Python模块导入方式疑问？

Question

import sqlalchemy
from sqlalchemy import create_engine
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy import Column, Integer, String
from sqlalchemy.orm import sessionmaker

sqlalchemy模块已经被导入，为什么下面还需要以“.”号连接导入子模块？
我有看到说某些特定的子模块必须以“.”号连接的形式明确标出才能够导入？如果是，那些子块有这种需要，我在哪里可以查找呢？

Answer 1

from a import b, b is a class, function or constant, from a.b import c, b is a package (module). Simply put, classes and functions can be used directly, and packages have their own classes and functions