为什么我只需要从模块导入子类而不从另一个实例导入它的基类和它的属性？答案

【问题标题】：Why did I need only to import child class from module without importing its base class and its properties from another instance?为什么我只需要从模块导入子类而不从另一个实例导入它的基类和它的属性？
【发布时间】：2020-12-14 00:21:37
【问题描述】：

Python 速成课程：基于项目的动手实践介绍编程

当我读这本书时，我有一个问题。

为什么我只需要从 'admin' 导入 'Admin'？

在我看来，“test.py”中的第一行代码意味着它只复制了“Admin”类代码。

因此，第二行和第三行代码不会成功运行，因为它没有结束代码块'from user import User'和类'Privileges'的定义。

任何帮助将不胜感激！

test.py

from admin import Admin  # line 1


my_admin = Admin('Jade', 'Lam', 'male', '22')  # line 2

my_admin.privileges.show_privileges()  # line 3
print('\n')

admin.py

from user import User


class Admin(User):
    def __init__(self, first_name, last_name, sex, age):
        super().__init__(first_name, last_name, sex, age)
        self.privileges = Privileges()


class Privileges():
    def __init__(self):
        self.privileges = ['can add post', 'can delete post',
                           'can ban user']

    def show_privileges(self):
        print("Admin's privileges: ")
        for privilege in self.privileges:
            print("--" + privilege)

user.py

class User():
    def __init__(self, first_name, last_name, sex, age):
        self.first_name = first_name.title()
        self.last_name = last_name.title()
        self.sex = sex
        self.age = age

    def describe_user(self):
        --snip--

    def greet_user(self):
        --snip--

【问题讨论】：

第一次导入模块时，会加载整个模块，但使用from admin import Admin 表单时，您只是在当前命名空间中保留了对名称 Admin 的引用。但是模块的整个代码仍然运行。
至少它的可执行语句。
谢谢！虽然不知道它的运行原理。
请参阅我的答案中的示例。

标签： python class import module

【解决方案1】：

请参见下面的示例。

# Consider this module:

# mod1.py:
# some outer level code
# gets executed

pass

# functions get defined but do not execute
# unless called from above


def f1():
    pass


def f2():
    pass


# classes get defined and actually execute but
# if they only have defs, again those get defined
# but don't run.


class A:
    pass


class B:
    pass


class C:
    pass


# now main.py:
# mod1 executes. Every name of mod1 also becomes  available
# under the prefix 'mod1.' This is like creating a mod1 namespace
# in outer scope of main.
import mod1

# mod1 only runs once on imports in this module, so this time it won't
# (it already has).
# While you can still access every name from mod1 using the mod1 prefix
# from the above import, below only the A name is kept in the outer scope
# of main.
from mod1 import A

# If A has dependencies on other mod1 names, that does not matter here.
# Those dependencies were solved on mod1 run, and all the names involved
# were accessed as needed at mod1 runtime. These names are actually in a
# different scope that belongs to mod1.

# After imports, main can have mod1 names in its scope, but mod1 never
# had access to main names. Unless it imports main, something it should
# not do to avoid circular imports.

希望你现在明白了。

【讨论】：