pydantic 欄位的預設值設定獲取當前時間

ponponon發表於2022-01-28

pydantic 欄位的預設值如何設定獲取當前時間

這種情況不要使用 default，而要用 default_factory
關於這兩種的區別說明如下:

:param default: since this is replacing the field’s default, its first argument is used
      to set the default, use ellipsis (``...``) to indicate the field is required
:param default_factory: callable that will be called when a default value is needed for this field
      If both `default` and `default_factory` are set, an error is raised.

來看看錯誤的例子，即通過 default 獲取當前時間：

from datetime import datetime, timezone
from pydantic import BaseModel, Field
from typing import Optional
import time


def get_utc_now_timestamp() -> datetime:
    return datetime.utcnow().replace(tzinfo=timezone.utc)


class Struct(BaseModel):
    releaseDate: Optional[datetime] = Field(
        default=get_utc_now_timestamp()
    )


print(Struct().releaseDate)
time.sleep(1)
print(Struct().releaseDate)

可以看到兩個時間是一樣的

2022-01-27 14:16:23.876755+00:00
2022-01-27 14:16:23.876755+00:00

再來看看 default_factory：

from datetime import datetime, timezone
from pydantic import BaseModel, Field
from typing import Optional
import time


def get_utc_now_timestamp() -> datetime:
    return datetime.utcnow().replace(tzinfo=timezone.utc)


class Struct(BaseModel):
    releaseDate: Optional[datetime] = Field(
        default_factory=get_utc_now_timestamp
    )


print(Struct().releaseDate)
time.sleep(1)
print(Struct().releaseDate)

可以看到，兩個時間相距了 1 秒鐘

2022-01-27 14:15:55.195409+00:00
2022-01-27 14:15:56.200775+00:00

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