Add support for functools.partial

Fixes #1484

This is currently the most popular mypy issue that does not need a PEP.
I'm sure there's stuff missing, but this should handle most cases.

mypy/checkexpr.py

@@ -1216,14 +1216,14 @@ def apply_function_plugin(
             assert callback is not None  # Assume that caller ensures this
             return callback(
                 FunctionContext(
-                    formal_arg_types,
-                    formal_arg_kinds,
-                    callee.arg_names,
-                    formal_arg_names,
-                    callee.ret_type,
-                    formal_arg_exprs,
-                    context,
-                    self.chk,
+                    arg_types=formal_arg_types,
+                    arg_kinds=formal_arg_kinds,
+                    callee_arg_names=callee.arg_names,
+                    arg_names=formal_arg_names,
+                    default_return_type=callee.ret_type,
+                    args=formal_arg_exprs,
+                    context=context,
+                    api=self.chk,
                 )
             )
         else:
@@ -1233,15 +1233,15 @@ def apply_function_plugin(
             object_type = get_proper_type(object_type)
             return method_callback(
                 MethodContext(
-                    object_type,
-                    formal_arg_types,
-                    formal_arg_kinds,
-                    callee.arg_names,
-                    formal_arg_names,
-                    callee.ret_type,
-                    formal_arg_exprs,
-                    context,
-                    self.chk,
+                    type=object_type,
+                    arg_types=formal_arg_types,
+                    arg_kinds=formal_arg_kinds,
+                    callee_arg_names=callee.arg_names,
+                    arg_names=formal_arg_names,
+                    default_return_type=callee.ret_type,
+                    args=formal_arg_exprs,
+                    context=context,
+                    api=self.chk,
                 )
             )
 

mypy/plugins/default.py

@@ -47,6 +47,10 @@ def get_function_hook(self, fullname: str) -> Callable[[FunctionContext], Type]
             return ctypes.array_constructor_callback
         elif fullname == "functools.singledispatch":
             return singledispatch.create_singledispatch_function_callback
+        elif fullname == "functools.partial":
+            import mypy.plugins.functools
+
+            return mypy.plugins.functools.partial_new_callback
 
         return None
 
@@ -118,6 +122,10 @@ def get_method_hook(self, fullname: str) -> Callable[[MethodContext], Type] | No
             return singledispatch.singledispatch_register_callback
         elif fullname == singledispatch.REGISTER_CALLABLE_CALL_METHOD:
             return singledispatch.call_singledispatch_function_after_register_argument
+        elif fullname == "functools.partial.__call__":
+            import mypy.plugins.functools
+
+            return mypy.plugins.functools.partial_call_callback
         return None
 
     def get_attribute_hook(self, fullname: str) -> Callable[[AttributeContext], Type] | None:

mypy/plugins/functools.py

@@ -3,11 +3,22 @@
 from __future__ import annotations
 
 from typing import Final, NamedTuple
+from mypy.argmap import map_actuals_to_formals
 
+import mypy.checker
 import mypy.plugin
-from mypy.nodes import ARG_POS, ARG_STAR2, Argument, FuncItem, Var
+from mypy.nodes import ARG_POS, ARG_STAR2, ArgKind, Argument, FuncItem, Var
 from mypy.plugins.common import add_method_to_class
-from mypy.types import AnyType, CallableType, Type, TypeOfAny, UnboundType, get_proper_type
+from mypy.types import (
+    AnyType,
+    CallableType,
+    Instance,
+    Type,
+    TypeOfAny,
+    UnboundType,
+    UninhabitedType,
+    get_proper_type,
+)
 
 functools_total_ordering_makers: Final = {"functools.total_ordering"}
 
@@ -102,3 +113,116 @@ def _analyze_class(ctx: mypy.plugin.ClassDefContext) -> dict[str, _MethodInfo |
                 comparison_methods[name] = None
 
     return comparison_methods
+
+
+def partial_new_callback(ctx: mypy.plugin.FunctionContext) -> Type:
+    """Infer a more precise return type for functools.partial"""
+    if not isinstance(ctx.api, mypy.checker.TypeChecker):  # use internals
+        return ctx.default_return_type
+    if len(ctx.arg_types) != 3:  # fn, *args, **kwargs
+        return ctx.default_return_type
+    if len(ctx.arg_types[0]) != 1:
+        return ctx.default_return_type
+
+    fn_type = ctx.arg_types[0][0]
+    if not isinstance(fn_type, CallableType):
+        return ctx.default_return_type
+
+    defaulted = fn_type.copy_modified(
+        arg_kinds=[
+            (
+                ArgKind.ARG_OPT
+                if k == ArgKind.ARG_POS
+                else (ArgKind.ARG_NAMED_OPT if k == ArgKind.ARG_NAMED else k)
+            )
+            for k in fn_type.arg_kinds
+        ]
+    )
+
+    actual_args = [a for param in ctx.args[1:] for a in param]
+    actual_arg_kinds = [a for param in ctx.arg_kinds[1:] for a in param]
+    actual_arg_names = [a for param in ctx.arg_names[1:] for a in param]
+    actual_types = [a for param in ctx.arg_types[1:] for a in param]
+
+    _, bound = ctx.api.expr_checker.check_call(
+        callee=defaulted,
+        args=actual_args,
+        arg_kinds=actual_arg_kinds,
+        arg_names=actual_arg_names,
+        context=ctx.context,
+    )
+    if not isinstance(bound, CallableType):
+        return ctx.default_return_type
+
+    formal_to_actual = map_actuals_to_formals(
+        actual_kinds=actual_arg_kinds,
+        actual_names=actual_arg_names,
+        formal_kinds=fn_type.arg_kinds,
+        formal_names=fn_type.arg_names,
+        actual_arg_type=lambda i: actual_types[i],
+    )
+
+    partial_kinds = []
+    partial_types = []
+    partial_names = []
+    # We need to fully apply any positional arguments (they cannot be respecified)
+    # However, keyword arguments can be respecified, so just give them a default
+    for i, actuals in enumerate(formal_to_actual):
+        arg_type = bound.arg_types[i]
+        if isinstance(arg_type, UninhabitedType):
+            arg_type = fn_type.arg_types[i]  # bit of a hack
+
+        if not actuals or fn_type.arg_kinds[i] in (ArgKind.ARG_STAR, ArgKind.ARG_STAR2):
+            partial_kinds.append(fn_type.arg_kinds[i])
+            partial_types.append(arg_type)
+            partial_names.append(fn_type.arg_names[i])
+        elif actuals:
+            if any(actual_arg_kinds[j] == ArgKind.ARG_POS for j in actuals):
+                continue
+            kind = actual_arg_kinds[actuals[0]]
+            if kind == ArgKind.ARG_NAMED:
+                kind = ArgKind.ARG_NAMED_OPT
+            partial_kinds.append(kind)
+            partial_types.append(arg_type)
+            partial_names.append(fn_type.arg_names[i])
+
+    ret_type = bound.ret_type
+    if isinstance(ret_type, UninhabitedType):
+        ret_type = fn_type.ret_type  # same kind of hack as above
+
+    partially_applied = fn_type.copy_modified(
+        arg_types=partial_types, arg_kinds=partial_kinds, arg_names=partial_names, ret_type=ret_type
+    )
+
+    ret = ctx.api.named_generic_type("functools.partial", [])
+    ret = ret.copy_with_extra_attr("__mypy_partial", partially_applied)
+    return ret
+
+
+def partial_call_callback(ctx: mypy.plugin.MethodContext) -> Type:
+    """Infer a more precise return type for functools.partial.__call__."""
+    if (
+        not isinstance(ctx.api, mypy.checker.TypeChecker)  # use internals
+        or not isinstance(ctx.type, Instance)
+        or ctx.type.type.fullname != "functools.partial"
+        or not ctx.type.extra_attrs
+        or "__mypy_partial" not in ctx.type.extra_attrs.attrs
+    ):
+        return ctx.default_return_type
+
+    partial_type = ctx.type.extra_attrs.attrs["__mypy_partial"]
+    if len(ctx.arg_types) != 2:  # *args, **kwargs
+        return ctx.default_return_type
+
+    args = [a for param in ctx.args for a in param]
+    arg_kinds = [a for param in ctx.arg_kinds for a in param]
+    arg_names = [a for param in ctx.arg_names for a in param]
+
+    result = ctx.api.expr_checker.check_call(
+        callee=partial_type,
+        args=args,
+        arg_kinds=arg_kinds,
+        arg_names=arg_names,
+        context=ctx.context,
+    )
+    return result[0]

test-data/unit/check-functools.test

@@ -144,3 +144,82 @@ def f(d: D[C]) -> None:
 
 d: D[int]  # E: Type argument "int" of "D" must be a subtype of "C"
 [builtins fixtures/dict.pyi]
+
+[case testFunctoolsPartialBasic]
+import functools
+
+def foo(a: int, b: str, c: int = 5) -> int: ...  # N: "foo" defined here
+
+p1 = functools.partial(foo)
+p1(1, "a", 3)  # OK
+p1(1, "a", c=3)  # OK
+p1(1, b="a", c=3)  # OK
+
+p2 = functools.partial(foo, 1)
+p2("a")  # OK
+p2("a", 3)  # OK
+p2("a", c=3)  # OK
+p2(1, 3)  # E: Argument 1 to "foo" has incompatible type "int"; expected "str"
+p2(1, "a", 3)  # E: Too many arguments for "foo" \
+               # E: Argument 1 to "foo" has incompatible type "int"; expected "str" \
+               # E: Argument 2 to "foo" has incompatible type "str"; expected "int"
+p2(a=1, b="a", c=3)  # E: Unexpected keyword argument "a" for "foo"
+
+p3 = functools.partial(foo, b="a")
+p3(1)  # OK
+p3(1, c=3)  # OK
+p3(a=1)  # OK
+p3(1, b="a", c=3)  # OK, keywords can be clobbered
+p3(1, 3)  # E: Too many positional arguments for "foo" \
+          # E: Argument 2 to "foo" has incompatible type "int"; expected "str"
+
+functools.partial(foo, "a")  # E: Argument 1 to "foo" has incompatible type "str"; expected "int"
+functools.partial(foo, b=1)  # E: Argument 1 to "foo" has incompatible type "int"; expected "str"
+functools.partial(1)  # E: Argument 1 to "partial" has incompatible type "int"; expected "Callable[..., Never]"
+[builtins fixtures/dict.pyi]
+
+[case testFunctoolsPartialStar]
+import functools
+
+def foo(a: int, b: str, *args: int, d: str, **kwargs: int) -> int: ...
+
+p1 = functools.partial(foo, 1, d="a", x=9)
+p1("a", 2, 3, 4)  # OK
+p1("a", 2, 3, 4, d="a")  # OK
+p1("a", 2, 3, 4, "a")  # E: Argument 5 to "foo" has incompatible type "str"; expected "int"
+p1("a", 2, 3, 4, x="a")  # E: Argument "x" to "foo" has incompatible type "str"; expected "int"
+
+p2 = functools.partial(foo, 1, "a")
+p2(2, 3, 4, d="a")  # OK
+p2("a")  # E: Missing named argument "d" for "foo" \
+         # E: Argument 1 to "foo" has incompatible type "str"; expected "int"
+p2(2, 3, 4)  # E: Missing named argument "d" for "foo"
+
+functools.partial(foo, 1, "a", "b", "c", d="a")  # E: Argument 3 to "foo" has incompatible type "str"; expected "int" \
+                                                 # E: Argument 4 to "foo" has incompatible type "str"; expected "int"
+
+[builtins fixtures/dict.pyi]
+
+[case testFunctoolsPartialGeneric]
+from typing import TypeVar
+import functools
+
+T = TypeVar("T")
+U = TypeVar("U")
+
+def foo(a: T, b: T) -> T: ...
+
+p1 = functools.partial(foo, 1)
+reveal_type(p1(2))  # N: Revealed type is "builtins.int"
+p1("a")  # E: Argument 1 to "foo" has incompatible type "str"; expected "int"
+
+p2 = functools.partial(foo, "a")
+p2(1)  # E: Argument 1 to "foo" has incompatible type "int"; expected "str"
+reveal_type(p2("a"))  # N: Revealed type is "builtins.str"
+
+def bar(a: T, b: U) -> U: ...
+
+p3 = functools.partial(bar, 1)
+reveal_type(p3(2))  # N: Revealed type is "builtins.int"
+reveal_type(p3("a"))  # N: Revealed type is "builtins.str"
+[builtins fixtures/dict.pyi]

test-data/unit/lib-stub/functools.pyi

@@ -1,4 +1,4 @@
-from typing import Generic, TypeVar, Callable, Any, Mapping, overload
+from typing import Generic, TypeVar, Callable, Any, Mapping, Self, overload
 
 _T = TypeVar("_T")
 
@@ -33,3 +33,7 @@ class cached_property(Generic[_T]):
     def __get__(self, instance: object, owner: type[Any] | None = ...) -> _T: ...
     def __set_name__(self, owner: type[Any], name: str) -> None: ...
     def __class_getitem__(cls, item: Any) -> Any: ...
+
+class partial(Generic[_T]):
+    def __new__(cls, __func: Callable[..., _T], *args: Any, **kwargs: Any) -> Self: ...
+    def __call__(__self, *args: Any, **kwargs: Any) -> _T: ...