Incomplete type annotations in BaseConfig yield mypy errors when Config is defined from BaseConfig

[mpkocher]

Checks

• I added a descriptive title to this issue
• I have searched (google, github) for similar issues and couldn't find anything
• I have read and followed the docs and still think this is a bug

Bug

1. The BaseConfig is missing type annotations for a few fields. Inheriting from BaseConfig and setting values yields type errors from mypy.

from pydantic import BaseConfig, BaseModel, Extra


class Record(BaseModel):
    alpha: str
    beta: str

    class Config(BaseConfig):
        title = "Record"
        extra = Extra.ignore
        max_anystr_length = 1234

yields

mypy --version && mypy example.py
mypy 0.812
example.py:9: error: Incompatible types in assignment (expression has type "str", base class "BaseConfig" defined the type as "None")  [assignment]
example.py:11: error: Incompatible types in assignment (expression has type "int", base class "BaseConfig" defined the type as "None")  [assignment]
Found 2 errors in 1 file (checked 1 source file)

I believe the type annotations should be:

from typing import Union, Optional

class BaseConfig:
    title: Optional[str] = None
    min_anystr_length: Optional[int] = None # can this just be int = 0 ?
    max_anystr_length: Optional[int] = None
    extra = Union[str, Extra] = Extra.ignore

Inheriting from BaseConfig is useful for autocomplete and avoids typos like the following.

from pydantic import BaseModel

class Point(BaseModel):
    x: int
    y: int
    
    class Config:
        extras = "forbid"

2. The Config docs look to be inconsistent with the default values defined in BaseConfig.

• the default for max_anystr_length is listed as 2 ** 16 in the docs, however, None is defined in BaseConfig (i.e., there is no limit)
• Setting max_anystr_length = 0 ignores the validation completely and has a different semantic meaning than any other positive integer. In other words, max_anystr_length = 0 is the same as max_anystr_length = None.
• Similarly, the default for min_anystr_length is listed as 0 in the docs, but the listed default is None in BaseConfig. If these are semantically the same, then perhaps set min_anystr_length:int = 0 as the default?
• In general, it might be useful to clarify the truthyness of these 0 vs None values

Output of python -c "import pydantic.utils; print(pydantic.utils.version_info())":

             pydantic version: 1.8.1
            pydantic compiled: False
               python version: 3.9.2 | packaged by conda-forge | (default, Feb 21 2021, 05:02:20)  [Clang 11.0.1 ]
                     platform: macOS-11.2.3-x86_64-i386-64bit
     optional deps. installed: ['typing-extensions']

[PrettyWood]

Hi @mpkocher
Thanks for reporting! I opened #2720
Just a remark for Config.extra. I think it's better to just type Extra instead of Union[Extra, str] if the goal is safety.
We could do Union[Extra, Literal["ignore", "forbid", "allow"]] but even with this extra = "forbid" wouldn't work. It would need to be enforced with Final or Literal.
WDYT?

[mpkocher]

@PrettyWood I agree that keeping the type as Extra would be a better approach if possible. The primary motivation for adding str was an attempt to be consistent with some of the docs.

https://pydantic-docs.helpmanual.io/usage/models/#model-creation-from-namedtuple-or-typeddict