Fields

The annotated pattern

from typing_extensions import Annotated

from pydantic import BaseModel, Field, WithJsonSchema


class Model(BaseModel):
    name: Annotated[str, Field(strict=True), WithJsonSchema({'extra': 'data'})]

Using this pattern has some advantages

• Using the f: <type> = Field(...) form can be confusing and might trick users into thinking f has a default value, while in reality it is still required.
• You can provide an arbitrary amount of metadata elements for a field. As shown in the example above, the Field() function only supports a limited set of constraints/metadata, and you may have to use different Pydantic utilities such as WithJsonSchema in some cases.
• Types can be made reusable (see the documentation on custom types using this pattern)

Field aliases

• Field(alias='foo')
• Field(validation_alias='foo')
• Field(serialization_alias='foo')

from pydantic import BaseModel, Field


class User(BaseModel):
    name: str = Field(alias='username')


user = User(username='johndoe')  
print(user)
#> name='johndoe'
print(user.model_dump(by_alias=True))  
#> {'username': 'johndoe'}