Pydantic: attribute named like hint with default causes 'error checking inheritance'

[bartekbrak]

Bug

For bugs/questions:

• OS: Ubuntu 18/04
• Python version import sys; print(sys.version): 3.7.3
• Pydantic version import pydantic; print(pydantic.VERSION): 0.25

from enum import Enum
from pydantic.dataclasses import dataclass

# this is real life example that used enums, enum is not the issue here
# same will happen with a class or otherwise

class SomeEnum(Enum):
    A = 0
    B = 1

@dataclass
class model:
    SomeEnum: SomeEnum = SomeEnum.A


model()

The problem is caused by having the same name for the hint and class attribute.
The problem only occurs when there is a default.

I know it is brave to name class attribute same as the hint but this is what I wanted and what worked for me without pydantic.

trace:

$ python a_bug.py
Traceback (most recent call last):
  File "/home/bartek/.pyenv/versions/rvo/lib/python3.7/site-packages/pydantic/validators.py", line 386, in find_validators
    if issubclass(type_, val_type):
TypeError: issubclass() arg 1 must be a class

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "a_bug.py", line 9, in <module>
    @dataclass
  File "/home/bartek/.pyenv/versions/rvo/lib/python3.7/site-packages/pydantic/dataclasses.py", line 127, in dataclass
    return wrap(_cls)
  File "/home/bartek/.pyenv/versions/rvo/lib/python3.7/site-packages/pydantic/dataclasses.py", line 122, in wrap
    return _process_class(cls, init, repr, eq, order, unsafe_hash, frozen, config)
  File "/home/bartek/.pyenv/versions/rvo/lib/python3.7/site-packages/pydantic/dataclasses.py", line 85, in _process_class
    cls.__name__, __config__=config, __module__=_cls.__module__, __validators__=validators, **fields
  File "/home/bartek/.pyenv/versions/rvo/lib/python3.7/site-packages/pydantic/main.py", line 574, in create_model
    return type(model_name, (__base__,), namespace)  # type: ignore
  File "/home/bartek/.pyenv/versions/rvo/lib/python3.7/site-packages/pydantic/main.py", line 197, in __new__
    config=config,
  File "/home/bartek/.pyenv/versions/rvo/lib/python3.7/site-packages/pydantic/fields.py", line 140, in infer
    schema=schema,
  File "/home/bartek/.pyenv/versions/rvo/lib/python3.7/site-packages/pydantic/fields.py", line 109, in __init__
    self.prepare()
  File "/home/bartek/.pyenv/versions/rvo/lib/python3.7/site-packages/pydantic/fields.py", line 175, in prepare
    self._populate_validators()
  File "/home/bartek/.pyenv/versions/rvo/lib/python3.7/site-packages/pydantic/fields.py", line 253, in _populate_validators
    else find_validators(self.type_, self.model_config.arbitrary_types_allowed)
  File "/home/bartek/.pyenv/versions/rvo/lib/python3.7/site-packages/pydantic/validators.py", line 390, in find_validators
    raise RuntimeError(f'error checking inheritance of {type_!r} (type: {display_as_type(type_)})') from e
RuntimeError: error checking inheritance of <SomeEnum.A: 0> (type: enum)

[bartekbrak]

For those who stumble upon this rare error, quoting the hint removes the error.

@dataclass
class model:
    SomeEnum: 'SomeEnum' = SomeEnum.A

[samuelcolvin]

thanks for reporting this, I'm not sure it's worth fixing. Better to use alias and name the field something else pep-8 compliant.

[samuelcolvin]

Sorry I was being dumb, this is not a problem with pydantic, but a fundamental of python / an error in your code.

Consider the following:

class Foo:
    pass

class Bar:
    f: Foo = 1

debug(Bar.__annotations__)

Outputs:

test.py:7 <module>
    Bar.__annotations__: {
        'f': <class '__main__.Foo'>,
    } (dict) len=1

Foo the annotation to f is the class Foo defined above.

However consider this case:

class Foo:
    pass

class Bar:
    Foo: Foo = 1

debug(Bar.__annotations__)

Outputs:

test.py:7 <module>
    Bar.__annotations__: {'Foo': 1} (dict) len=1

In other words, here Foo the annotation to Foo the class property takes the value of Foo the local variable, and becomes 1.

This is not a problem with pydantic.