False positive error when subclassing List type

@sobolevn

Bug Report

This bug is experienced in classes lib

To Reproduce

from typing import List


class ListOfStrMeta(type):
    def __instancecheck__(cls, other) -> bool:
        return (
            isinstance(other, list) and
            bool(other) and
            all(isinstance(list_item, str) for list_item in other)
        )


class ListOfStr(List[str], metaclass=ListOfStrMeta):
    ...

Expected Behavior

No errors should appear

Actual Behavior

Error from the code above:

poetry run mypy ex.py
ex.py:13: error: Metaclass conflict: the metaclass of a derived class must be a (non-strict) subclass of the metaclasses of all its bases  [misc]
Found 1 error in 1 file (checked 1 source file)

Your Environment

Mypy version used: 1.2.0
Mypy command-line flags: N/A
Mypy configuration options from mypy.ini (and other config files):

[mypy]
# Plugins, includes custom:
plugins =
  classes.contrib.mypy.classes_plugin

disable_error_code = empty-body

allow_redefinition = false
check_untyped_defs = true
disallow_any_explicit = true
# disallow_any_generics = true
disallow_untyped_calls = true
ignore_errors = false
ignore_missing_imports = true
implicit_reexport = false
local_partial_types = true
strict_optional = true
strict_equality = true
no_implicit_optional = true
warn_no_return = true
warn_unused_ignores = true
warn_redundant_casts = true
warn_unused_configs = true
warn_unreachable = true

Python version used: 3.10.10

cc @sobolevn

@ilevkivskyi

This is probably due to #6042:

mypy/mypy/semanal.py

Lines 2363 to 2372 in 0f09be4

    
           if any(info.is_protocol for info in defn.info.mro): 
        
               if ( 
        
                   not defn.info.metaclass_type 
        
                   or defn.info.metaclass_type.type.fullname == "builtins.type" 
        
               ): 
        
                   # All protocols and their subclasses have ABCMeta metaclass by default. 
        
                   # TODO: add a metaclass conflict check if there is another metaclass. 
        
                   abc_meta = self.named_type_or_none("abc.ABCMeta", []) 
        
                   if abc_meta is not None:  # May be None in tests with incomplete lib-stub. 
        
                       defn.info.metaclass_type = abc_meta

Can take a look @ilevkivskyi?

Since list derives from a handful of protocols (e.g. Sequence), its metaclass is ABCMeta.

Changing

-class ListOfStrMeta(typing):
+class ListOfStrMeta(ABCMeta):

eliminates the error, though it's probably wrong since type(list) is type.

Yes, this is due to the fact that typeshed has Sequence in the base classes of list.
While runtime does not have it.

#17713 makes the error here go away

Navigation Menu

Repository navigation

False positive error when subclassing `List` type #15040

Metadata

Assignees

Labels

Projects

Milestone

Relationships

Development

	if any(info.is_protocol for info in defn.info.mro):
	if (
	not defn.info.metaclass_type
	or defn.info.metaclass_type.type.fullname == "builtins.type"
	):
	# All protocols and their subclasses have ABCMeta metaclass by default.
	# TODO: add a metaclass conflict check if there is another metaclass.
	abc_meta = self.named_type_or_none("abc.ABCMeta", [])
	if abc_meta is not None: # May be None in tests with incomplete lib-stub.
	defn.info.metaclass_type = abc_meta

False positive error when subclassing List type #15040

Description

Activity

ikonst commented on Apr 12, 2023

sobolevn commented on Apr 12, 2023

hauntsaninja commented on Jun 20, 2025

Metadata

Metadata

Assignees

Labels

Projects

Milestone

Relationships

Development

Issue actions

False positive error when subclassing `List` type #15040