2024-09-24-13-32-16.gh-issue-124176.6hmOPz.rst

@@ -0,0 +1,4 @@
+Add support for :func:`dataclasses.dataclass` in
+:func:`unittest.mock.create_autospec`. Now ``create_autospec`` will check
+for potential dataclasses and use :func:`dataclasses.fields` function to
+retrieve the spec information.