Support Field(default_factory) in validate_arguments (#2176)

* Support Field(default_factory) in validate_arguments

* Added docs for validate_arguments with Field

changes/2176-thomascobb.md

@@ -0,0 +1,2 @@
+Allow `Field` with a `default_factory` to be used as an argument to a function
+decorated with `validate_arguments`

docs/examples/validation_decorator_field.py

@@ -0,0 +1,22 @@
+from datetime import datetime
+from pydantic import validate_arguments, Field, ValidationError
+from pydantic.typing import Annotated
+
+
+@validate_arguments
+def how_many(num: Annotated[int, Field(gt=10)]):
+    return num
+
+
+try:
+    how_many(1)
+except ValidationError as e:
+    print(e)
+
+
+@validate_arguments
+def when(dt: datetime = Field(default_factory=datetime.now)):
+    return dt
+
+
+print(type(when()))

docs/usage/validation_decorator.md

@@ -55,6 +55,18 @@ To demonstrate all the above parameter types:
 ```
 _(This script is complete, it should run "as is")_
 
+## Using Field to describe function arguments
+
+[Field](schema.md#field-customisation) can also be used with `validate_arguments` to provide extra information about
+the field and validations. In general it should be used in a type hint with
+[Annotated](schema.md#typingannotated-fields), unless `default_factory` is specified, in which case it should be used
+as the default value of the field:
+
+```py
+{!.tmp_examples/validation_decorator_field.py!}
+```
+_(This script is complete, it should run "as is")_
+
 ## Usage with mypy
 
 The `validate_arguments` decorator should work "out of the box" with [mypy](http://mypy-lang.org/) since it's
@@ -93,13 +105,13 @@ _(This script is complete, it should run "as is")_
 
 ## Custom Config
 
-The model behind `validate_arguments` can be customised using a config setting which is equivalent to 
+The model behind `validate_arguments` can be customised using a config setting which is equivalent to
 setting the `Config` sub-class in normal models.
 
 !!! warning
     The `fields` and `alias_generator` properties of `Config` which allow aliases to be configured are not supported
     yet with `@validate_arguments`, using them will raise an error.
-    
+
 Configuration is set using the `config` keyword argument to the decorator, it may be either a config class
 or a dict of properties which are converted to a class later.
 
@@ -154,7 +166,7 @@ in future.
 ### Config and Validators
 
 `fields` and `alias_generator` on custom [`Config`](model_config.md) are not supported, see [above](#custom-config).
- 
+
 Neither are [validators](validators.md).
 
 ### Model fields and reserved arguments

pydantic/decorator.py

@@ -184,7 +184,7 @@ class ValidatedFunction:
         return values
 
     def execute(self, m: BaseModel) -> Any:
-        d = {k: v for k, v in m._iter() if k in m.__fields_set__}
+        d = {k: v for k, v in m._iter() if k in m.__fields_set__ or m.__fields__[k].default_factory}
         var_kwargs = d.pop(self.v_kwargs_name, {})
 
         if self.v_args_name in d:

tests/test_decorator.py

@@ -3,12 +3,14 @@ import inspect
 import sys
 from pathlib import Path
 from typing import List
+from unittest.mock import ANY
 
 import pytest
 
-from pydantic import BaseModel, ValidationError, validate_arguments
+from pydantic import BaseModel, Field, ValidationError, validate_arguments
 from pydantic.decorator import ValidatedFunction
 from pydantic.errors import ConfigError
+from pydantic.typing import Annotated
 
 skip_pre_38 = pytest.mark.skipif(sys.version_info < (3, 8), reason='testing >= 3.8 behaviour only')
 
@@ -142,6 +144,24 @@ def test_var_args_kwargs(validated):
     assert foo(1, 2, kwargs=4, e=5) == "a=1, b=2, args=(), d=3, kwargs={'kwargs': 4, 'e': 5}"
 
 
+def test_field_can_provide_factory() -> None:
+    @validate_arguments
+    def foo(a: int, b: int = Field(default_factory=lambda: 99), *args: int) -> int:
+        """mypy is happy with this"""
+        return a + b + sum(args)
+
+    assert foo(3) == 102
+    assert foo(1, 2, 3) == 6
+
+
+@pytest.mark.skipif(not Annotated, reason='typing_extensions not installed')
+def test_annotated_field_can_provide_factory() -> None:
+    @validate_arguments
+    def foo2(a: int, b: Annotated[int, Field(default_factory=lambda: 99)] = ANY, *args: int) -> int:
+        """mypy reports Incompatible default for argument "b" if we don't supply ANY as default"""
+        return a + b + sum(args)
+
+
 @skip_pre_38
 def test_positional_only(create_module):
     module = create_module(