Handle functools.Placeholder in partial

Fixes #21313.

Author: MestreY0d4-Uninter

mypy/plugins/functools.py

@@ -9,13 +9,17 @@
 import mypy.semanal
 from mypy.argmap import map_actuals_to_formals
 from mypy.erasetype import erase_typevars
+from mypy.expandtype import expand_type
+from mypy.infer import infer_type_arguments
 from mypy.nodes import (
     ARG_POS,
     ARG_STAR2,
     SYMBOL_FUNCBASE_TYPES,
     ArgKind,
     Argument,
     CallExpr,
+    Expression,
+    MemberExpr,
     NameExpr,
     Var,
 )
@@ -30,6 +34,7 @@
     ParamSpecType,
     Type,
     TypeOfAny,
+    TypeVarId,
     TypeVarType,
     UnboundType,
     UnionType,
@@ -41,6 +46,7 @@
 _ORDERING_METHODS: Final = {"__lt__", "__le__", "__gt__", "__ge__"}
 
 PARTIAL: Final = "functools.partial"
+PLACEHOLDER: Final = "functools.Placeholder"
 
 
 class _MethodInfo:
@@ -134,6 +140,10 @@ def _analyze_class(ctx: mypy.plugin.ClassDefContext) -> dict[str, _MethodInfo |
     return comparison_methods
 
 
+def _is_functools_placeholder(expr: Expression) -> bool:
+    return isinstance(expr, (NameExpr, MemberExpr)) and expr.fullname == PLACEHOLDER
+
+
 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
@@ -184,6 +194,7 @@ def handle_partial_with_callee(ctx: mypy.plugin.FunctionContext, callee: Type) -
     actual_arg_kinds = []
     actual_arg_names = []
     actual_types = []
+    placeholder_actuals = []
     seen_args = set()
     for i, param in enumerate(ctx.args[1:], start=1):
         for j, a in enumerate(param):
@@ -198,6 +209,9 @@ def handle_partial_with_callee(ctx: mypy.plugin.FunctionContext, callee: Type) -
             actual_arg_kinds.append(ctx.arg_kinds[i][j])
             actual_arg_names.append(ctx.arg_names[i][j])
             actual_types.append(ctx.arg_types[i][j])
+            placeholder_actuals.append(
+                ctx.arg_kinds[i][j].is_positional() and _is_functools_placeholder(a)
+            )
 
     formal_to_actual = map_actuals_to_formals(
         actual_kinds=actual_arg_kinds,
@@ -215,8 +229,20 @@ def handle_partial_with_callee(ctx: mypy.plugin.FunctionContext, callee: Type) -
             continue
         can_infer_ids.update({tv.id for tv in get_all_type_vars(arg_type)})
 
+    defaulted_arg_types = list(fn_type.arg_types)
+    for i, actuals in enumerate(formal_to_actual):
+        if any(placeholder_actuals[j] for j in actuals):
+            # functools.Placeholder is a positional sentinel introduced in Python 3.14.
+            # It occupies the formal slot but does not bind it, so make the validation
+            # call accept the sentinel while preserving the original type for the
+            # resulting partial signature below.
+            defaulted_arg_types[i] = actual_types[
+                next(j for j in actuals if placeholder_actuals[j])
+            ]
+
     # special_sig="partial" allows omission of args/kwargs typed with ParamSpec
     defaulted = fn_type.copy_modified(
+        arg_types=defaulted_arg_types,
         arg_kinds=[
             (
                 ArgKind.ARG_OPT
@@ -273,10 +299,25 @@ def handle_partial_with_callee(ctx: mypy.plugin.FunctionContext, callee: Type) -
     partial_kinds = []
     partial_types = []
     partial_names = []
+    inferred_type_vars: dict[TypeVarId, Type] = {}
+    if len(bound.arg_types) == len(fn_type.arg_types):
+        for i, actuals in enumerate(formal_to_actual):
+            if not actuals or any(placeholder_actuals[j] for j in actuals):
+                continue
+            inferred_args = infer_type_arguments(
+                fn_type.variables, fn_type.arg_types[i], bound.arg_types[i]
+            )
+            for type_var, inferred_arg in zip(fn_type.variables, inferred_args):
+                if inferred_arg is not None and mypy.checker.is_valid_inferred_type(
+                    inferred_arg, ctx.api.options
+                ):
+                    inferred_type_vars[type_var.id] = inferred_arg
     # 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):
-        if len(bound.arg_types) == len(fn_type.arg_types):
+        if any(placeholder_actuals[j] for j in actuals):
+            arg_type = expand_type(fn_type.arg_types[i], inferred_type_vars)
+        elif len(bound.arg_types) == len(fn_type.arg_types):
             arg_type = bound.arg_types[i]
             if not mypy.checker.is_valid_inferred_type(arg_type, ctx.api.options):
                 arg_type = fn_type.arg_types[i]  # bit of a hack
@@ -285,10 +326,16 @@ def handle_partial_with_callee(ctx: mypy.plugin.FunctionContext, callee: Type) -
             # true when PEP 646 things are happening. See testFunctoolsPartialTypeVarTuple
             arg_type = fn_type.arg_types[i]
 
-        if not actuals or fn_type.arg_kinds[i] in (ArgKind.ARG_STAR, ArgKind.ARG_STAR2):
+        if (
+            not actuals
+            or fn_type.arg_kinds[i] in (ArgKind.ARG_STAR, ArgKind.ARG_STAR2)
+            or any(placeholder_actuals[j] for j in actuals)
+        ):
             partial_kinds.append(fn_type.arg_kinds[i])
             partial_types.append(arg_type)
-            partial_names.append(fn_type.arg_names[i])
+            partial_names.append(
+                None if any(placeholder_actuals[j] for j in actuals) else fn_type.arg_names[i]
+            )
         else:
             assert actuals
             if any(actual_arg_kinds[j] in (ArgKind.ARG_POS, ArgKind.ARG_STAR) for j in actuals):

test-data/unit/check-functools.test

@@ -726,3 +726,53 @@ def outer_c(arg: Tc) -> None:
     use_int_callable(partial(inner, b=""))  # E: Argument 1 to "use_int_callable" has incompatible type "partial[str]"; expected "Callable[[int], int]" \
                                             # N: "partial[str].__call__" has type "def __call__(__self, *args: Any, **kwargs: Any) -> str"
 [builtins fixtures/tuple.pyi]
+
+[case testFunctoolsPartialPlaceholder]
+import functools
+from functools import partial, Placeholder as _
+from typing import TypeVar
+
+T = TypeVar("T")
+
+
+def foo(a: int, b: str, c: bool) -> tuple[int, str, bool]: ...
+
+
+p = partial(foo, _, "x", _)
+reveal_type(p)  # N: Revealed type is "functools.partial[tuple[builtins.int, builtins.str, builtins.bool]]"
+reveal_type(p(1, True))  # N: Revealed type is "tuple[builtins.int, builtins.str, builtins.bool]"
+p("bad", True)  # E: Argument 1 to "foo" has incompatible type "str"; expected "int"
+p(1, 1)  # E: Argument 2 to "foo" has incompatible type "int"; expected "bool"
+p(a=1, c=True)  # E: Unexpected keyword argument "a" for "foo" \
+                # E: Unexpected keyword argument "c" for "foo"
+
+
+def same(a: T, b: T) -> T: ...
+def same_list(a: T, b: list[T]) -> T: ...
+
+
+generic = partial(same, _, 1)
+reveal_type(generic)  # N: Revealed type is "functools.partial[builtins.int]"
+generic(2)
+generic("bad")  # E: Argument 1 to "same" has incompatible type "str"; expected "int"
+
+nested_generic = partial(same_list, _, [1])
+reveal_type(nested_generic)  # N: Revealed type is "functools.partial[builtins.int]"
+nested_generic(2)
+nested_generic("bad")  # E: Argument 1 to "same_list" has incompatible type "str"; expected "int"
+
+module_attr = partial(foo, functools.Placeholder, "x", functools.Placeholder)
+reveal_type(module_attr(1, True))  # N: Revealed type is "tuple[builtins.int, builtins.str, builtins.bool]"
+partial(foo, a=_)  # E: Argument "a" to "foo" has incompatible type "_PlaceholderType"; expected "int"
+[file functools.pyi]
+from typing import Any, Callable, Final, Generic, TypeVar
+
+_T = TypeVar("_T")
+
+class _PlaceholderType: ...
+Placeholder: Final[_PlaceholderType]
+
+class partial(Generic[_T]):
+    def __new__(cls, func: Callable[..., _T], /, *args: Any, **kwargs: Any) -> partial[_T]: ...
+    def __call__(self, *args: Any, **kwargs: Any) -> _T: ...
+[builtins fixtures/tuple.pyi]