Improve BaseConverter mapping structuring (#496)

HISTORY.md

@@ -49,6 +49,7 @@ can now be used as decorators and have gained new features.
   ([#460](https://github.com/python-attrs/cattrs/issues/460) [#467](https://github.com/python-attrs/cattrs/pull/467))
 - `typing_extensions.Any` is now supported and handled like `typing.Any`.
   ([#488](https://github.com/python-attrs/cattrs/issues/488) [#490](https://github.com/python-attrs/cattrs/pull/490))
+- The BaseConverter now properly generates detailed validation errors for mappings.
 - [PEP 695](https://peps.python.org/pep-0695/) generics are now tested.
   ([#452](https://github.com/python-attrs/cattrs/pull/452))
 - Imports are now sorted using Ruff.

src/cattrs/converters.py

@@ -853,6 +853,38 @@ def _structure_dict(self, obj: Mapping[T, V], cl: Any) -> dict[T, V]:
         if is_bare(cl) or cl.__args__ == (Any, Any):
             return dict(obj)
         key_type, val_type = cl.__args__
+
+        if self.detailed_validation:
+            key_handler = self._structure_func.dispatch(key_type)
+            val_handler = self._structure_func.dispatch(val_type)
+            errors = []
+            res = {}
+
+            for k, v in obj.items():
+                try:
+                    value = val_handler(v, val_type)
+                except Exception as exc:
+                    msg = IterableValidationNote(
+                        f"Structuring mapping value @ key {k!r}", k, val_type
+                    )
+                    exc.__notes__ = [*getattr(exc, "__notes__", []), msg]
+                    errors.append(exc)
+                    continue
+
+                try:
+                    key = key_handler(k, key_type)
+                    res[key] = value
+                except Exception as exc:
+                    msg = IterableValidationNote(
+                        f"Structuring mapping key @ key {k!r}", k, key_type
+                    )
+                    exc.__notes__ = [*getattr(exc, "__notes__", []), msg]
+                    errors.append(exc)
+
+            if errors:
+                raise IterableValidationError(f"While structuring {cl!r}", errors, cl)
+            return res
+
         if key_type in ANIES:
             val_conv = self._structure_func.dispatch(val_type)
             return {k: val_conv(v, val_type) for k, v in obj.items()}

src/cattrs/gen/__init__.py

@@ -879,12 +879,12 @@ def make_mapping_structure_fn(
             globs["enumerate"] = enumerate
 
             lines.append("  res = {}; errors = []")
-            lines.append("  for ix, (k, v) in enumerate(mapping.items()):")
+            lines.append("  for k, v in mapping.items():")
             lines.append("    try:")
             lines.append(f"      value = {v_s}")
             lines.append("    except Exception as e:")
             lines.append(
-                "      e.__notes__ = getattr(e, '__notes__', []) + [IterableValidationNote('Structuring mapping value @ key ' + repr(k), k, val_type)]"
+                "      e.__notes__ = getattr(e, '__notes__', []) + [IterableValidationNote(f'Structuring mapping value @ key {k!r}', k, val_type)]"
             )
             lines.append("      errors.append(e)")
             lines.append("      continue")
@@ -893,7 +893,7 @@ def make_mapping_structure_fn(
             lines.append("      res[key] = value")
             lines.append("    except Exception as e:")
             lines.append(
-                "      e.__notes__ = getattr(e, '__notes__', []) + [IterableValidationNote('Structuring mapping key @ key ' + repr(k), k, key_type)]"
+                "      e.__notes__ = getattr(e, '__notes__', []) + [IterableValidationNote(f'Structuring mapping key @ key {k!r}', k, key_type)]"
             )
             lines.append("      errors.append(e)")
             lines.append("  if errors:")

tests/test_validation.py

@@ -104,14 +104,12 @@ def test_deque_validation():
     ]
 
 
-@given(...)
-def test_mapping_validation(detailed_validation: bool):
+def test_mapping_validation(converter):
     """Proper validation errors are raised structuring mappings."""
-    c = Converter(detailed_validation=detailed_validation)
 
-    if detailed_validation:
+    if converter.detailed_validation:
         with pytest.raises(IterableValidationError) as exc:
-            c.structure({"1": 1, "2": "b", "c": 3}, Dict[int, int])
+            converter.structure({"1": 1, "2": "b", "c": 3}, Dict[int, int])
 
         assert repr(exc.value.exceptions[0]) == repr(
             ValueError("invalid literal for int() with base 10: 'b'")
@@ -128,7 +126,7 @@ def test_mapping_validation(detailed_validation: bool):
         ]
     else:
         with pytest.raises(ValueError):
-            c.structure({"1": 1, "2": "b", "c": 3}, Dict[int, int])
+            converter.structure({"1": 1, "2": "b", "c": 3}, Dict[int, int])
 
 
 @given(...)