fix mypy with default_factory=list etc.

changes/4457-samuelcolvin.md

@@ -0,0 +1 @@
+Fix mypy plugin when using bare types like `list` and `dict` as `default_factory`.

pydantic/mypy.py

@@ -82,7 +82,8 @@ def parse_mypy_version(version: str) -> Tuple[int, ...]:
     return tuple(int(part) for part in version.split('+', 1)[0].split('.'))
 
 
-BUILTINS_NAME = 'builtins' if parse_mypy_version(mypy_version) >= (0, 930) else '__builtins__'
+MYPY_VERSION_TUPLE = parse_mypy_version(mypy_version)
+BUILTINS_NAME = 'builtins' if MYPY_VERSION_TUPLE >= (0, 930) else '__builtins__'
 
 
 def plugin(version: str) -> 'TypingType[Plugin]':
@@ -162,14 +163,22 @@ def _pydantic_field_callback(self, ctx: FunctionContext) -> 'Type':
             # Functions which use `ParamSpec` can be overloaded, exposing the callable's types as a parameter
             # Pydantic calls the default factory without any argument, so we retrieve the first item
             if isinstance(default_factory_type, Overloaded):
-                if float(mypy_version) > 0.910:
+                if MYPY_VERSION_TUPLE > (0, 910):
                     default_factory_type = default_factory_type.items[0]
                 else:
                     # Mypy0.910 exposes the items of overloaded types in a function
                     default_factory_type = default_factory_type.items()[0]  # type: ignore[operator]
 
             if isinstance(default_factory_type, CallableType):
-                return default_factory_type.ret_type
+                ret_type = default_factory_type.ret_type
+                # mypy doesn't think `ret_type` has `args`, you'd think mypy should know,
+                # add this check in case it varies by version
+                args = getattr(ret_type, 'args', None)
+                if args:
+                    if all(isinstance(arg, TypeVarType) for arg in args):
+                        # Looks like the default factory is a type like `list` or `dict`, replace all args with `Any`
+                        ret_type.args = tuple(default_any_type for _ in args)  # type: ignore[attr-defined]
+                return ret_type
 
         return default_any_type
 

tests/mypy/modules/plugin_default_factory.py

@@ -0,0 +1,21 @@
+"""
+See https://github.com/pydantic/pydantic/issues/4457
+"""
+
+from typing import Dict, List
+
+from pydantic import BaseModel, Field
+
+
+def new_list() -> List[int]:
+    return []
+
+
+class Model(BaseModel):
+    l1: List[str] = Field(default_factory=list)
+    l2: List[int] = Field(default_factory=new_list)
+    l3: List[str] = Field(default_factory=lambda: list())
+    l4: Dict[str, str] = Field(default_factory=dict)
+    l5: int = Field(default_factory=lambda: 123)
+    l6_error: List[str] = Field(default_factory=new_list)
+    l7_error: int = Field(default_factory=list)

tests/mypy/outputs/plugin-fail-strict.txt

@@ -36,9 +36,8 @@
 219: error: Property "y" defined in "FrozenModel" is read-only  [misc]
 240: error: Incompatible types in assignment (expression has type "None", variable has type "int")  [assignment]
 241: error: Incompatible types in assignment (expression has type "None", variable has type "int")  [assignment]
-244: error: Incompatible types in assignment (expression has type "Set[_T]", variable has type "str")  [assignment]
+244: error: Incompatible types in assignment (expression has type "Set[Any]", variable has type "str")  [assignment]
 245: error: Incompatible types in assignment (expression has type "str", variable has type "int")  [assignment]
-246: error: Incompatible types in assignment (expression has type "List[_T]", variable has type "List[int]")  [assignment]
 247: error: Argument "default_factory" to "Field" has incompatible type "int"; expected "Optional[Callable[[], Any]]"  [arg-type]
 250: error: Field default and default_factory cannot be specified together  [pydantic-field]
 260: error: Missing positional argument "self" in call to "instance_method" of "ModelWithAnnotatedValidator"  [call-arg]

tests/mypy/outputs/plugin-fail.txt

@@ -25,9 +25,8 @@
 219: error: Property "y" defined in "FrozenModel" is read-only  [misc]
 240: error: Incompatible types in assignment (expression has type "None", variable has type "int")  [assignment]
 241: error: Incompatible types in assignment (expression has type "None", variable has type "int")  [assignment]
-244: error: Incompatible types in assignment (expression has type "Set[_T]", variable has type "str")  [assignment]
+244: error: Incompatible types in assignment (expression has type "Set[Any]", variable has type "str")  [assignment]
 245: error: Incompatible types in assignment (expression has type "str", variable has type "int")  [assignment]
-246: error: Incompatible types in assignment (expression has type "List[_T]", variable has type "List[int]")  [assignment]
 247: error: Argument "default_factory" to "Field" has incompatible type "int"; expected "Optional[Callable[[], Any]]"  [arg-type]
 250: error: Field default and default_factory cannot be specified together  [pydantic-field]
 260: error: Missing positional argument "self" in call to "instance_method" of "ModelWithAnnotatedValidator"  [call-arg]

tests/mypy/outputs/plugin_default_factory.txt

@@ -0,0 +1,2 @@
+20: error: Incompatible types in assignment (expression has type "List[int]", variable has type "List[str]")  [assignment]
+21: error: Incompatible types in assignment (expression has type "List[Any]", variable has type "int")  [assignment]

tests/mypy/test_mypy.py

@@ -49,6 +49,7 @@
     ('pyproject-plugin-strict.toml', 'plugin_fail.py', 'plugin-fail-strict.txt'),
     ('pyproject-plugin-strict.toml', 'fail_defaults.py', 'fail_defaults.txt'),
     ('mypy-plugin-strict.ini', 'settings_config.py', None),
+    ('mypy-plugin-strict.ini', 'plugin_default_factory.py', 'plugin_default_factory.txt'),
 ]
 executable_modules = list({fname[:-3] for _, fname, out_fname in cases if out_fname is None})