invalid-inheritance error when a pydantic BaseModel extends a Protocol

Describe the Bug

from typing import Protocol

from pydantic import BaseModel


class Foo(Protocol):
    a: int


class Bar(BaseModel, Foo):  # error: invalid-inheritance
    a: int = 1

Class `Bar` has metaclass `ModelMetaclass` from base class `BaseModel` which is not compatible with metaclass `_ProtocolMeta` from base class `Foo`

Sandbox Link

No response

(Only applicable for extension issues) IDE Information

No response

Thanks for the report! Python itself rejects this class with TypeError: metaclass conflict, since pydantic's ModelMetaclass and the _ProtocolMeta behind Protocol are unrelated subclasses of ABCMeta (sandbox), so I'm closing this as working as intended. Protocols are structural, though, so if you drop Foo from the bases, Bar is still accepted anywhere a Foo is expected.

@DetachHead

I'm going to reopen this one for now and take it over, since I think there's probably a valid ask here, but I'm not sure what it is yet.

@DetachHead, Hassan is right that it looks like it errors when run by a Python interpreter. Is there a different example you have where this would be a false positive?

wow, i had no idea this doesnt work at runtime... just another thing to add to the long list of everything wrong with Protocols in python

the reason i never ran into this before is because pyright doesn't handle extending Protocols properly at all, so i had to use my extremely hacky @implements decorator instead. but that doesn't work in pyrefly due to #5536 and #1180

i dont mind if you close this because it doesnt really seem fixable in pyrefly, unless we can have a separate pyrefly_extensions module with runtime fixes for some of these fundamentally broken typing features

wow, i had no idea this doesnt work at runtime... just another thing to add to DetachHead/basedpyright#294 (comment)

another special casing situation to consider for protocols 🤣

the reason i never ran into this before is because pyright doesn't handle extending Protocols properly at all, so i had to use my DetachHead/basedpyright#775 (comment). but that doesn't work in pyrefly due to #5536 and #1180

wait, I never knew pyright didn't handle protocol at all, that's crazy! hopefully #5536 and #1180 will be fixed this week, and that it will support your @implements decorator. I'll put out a new release once it's landed

i dont mind if you close this because it doesnt really seem fixable in pyrefly, unless we can have a separate pyrefly_extensions module with runtime fixes for some of these fundamentally broken typing features

I may open a new issue or discussion post with possible ideas for things to include in a pyrefly_extensions module and add this in

Edit: #5600 created

Navigation Menu

Repository navigation

`invalid-inheritance` error when a pydantic `BaseModel` extends a `Protocol` #5521

Describe the Bug

Sandbox Link

(Only applicable for extension issues) IDE Information

Metadata

Assignees

Labels

Type

Projects

Milestone

Relationships

Development

invalid-inheritance error when a pydantic BaseModel extends a Protocol #5521

Description

Describe the Bug

Sandbox Link

(Only applicable for extension issues) IDE Information

Activity

shahhamdihassan commented on Nov 5, 2026

connernilsen commented on Nov 6, 2026

DetachHead commented on Nov 9, 2026

connernilsen commented on Nov 9, 2026

Metadata

Metadata

Assignees

Labels

Type

Projects

Milestone

Relationships

Development

Issue actions

`invalid-inheritance` error when a pydantic `BaseModel` extends a `Protocol` #5521