Python名称空间“ import X”和“ from X import”

Question

The following code works:

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

However, the following:

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

throws: module 'sqlalchemy' has no attribute 'ext' . How can that be?

Answer 1

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()

The import system doesn't load the sqlalchemy.ext submodule, because you didn't ask for it.

When you do

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

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

You also could have done

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

which would also load sqlalchemy.ext and sqlalchemy.ext.declarative .

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.