Computed_fields outputs a property object instead of the value

[Galsor]

Initial Checks

• I confirm that I'm using Pydantic V2

Description

Hello,

When a model inherits from another model, overriding one of its fields with a computed_field, the overridden computed field returns a property object instead of the value produced by the function.

# This works fine
class Plane(BaseModel):
 
@computed_field
def name(self) -> str:
   return 'A310'

Plane().name
>>> 'A310' 

# This doesn't
class Aircraft(BaseModel):
   name: str

class Plane(Aircraft):
   category: str = 'A'
   code: str = '310'

   @computed_field
   def name(self) -> str:
      return self.category + self.code

Plane().name
>>> '<property object at 0x7ff1ed10b8d0>'

I am running pydantic==2.3.0 (pydantic_core==2.6.3)

Example Code

from pydantic import BaseModel, computed_field

class Aircraft(BaseModel):
   name: str

class Plane(Aircraft):
   category: str = 'A'
   code: str = '310'

   @computed_field
   def name(self) -> str:
      return self.category + self.code

Python, Pydantic & OS Version

pydantic version: 2.3.0
        pydantic-core version: 2.6.3
          pydantic-core build: profile=release pgo=true
                 install path: /workspaces/project/.venv/lib/python3.10/site-packages/pydantic
               python version: 3.10.8 (main, Aug 19 2023, 00:31:12) [GCC 9.4.0]
                     platform: Linux-5.15.0-1041-azure-x86_64-with-glibc2.31
     optional deps. installed: ['typing-extensions']

[sydney-runkle]

Hi,

Thanks for your question. We recently addressed this issue in this PR, and these changes should get released with the next version. The changes make it such that an error is raised when a computed_field is defined in a subclass such that it overrides a field from a superclass.

We decided to raise an error here because this behavior is both prohibited by dataclasses and discouraged by mypy.

If you're looking for a workaround, using a model validator with mode="after" as shown in this example could help.

Let me know if you have any more questions! 😄

[Galsor]

Ok...
I am interested to understand this choice of forbidding this behavior. Is there any discussion or resources somewhere to justify this choice?

[sydney-runkle]

Sure thing - we've been discussing this decision in #7250.

[sydney-runkle]

Closing in favor of continuing discussion on #7250.