For forbid_extra_keys raise custom ForbiddenExtraKeyError

HISTORY.rst

@@ -18,7 +18,8 @@ History
   (`#231 <https://github.com/python-attrs/cattrs/pull/231>`_)
 * Fix unstructuring all tuples - unannotated, variable-length, homogenous and heterogenous - to `list`.
   (`#226 <https://github.com/python-attrs/cattrs/issues/226>`_)
-
+* For ``forbid_extra_keys`` raise custom ``ForbiddenExtraKeyError`` instead of generic ``Exception``.
+  (`#255 <https://github.com/python-attrs/cattrs/pull/225>`_)
 
 1.10.0 (2022-01-04)
 -------------------

docs/customizing.rst

@@ -108,7 +108,7 @@ creating structure hooks with ``make_dict_structure_fn``.
     >>> c.structure({"nummber": 2}, TestClass)
     Traceback (most recent call last):
     ...
-    Exception: Extra fields in constructor for TestClass: nummber
+    ForbiddenExtraKeyError: Extra fields in constructor for TestClass: nummber
     >>> hook = make_dict_structure_fn(TestClass, c, _cattrs_forbid_extra_keys=False)
     >>> c.register_structure_hook(TestClass, hook)
     >>> c.structure({"nummber": 2}, TestClass)

docs/structuring.rst

@@ -473,7 +473,7 @@ Here's a small example showing how to use factory hooks to apply the `forbid_ext
     >>> c.structure({"an_int": 1, "else": 2}, E)
     Traceback (most recent call last):
     ...
-    Exception: Extra fields in constructor for E: else
+    ForbiddenExtraKeyError: Extra fields in constructor for E: else
 
 
 A complex use case for hook factories is described over at :ref:`Using factory hooks`.

src/cattrs/errors.py

@@ -1,4 +1,4 @@
-from typing import Type
+from typing import Optional, Set, Type
 
 from cattr._compat import ExceptionGroup
 
@@ -33,3 +33,25 @@ class ClassValidationError(BaseValidationError):
     """Raised when validating a class if any attributes are invalid."""
 
     pass
+
+
+class ForbiddenExtraKeysError(Exception):
+    """Raised when `forbid_extra_keys` is activated and such extra keys are detected during structuring.
+
+    The attribute `extra_fields` is a sequence of those extra keys, which were the cause of this error,
+    and `cl` is the class which was structured with those extra keys.
+    """
+
+    def __init__(
+        self, message: Optional[str], cl: Type, extra_fields: Set[str]
+    ) -> None:
+        self.cl = cl
+        self.extra_fields = extra_fields
+
+        msg = (
+            message
+            if message
+            else f"Extra fields in constructor for {cl.__name__}: {', '.join(extra_fields)}"
+        )
+
+        super().__init__(msg)

src/cattrs/gen.py

@@ -17,7 +17,11 @@
     is_generic,
 )
 from cattr._generics import deep_copy_with
-from cattrs.errors import ClassValidationError, IterableValidationError
+from cattrs.errors import (
+    ClassValidationError,
+    ForbiddenExtraKeysError,
+    IterableValidationError,
+)
 
 if TYPE_CHECKING:  # pragma: no cover
     from cattr.converters import Converter
@@ -258,6 +262,10 @@ def make_dict_structure_fn(
         resolve_types(cl)
 
     allowed_fields = set()
+    if _cattrs_forbid_extra_keys:
+        globs["__c_a"] = allowed_fields
+        globs["__c_feke"] = ForbiddenExtraKeysError
+
     if _cattrs_detailed_validation:
         lines.append("  res = {}")
         lines.append("  errors = []")
@@ -323,6 +331,14 @@ def make_dict_structure_fn(
                 f"{i}e.__note__ = 'Structuring class {cl.__qualname__} @ attribute {an}'"
             )
             lines.append(f"{i}errors.append(e)")
+
+        if _cattrs_forbid_extra_keys:
+            post_lines += [
+                "  unknown_fields = set(o.keys()) - __c_a",
+                "  if unknown_fields:",
+                "    errors.append(__c_feke('', __cl, unknown_fields))",
+            ]
+
         post_lines.append(
             f"  if errors: raise __c_cve('While structuring {cl.__name__}', errors, __cl)"
         )
@@ -445,15 +461,12 @@ def make_dict_structure_fn(
             ["  return __cl("] + [f"    {line}" for line in invocation_lines] + ["  )"]
         )
 
-    if _cattrs_forbid_extra_keys:
-        globs["__c_a"] = allowed_fields
-        post_lines += [
-            "  unknown_fields = set(o.keys()) - __c_a",
-            "  if unknown_fields:",
-            "    raise Exception(",
-            f"      'Extra fields in constructor for {cl_name}: ' + ', '.join(unknown_fields)"
-            "    )",
-        ]
+        if _cattrs_forbid_extra_keys:
+            post_lines += [
+                "  unknown_fields = set(o.keys()) - __c_a",
+                "  if unknown_fields:",
+                "    raise __c_feke('', __cl, unknown_fields)",
+            ]
 
     # At the end, we create the function header.
     internal_arg_line = ", ".join([f"{i}={i}" for i in internal_arg_parts])

tests/metadata/test_genconverter.py

@@ -20,6 +20,7 @@
 from cattr import UnstructureStrategy
 from cattr._compat import is_py39_plus, is_py310_plus
 from cattr.gen import make_dict_structure_fn, override
+from cattrs.errors import ClassValidationError, ForbiddenExtraKeysError
 
 from . import (
     nested_typed_classes,
@@ -87,9 +88,14 @@ def test_forbid_extra_keys(cls_and_vals):
     while bad_key in unstructured:
         bad_key += "A"
     unstructured[bad_key] = 1
-    with pytest.raises(Exception):
+    with pytest.raises(ClassValidationError) as cve:
         converter.structure(unstructured, cl)
 
+    assert len(cve.value.exceptions) == 1
+    assert isinstance(cve.value.exceptions[0], ForbiddenExtraKeysError)
+    assert cve.value.exceptions[0].cl is cl
+    assert cve.value.exceptions[0].extra_fields == {bad_key}
+
 
 @given(simple_typed_attrs(defaults=True))
 def test_forbid_extra_keys_defaults(attr_and_vals):
@@ -102,9 +108,14 @@ def test_forbid_extra_keys_defaults(attr_and_vals):
     inst = cl()
     unstructured = converter.unstructure(inst)
     unstructured["aa"] = unstructured.pop("a")
-    with pytest.raises(Exception):
+    with pytest.raises(ClassValidationError) as cve:
         converter.structure(unstructured, cl)
 
+    assert len(cve.value.exceptions) == 1
+    assert isinstance(cve.value.exceptions[0], ForbiddenExtraKeysError)
+    assert cve.value.exceptions[0].cl is cl
+    assert cve.value.exceptions[0].extra_fields == {"aa"}
+
 
 def test_forbid_extra_keys_nested_override():
     @attr.s
@@ -122,17 +133,30 @@ class A:
     converter.structure(unstructured, A)
     # if we break it in the subclass, we need it to raise
     unstructured["c"]["aa"] = 5
-    with pytest.raises(Exception):
+    with pytest.raises(ClassValidationError) as cve:
         converter.structure(unstructured, A)
+
+    assert len(cve.value.exceptions) == 1
+    assert isinstance(cve.value.exceptions[0], ClassValidationError)
+    assert len(cve.value.exceptions[0].exceptions) == 1
+    assert isinstance(cve.value.exceptions[0].exceptions[0], ForbiddenExtraKeysError)
+    assert cve.value.exceptions[0].exceptions[0].cl is C
+    assert cve.value.exceptions[0].exceptions[0].extra_fields == {"aa"}
+
     # we can "fix" that by disabling forbid_extra_keys on the subclass
     hook = make_dict_structure_fn(C, converter, _cattrs_forbid_extra_keys=False)
     converter.register_structure_hook(C, hook)
     converter.structure(unstructured, A)
     # but we should still raise at the top level
     unstructured["b"] = 6
-    with pytest.raises(Exception):
+    with pytest.raises(ClassValidationError) as cve:
         converter.structure(unstructured, A)
 
+    assert len(cve.value.exceptions) == 1
+    assert isinstance(cve.value.exceptions[0], ForbiddenExtraKeysError)
+    assert cve.value.exceptions[0].cl is A
+    assert cve.value.exceptions[0].extra_fields == {"b"}
+
 
 @given(nested_typed_classes(defaults=True, min_attrs=1), unstructure_strats, booleans())
 def test_nested_roundtrip(cls_and_vals, strat, omit_if_default):

tests/test_gen_dict.py

@@ -7,6 +7,7 @@
 
 from cattr._compat import adapted_fields, fields
 from cattrs import Converter
+from cattrs.errors import ClassValidationError, ForbiddenExtraKeysError
 from cattrs.gen import make_dict_structure_fn, make_dict_unstructure_fn, override
 
 from . import nested_classes, simple_classes
@@ -226,9 +227,15 @@ class A:
 
     assert new_inst == A(1, "str")
 
-    with pytest.raises(Exception):
+    with pytest.raises(ClassValidationError) as cve:
         converter.structure({"b": 1, "c": "str"}, A)
 
+    assert len(cve.value.exceptions) == 2
+    assert isinstance(cve.value.exceptions[0], KeyError)
+    assert isinstance(cve.value.exceptions[1], ForbiddenExtraKeysError)
+    assert cve.value.exceptions[1].cl is A
+    assert cve.value.exceptions[1].extra_fields == {"c"}
+
 
 def test_omitting():
     converter = Converter()