問題描述
以下代碼有效:
from sqlalchemy.ext.declarative import declarative_base
Base = declarative_base()
但是，以下內容:
import sqlalchemy
Base = sqlalchemy.ext.declarative.declarative_base()
拋出:模塊sqlalchemy"沒有屬性ext".怎么可能?
推薦答案
sqlalchemy 是一個包，導入一個包并不會自動加載它的子模塊.當你這樣做時
sqlalchemy is a package, and importing a package doesn't automatically load its submodules. When you do
import sqlalchemy
Base = sqlalchemy.ext.declarative.declarative_base()
導入系統不會加載 sqlalchemy.ext 子模塊，因為您沒有要求它.
The import system doesn't load the sqlalchemy.ext submodule, because you didn't ask for it.
當你這樣做時
from sqlalchemy.ext.declarative import declarative_base
Base = declarative_base()
您已明確要求加載 sqlalchemy.ext 和 sqlalchemy.ext.declarative，因此訪問有效.
You've explicitly requested that sqlalchemy.ext and sqlalchemy.ext.declarative get loaded, so the access works.
你也可以這樣做
import sqlalchemy.ext.declarative
Base = sqlalchemy.ext.declarative.declarative_base()
這也將加載 sqlalchemy.ext 和 sqlalchemy.ext.declarative.
有些包會在 __init__.py 中自動加載其子模塊，因此您不必顯式導入子模塊.不過，這不是您應該依賴的東西.
Some packages automatically load their submodules inside their __init__.py, so you don't have to import submodules explicitly. This isn't something you should rely on, though.
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