以下代码有效:
from sqlalchemy.ext.declarative import declarative_base
Base = declarative_base()
但是，以下内容:
import sqlalchemy
Base = sqlalchemy.ext.declarative.declarative_base()
抛出:模块sqlalchemy"没有属性ext".怎么可能?
sqlalchemy 是一个包，导入一个包并不会自动加载它的子模块.当你这样做时
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()
导入系统不会加载 sqlalchemy.ext 子模块，因为您没有要求它.
The import system doesn't load the sqlalchemy.ext submodule, because you didn't ask for it.
当你这样做时
from sqlalchemy.ext.declarative import declarative_base
Base = declarative_base()
您已明确要求加载 sqlalchemy.ext 和 sqlalchemy.ext.declarative，因此访问有效.
You've explicitly requested that sqlalchemy.ext and sqlalchemy.ext.declarative get loaded, so the access works.
你也可以这样做
import sqlalchemy.ext.declarative
Base = sqlalchemy.ext.declarative.declarative_base()
这也将加载 sqlalchemy.ext 和 sqlalchemy.ext.declarative.
有些包会在 __init__.py 中自动加载其子模块，因此您不必显式导入子模块.不过，这不是您应该依赖的东西.
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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