typeddicts: improve error message on invalid input (#617)

src/cattrs/gen/typeddicts.py

@@ -2,6 +2,7 @@
 
 import re
 import sys
+from collections.abc import Mapping
 from typing import TYPE_CHECKING, Any, Callable, Literal, TypedDict, TypeVar
 
 from attrs import NOTHING, Attribute
@@ -307,15 +308,18 @@ def make_dict_structure_fn(
         globs["__c_feke"] = ForbiddenExtraKeysError
 
     if _cattrs_detailed_validation:
-        # When running under detailed validation, be extra careful about copying
-        # so that the correct error is raised if the input isn't a dict.
-        lines.append("  try:")
-        lines.append("    res = o.copy()")
-        lines.append("  except Exception as exc:")
+        # When running under detailed validation, be extra careful about the
+        # input type so that the correct error is raised if the input isn't a dict.
+        internal_arg_parts["__c_mapping"] = Mapping
+        lines.append("  if not isinstance(o, __c_mapping):")
+        te = "TypeError(f'expected a mapping, not {o.__class__.__name__}')"
         lines.append(
-            f"    raise __c_cve('While structuring ' + {cl.__name__!r}, [exc], __cl)"
+            f"    raise __c_cve('While structuring ' + {cl.__name__!r}, [{te}], __cl)"
         )
 
+    lines.append("  res = o.copy()")
+
+    if _cattrs_detailed_validation:
         lines.append("  errors = []")
         internal_arg_parts["__c_cve"] = ClassValidationError
         internal_arg_parts["__c_avn"] = AttributeValidationNote
@@ -389,7 +393,6 @@ def make_dict_structure_fn(
             f"  if errors: raise __c_cve('While structuring ' + {cl.__name__!r}, errors, __cl)"
         )
     else:
-        lines.append("  res = o.copy()")
         non_required = []
 
         # The first loop deals with required args.

src/cattrs/v.py

@@ -47,12 +47,6 @@ def format_exception(exc: BaseException, type: Union[type, None]) -> str:
     ):
         # This was supposed to be a mapping (and have .items()) but it something else.
         res = "expected a mapping"
-    elif isinstance(exc, AttributeError) and exc.args[0].endswith(
-        "object has no attribute 'copy'"
-    ):
-        # This was supposed to be a mapping (and have .copy()) but it something else.
-        # Used for TypedDicts.
-        res = "expected a mapping"
     else:
         res = f"unknown error ({exc})"
 

tests/test_typeddicts.py

@@ -517,4 +517,13 @@ def test_nondict_input():
     with raises(ClassValidationError) as exc:
         converter.structure(1, TypedDictA)
 
-    assert transform_error(exc.value) == ["expected a mapping @ $"]
+    assert transform_error(exc.value) == [
+        "invalid type (expected a mapping, not int) @ $"
+    ]
+
+    with raises(ClassValidationError) as exc:
+        converter.structure([1], TypedDictA)
+
+    assert transform_error(exc.value) == [
+        "invalid type (expected a mapping, not list) @ $"
+    ]

tests/test_v.py

@@ -323,7 +323,9 @@ class D(TypedDict):
     try:
         c.structure({"c": 1}, D)
     except Exception as exc:
-        assert transform_error(exc) == ["expected a mapping @ $.c"]
+        assert transform_error(exc) == [
+            "invalid type (expected a mapping, not int) @ $.c"
+        ]
 
     try:
         c.structure({"c": {"a": "str"}}, D)