如何在Pydantic中使用一个类的属性作为另一个类的类型？

Great question! When you need to reuse a specific field's type (and optionally its default value) from another Pydantic model, there are a few clean, maintainable ways to do this without duplicating code. Here are the most common approaches for your example:

1. Access the Class Annotations Directly #

Python classes store their type hints in the __annotations__ dictionary, which you can access directly to pull the type from model A:

from pydantic import BaseModel
from typing import Optional

class A(BaseModel):
    a: int
    b: Optional[str]
    c: bool = False

class B(BaseModel):
    a: str
    b: Optional[str]
    c: A.__annotations__['c'] = A.c  # Reuses type AND default value from A's 'c'

This works because A.__annotations__['c'] gives you the exact type hint defined for c in class A. We also set the default to A.c to match the original default value if you want consistency across models.

2. Use Pydantic's Field Metadata #

Pydantic models keep detailed information about each field in the __fields__ attribute. This is especially useful if the type was inferred by Pydantic or includes validation logic:

class B(BaseModel):
    a: str
    b: Optional[str]
    c: A.__fields__['c'].type_ = A.__fields__['c'].default

Here, A.__fields__['c'].type_ returns the resolved type of the field, and default pulls the original default value. This method is more Pydantic-native and handles edge cases like custom types or validators better.

3. Resolve Type Hints with typing.get_type_hints #

If your model uses forward references (string-based type hints), typing.get_type_hints will resolve them correctly, making this a safe, robust option:

from typing import Optional, get_type_hints
from pydantic import BaseModel

class A(BaseModel):
    a: int
    b: Optional[str]
    c: bool = False

class B(BaseModel):
    a: str
    b: Optional[str]
    c: get_type_hints(A)['c'] = A.c

get_type_hints returns a dictionary of fully resolved type hints for the class, so you don't have to worry about unresolved references.

Bonus: Reusing Multiple Fields #

If you ever need to reuse multiple fields instead of just one, consider creating a mixin class or composing models. For example:

class CMixin(BaseModel):
    c: bool = False

class A(CMixin, BaseModel):
    a: int
    b: Optional[str]

class B(CMixin, BaseModel):
    a: str
    b: Optional[str]

This way, any changes to the c field in the mixin will automatically apply to both A and B.

内容的提问来源于stack exchange，提问作者shantanu rahut