Merge branch 'master' into pos_only-in-stubgen

Author: InvincibleRMC

docs/source/error_code_list.rst

@@ -215,6 +215,35 @@ You can use :py:class:`~collections.abc.Callable` as the type for callable objec
         for x in objs:
             f(x)
 
+.. _code-metaclass:
+
+Check the validity of a class's metaclass [metaclass]
+-----------------------------------------------------
+
+Mypy checks whether the metaclass of a class is valid. The metaclass
+must be a subclass of ``type``. Further, the class hierarchy must yield
+a consistent metaclass. For more details, see the
+`Python documentation <https://docs.python.org/3.13/reference/datamodel.html#determining-the-appropriate-metaclass>`_
+
+Note that mypy's metaclass checking is limited and may produce false-positives.
+See also :ref:`limitations`.
+
+Example with an error:
+
+.. code-block:: python
+
+    class GoodMeta(type):
+        pass
+
+    class BadMeta:
+        pass
+
+    class A1(metaclass=GoodMeta):  # OK
+        pass
+
+    class A2(metaclass=BadMeta):  # Error:  Metaclasses not inheriting from "type" are not supported  [metaclass]
+        pass
+
 .. _code-var-annotated:
 
 Require annotation if variable type is unclear [var-annotated]

docs/source/metaclasses.rst

@@ -90,3 +90,28 @@ so it's better not to combine metaclasses and class hierarchies:
 * ``Self`` is not allowed as annotation in metaclasses as per `PEP 673`_.
 
 .. _PEP 673: https://peps.python.org/pep-0673/#valid-locations-for-self
+
+For some builtin types, mypy may think their metaclass is :py:class:`abc.ABCMeta`
+even if it is :py:class:`type` at runtime. In those cases, you can either:
+
+* use :py:class:`abc.ABCMeta` instead of :py:class:`type` as the
+  superclass of your metaclass if that works in your use-case
+* mute the error with ``# type: ignore[metaclass]``
+
+.. code-block:: python
+
+    import abc
+
+    assert type(tuple) is type  # metaclass of tuple is type at runtime
+
+    # The problem:
+    class M0(type): pass
+    class A0(tuple, metaclass=M0): pass  # Mypy Error: metaclass conflict
+
+    # Option 1: use ABCMeta instead of type
+    class M1(abc.ABCMeta): pass
+    class A1(tuple, metaclass=M1): pass
+
+    # Option 2: mute the error
+    class M2(type): pass
+    class A2(tuple, metaclass=M2): pass  # type: ignore[metaclass]

mypy/checker.py

@@ -2959,7 +2959,11 @@ def check_metaclass_compatibility(self, typ: TypeInfo) -> None:
                 "Metaclass conflict: the metaclass of a derived class must be "
                 "a (non-strict) subclass of the metaclasses of all its bases",
                 typ,
+                code=codes.METACLASS,
             )
+            explanation = typ.explain_metaclass_conflict()
+            if explanation:
+                self.note(explanation, typ, code=codes.METACLASS)
 
     def visit_import_from(self, node: ImportFrom) -> None:
         for name, _ in node.names:

mypy/errorcodes.py

@@ -270,6 +270,7 @@ def __hash__(self) -> int:
     "General",
     default_enabled=False,
 )
+METACLASS: Final[ErrorCode] = ErrorCode("metaclass", "Ensure that metaclass is valid", "General")
 
 # Syntax errors are often blocking.
 SYNTAX: Final[ErrorCode] = ErrorCode("syntax", "Report syntax errors", "General")

mypy/nodes.py

@@ -3402,6 +3402,43 @@ def calculate_metaclass_type(self) -> mypy.types.Instance | None:
 
         return winner
 
+    def explain_metaclass_conflict(self) -> str | None:
+        # Compare to logic in calculate_metaclass_type
+        declared = self.declared_metaclass
+        if declared is not None and not declared.type.has_base("builtins.type"):
+            return None
+        if self._fullname == "builtins.type":
+            return None
+
+        winner = declared
+        if declared is None:
+            resolution_steps = []
+        else:
+            resolution_steps = [f'"{declared.type.fullname}" (metaclass of "{self.fullname}")']
+        for super_class in self.mro[1:]:
+            super_meta = super_class.declared_metaclass
+            if super_meta is None or super_meta.type is None:
+                continue
+            if winner is None:
+                winner = super_meta
+                resolution_steps.append(
+                    f'"{winner.type.fullname}" (metaclass of "{super_class.fullname}")'
+                )
+                continue
+            if winner.type.has_base(super_meta.type.fullname):
+                continue
+            if super_meta.type.has_base(winner.type.fullname):
+                winner = super_meta
+                resolution_steps.append(
+                    f'"{winner.type.fullname}" (metaclass of "{super_class.fullname}")'
+                )
+                continue
+            # metaclass conflict
+            conflict = f'"{super_meta.type.fullname}" (metaclass of "{super_class.fullname}")'
+            return f"{' > '.join(resolution_steps)} conflicts with {conflict}"
+
+        return None
+
     def is_metaclass(self, *, precise: bool = False) -> bool:
         return (
             self.has_base("builtins.type")

mypy/semanal.py

@@ -2702,7 +2702,7 @@ def infer_metaclass_and_bases_from_compat_helpers(self, defn: ClassDef) -> None:
         if len(metas) == 0:
             return
         if len(metas) > 1:
-            self.fail("Multiple metaclass definitions", defn)
+            self.fail("Multiple metaclass definitions", defn, code=codes.METACLASS)
             return
         defn.metaclass = metas.pop()
 
@@ -2758,7 +2758,11 @@ def get_declared_metaclass(
             elif isinstance(metaclass_expr, MemberExpr):
                 metaclass_name = get_member_expr_fullname(metaclass_expr)
             if metaclass_name is None:
-                self.fail(f'Dynamic metaclass not supported for "{name}"', metaclass_expr)
+                self.fail(
+                    f'Dynamic metaclass not supported for "{name}"',
+                    metaclass_expr,
+                    code=codes.METACLASS,
+                )
                 return None, False, True
             sym = self.lookup_qualified(metaclass_name, metaclass_expr)
             if sym is None:
@@ -2769,6 +2773,7 @@ def get_declared_metaclass(
                     self.fail(
                         f'Class cannot use "{sym.node.name}" as a metaclass (has type "Any")',
                         metaclass_expr,
+                        code=codes.METACLASS,
                     )
                 return None, False, True
             if isinstance(sym.node, PlaceholderNode):
@@ -2786,11 +2791,15 @@ def get_declared_metaclass(
                     metaclass_info = target.type
 
             if not isinstance(metaclass_info, TypeInfo) or metaclass_info.tuple_type is not None:
-                self.fail(f'Invalid metaclass "{metaclass_name}"', metaclass_expr)
+                self.fail(
+                    f'Invalid metaclass "{metaclass_name}"', metaclass_expr, code=codes.METACLASS
+                )
                 return None, False, False
             if not metaclass_info.is_metaclass():
                 self.fail(
-                    'Metaclasses not inheriting from "type" are not supported', metaclass_expr
+                    'Metaclasses not inheriting from "type" are not supported',
+                    metaclass_expr,
+                    code=codes.METACLASS,
                 )
                 return None, False, False
             inst = fill_typevars(metaclass_info)

mypy/typeshed/stdlib/VERSIONS

@@ -95,6 +95,7 @@ asyncio.staggered: 3.8-
 asyncio.taskgroups: 3.11-
 asyncio.threads: 3.9-
 asyncio.timeouts: 3.11-
+asyncio.tools: 3.14-
 asyncio.trsock: 3.8-
 asyncore: 3.0-3.11
 atexit: 3.0-

mypy/typeshed/stdlib/_csv.pyi

@@ -90,6 +90,7 @@ else:
 
 def writer(
     csvfile: SupportsWrite[str],
+    /,
     dialect: _DialectLike = "excel",
     *,
     delimiter: str = ",",
@@ -103,6 +104,7 @@ def writer(
 ) -> _writer: ...
 def reader(
     csvfile: Iterable[str],
+    /,
     dialect: _DialectLike = "excel",
     *,
     delimiter: str = ",",

mypy/typeshed/stdlib/_zstd.pyi

@@ -52,6 +52,7 @@ class ZstdCompressor:
         self, /, data: ReadableBuffer, mode: _ZstdCompressorContinue | _ZstdCompressorFlushBlock | _ZstdCompressorFlushFrame = 0
     ) -> bytes: ...
     def flush(self, /, mode: _ZstdCompressorFlushBlock | _ZstdCompressorFlushFrame = 2) -> bytes: ...
+    def set_pledged_input_size(self, size: int | None, /) -> None: ...
     @property
     def last_mode(self) -> _ZstdCompressorContinue | _ZstdCompressorFlushBlock | _ZstdCompressorFlushFrame: ...