02. Fields
Field 函数用于自定义字段，为其添加元数据
默认值 default
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| from pydantic import BaseModel, Field
class User(BaseModel):
name: str = Field(default='John Doe')
user = User()
print(user)
#> name='John Doe'
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还可以使用 default_factory 定义一个可变的默认值
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| from uuid import uuid4
from pydantic import BaseModel, Field
class User(BaseModel):
id: str = Field(default_factory=lambda: uuid4().hex)
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If you use typing.Optional, it doesn’t mean that the field has a default value of None!
与 Annotated 合用
Field 可以写进 Annotated 哦（不过有啥用嘛）
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| from uuid import uuid4
from typing_extensions import Annotated
from pydantic import BaseModel, Field
class User(BaseModel):
id: Annotated[str, Field(default_factory=lambda: uuid4().hex)]
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default_factory 只能写在 Annotated 里面，而 default 无法写在里面，只能直接在外面通过赋值的形式写
字段别名 alias
有三种方法定义一个 alias：
Field(..., alias='foo')Field(..., validation_alias='foo')Field(..., serialization_alias='foo')
alias 在验证和序列化时都被用到，除非用第二、三种方法分别开来
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| 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'}
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by_alias=True 一定要显式地写出来！默认是 False
If you want to use an alias only for validation, you can use the validation_alias parameter:
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| from pydantic import BaseModel, Field
class User(BaseModel):
name: str = Field(..., validation_alias='username')
user = User(username='johndoe')
print(user)
#> name='johndoe'
print(user.model_dump(by_alias=True))
#> {'name': 'johndoe'}
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If you only want to define an alias for serialization, you can use the serialization_alias parameter:
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| from pydantic import BaseModel, Field
class User(BaseModel):
name: str = Field(..., serialization_alias='username')
user = User(name='johndoe')
print(user)
#> name='johndoe'
print(user.model_dump(by_alias=True))
#> {'username': 'johndoe'}
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数字条件约束
可以用以下参数来限制数字型字段的值：
gt - greater thanlt - less thange - greater than or equal tole - less than or equal tomultiple_of - a multiple of the given number 给定数字的倍数allow_inf_nan - allow 'inf', '-inf', 'nan' values
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| from pydantic import BaseModel, Field
class Foo(BaseModel):
positive: int = Field(gt=0)
non_negative: int = Field(ge=0)
negative: int = Field(lt=0)
non_positive: int = Field(le=0)
even: int = Field(multiple_of=2)
love_for_pydantic: float = Field(allow_inf_nan=True)
foo = Foo(
positive=1,
non_negative=0,
negative=-1,
non_positive=0,
even=2,
love_for_pydantic=float('inf'),
)
print(foo)
"""
positive=1 non_negative=0 negative=-1 non_positive=0 even=2 love_for_pydantic=inf
"""
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对那些混合类型来说，使用数字约束可能会出错。使用 Annotated 来避免这种情况
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| from typing import Optional
from typing_extensions import Annotated
from pydantic import BaseModel, Field
class Foo(BaseModel):
positive: Optional[Annotated[int, Field(gt=0)]]
# Can error in some cases, not recommended:
non_negative: Optional[int] = Field(ge=0)
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