ImportError: cannot import name 'ModelMetaclass' from 'pydantic.main'

How to fix ImportError: cannot import name 'ModelMetaclass' from 'pydantic.main'

ModelMetaclass is not public API in v2. If you just need a model's metaclass, use type(YourModel) instead of importing it. If a library does this (motor-odm did), upgrade the library. The class does exist at pydantic._internal._model_construction, but that is private and can move between releases.

The exact error

ImportError: cannot import name 'ModelMetaclass' from 'pydantic.main'

Fix it

1. Replace from pydantic.main import ModelMetaclass usage: type(MyModel) gives you the metaclass.

Expected: no import needed

1. If a library does this (motor-odm did), upgrade the library.

Expected: fixed upstream

1. Last resort: import from pydantic._internal._model_construction.

Expected: works but pinned to internals

When this applies

v2 code importing ModelMetaclass from pydantic.main

When this does NOT apply

normal BaseModel subclassing (you never need the metaclass)

Versions

pydantic v2

Why it happens

v2 rebuilt the internals; ModelMetaclass left the public namespace.

Edge cases

Depending on _internal ties you to exact pydantic versions; avoid in libraries.