Python 命名空间“import X"和“from X import" [英] Python namespace 'import X' and 'from X import'

问题描述

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?

推荐答案

sqlalchemy 是一个包，导入包不会自动加载其子模块.当你这样做

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

from sqlalchemy.ext.declarative import declarative_base
Base = declarative_base()

您已明确请求加载 sqlalchemy.ext 和 sqlalchemy.ext.declarative，以便访问有效.

You've explicitly requested that sqlalchemy.ext and sqlalchemy.ext.declarative get loaded, so the access works.

你也可以做到

import sqlalchemy.ext.declarative
Base = sqlalchemy.ext.declarative.declarative_base()

这也会加载 sqlalchemy.ext 和 sqlalchemy.ext.declarative.

某些包会自动在其 __init__.py 中加载它们的子模块，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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