Pydantic 2.6.4 -> 2.7.0: expected enum but got str serialized value may not be as expected

[hhartzer]

I'd like to open this as an issue, but I don't have a reproducible example yet. Hoping someone else may have seen this.

FastAPI 0.101.1, sqlmodel 0.0.16, Python 3.11.

Going from Pydantic 2.6.4 to 2.7.0, some heavy endpoints, maybe somewhat randomly (and more often than not), will spew expected enum but got str serialized value may not be as expected over and over. It seems to be able to lock thing up for minutes at a time, if left alone.

I do have some enums. I suspect this is related to these:

#9045

#9064

Which showed up in 2.7.0.

When I reverted (solely) back to 2.6.4, all was well.

This might definitely be my bug in some way, but I feel like this is a breaking change to some degree or another.

[inyutin]

Interestingly, we have very same issue on pydantic 2.6.4 🤔
Python 3.11, fastapi 0.110.1

[hhartzer]

Did your pydantic-core version change with it?

[tamird]

Indeed we are on pydantic-core 2.18.1.

[hhartzer]

I think that pydantic 2.7.0 is supposed to go with pydantic-core 2.18.1, while pydantic 2.6.4 should go with 2.16.3.

Can you downgrade pydantic-core and see if it fixes this?

[tamird]

I found the problem and fixed it. We were indeed doing the wrong thing.

[junoriosity]

@hhartzer I also encountered this issue, is there a way to overcome that impediment?

[hhartzer]

You can downgrade. I think the best thing would be to make a small, reproducible example so we can make a bug report with it. I haven't done that yet.

[miketheman]

Here's an example that's reproducible, but I suspect it's an operator error.

from enum import Enum

from pydantic import BaseModel


class FruitEnum(str, Enum):
    pear = 'pear'
    banana = 'banana'


class CookingModel(BaseModel):
    fruit: FruitEnum | None = None
  
# no warnings emitted since the enum is used in the model construction
print(CookingModel(fruit='banana').model_dump_json(indent=2))

cm = CookingModel()
cm.fruit = 'pear'
print(cm.model_dump_json(indent=2))
# warning: "Expected `enum` but got `str` - serialized value may not be as expected"

After starting a bug report, I realized that it's partially my fault, since I'm assigning the value of fruit outside the construction step, which probably skips validation.

Changing to:

cm.fruit = FruitEnum('pear')

resolves the warning, as it's using the string 'pear' to look up the enum value from FruitEnum and returns the correct type for the dump renderer.

[hhartzer]

Awesome, thank you!

I do feel like if it's a str, Enum, a matching str should be counted as well.