Relax signature checks to better support builtins and C extension fun…

…ctions as validators (#7101)

Co-authored-by: David Montague <35119617+dmontagu@users.noreply.github.com>

pydantic/_internal/_decorators.py

@@ -493,7 +493,7 @@ def build(model_dc: type[Any]) -> DecoratorInfos:  # noqa: C901 (ignore complexi
 
 
 def inspect_validator(validator: Callable[..., Any], mode: FieldValidatorModes) -> bool:
-    """Look at a field or model validator function and determine if it whether it takes an info argument.
+    """Look at a field or model validator function and determine whether it takes an info argument.
 
     An error is raised if the function has an invalid signature.
 
@@ -504,10 +504,13 @@ def inspect_validator(validator: Callable[..., Any], mode: FieldValidatorModes)
     Returns:
         Whether the validator takes an info argument.
     """
-    if getattr(validator, '__module__', None) == 'builtins':
-        # int, str, etc.
+    try:
+        sig = signature(validator)
+    except ValueError:
+        # builtins and some C extensions don't have signatures
+        # assume that they don't take an info argument and only take a single argument
+        # e.g. `str.strip` or `datetime.datetime`
         return False
-    sig = signature(validator)
     n_positional = count_positional_params(sig)
     if mode == 'wrap':
         if n_positional == 3:

tests/test_validators.py

@@ -61,11 +61,15 @@ class Model(BaseModel):
 def test_annotated_validator_builtin() -> None:
     """https://github.com/pydantic/pydantic/issues/6752"""
     TruncatedFloat = Annotated[float, BeforeValidator(int)]
+    DateTimeFromIsoFormat = Annotated[datetime, BeforeValidator(datetime.fromisoformat)]
 
     class Model(BaseModel):
         x: TruncatedFloat
+        y: DateTimeFromIsoFormat
 
-    assert Model(x=1.234).x == 1
+    m = Model(x=1.234, y='2011-11-04T00:05:23')
+    assert m.x == 1
+    assert m.y == datetime(2011, 11, 4, 0, 5, 23)
 
 
 def test_annotated_validator_plain() -> None: