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python 函数、变量中单下划线和双下划线的区别详解

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一、_func 单下划线开头 --口头私有变量

1.1、在模块中使用单下划线开头

在Python中，通过单下划线_来实现模块级别的私有化，变量除外。一般约定以单下划线开头的函数为模块私有的，也就是说from moduleName import * 将不会引入以单下划线开头的函数。模块中使用单下划线开头定义函数、全局变量和类均适用，但可以用：from module import _func形式单独导入。

实例如下:

lerarn_under_line.py

# coding=utf-8
course = "math"
_credit = 4


def call_var():
    print "course:%s" % course
    print "_credit:%d" % _credit


def _call_var():
    print "带下划线course:%s" % course
    print "带下划线_credit:%d" % _credit


def __call_var():
    print "带双下划线course:%s" % course
    print "带双下划线_credit:%d" % _credit

import lerarn_under_line

>>> import lerarn_under_line
>>> lerarn_under_line.call_var
<function call_var at 0x10aa61850>
>>> lerarn_under_line.call_var()
course:math
_credit:4
>>> lerarn_under_line._call_var()   # 单下划线可以调用
带下划线course:math
带下划线_credit:4
>>> >>> lerarn_under_line.__call_var()   # 双下划线不可调用
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'module' object has no attribute '__call_var'

from lerarn_under_line import *

>>> from lerarn_under_line import *
>>> course
'math'
>>> _credit
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name '_credit' is not defined
>>> call_var()
course:math
_credit:4
>>> _call_var()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name '_call_var' is not defined
>>> __call_var()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name '__call_var' is not defined

from module import _func

>>> from lerarn_under_line import course
>>> course
'math'
>>> from lerarn_under_line import _credit
>>> _credit
4
>>> from lerarn_under_line import call_var
>>> call_var()
course:math
_credit:4
>>> from lerarn_under_line import _call_var
>>> _call_var()
带下划线course:math
带下划线_credit:4
>>> from lerarn_under_line import __call_var
>>> __call_var()
带双下划线course:math
带双下划线_credit:4

1.2、在类中使用单下划线开头

lerarn_under_line.py

class Course(object):
    def __init__(self, name):
        self.name = name

    def credit(self):
        if self.name == 'math':
            print "%s的credit 为%d" % (self.name, 4)
        else:
            print "%s的credit 为%d" % (self.name, 2)

    def _credit(self):
        if self.name == 'math':
            print "%s的credit 为%d" % (self.name, 4)
        else:
            print "%s的credit 为%d" % (self.name, 2)

    def __credit(self):
        if self.name == 'math':
            print "%s的credit 为%d" % (self.name, 4)
        else:
            print "%s的credit 为%d" % (self.name, 2)

import lerarn_under_line

>>> import lerarn_under_line
>>> a=Course('math')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'Course' is not defined

from lerarn_under_line import *

>>> from lerarn_under_line import *
>>> a=Course('math')
>>> a.credit()
math的credit 为4
>>> a._credit()
math的credit 为4
>>> a.__credit()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'Course' object has no attribute '__credit'

from lerarn_under_line import Course

>>> from lerarn_under_line import Course
>>> a=Course('math')
>>> a.__credit()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'Course' object has no attribute '__credit'
>>> a._credit()
math的credit 为4
>>> a.credit()
math的credit 为4

综上，单下划线开头的函数表示是口头实例私有变量，是可访问的，但是也不要随意访问，即所谓防君子不防小人。

二、__func 双下划线开头的函数 --私有变量

2.1、在模块中使用双下划线开头

在实例中，带双下划线的类变量、实例变量、方法不能被直接访问。但有办法间接访问。如1.1中的from module import __func

>>> from lerarn_under_line import *
>>> __call_var()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name '__call_var' is not defined

>>> import lerarn_under_line
>>> lerarn_under_line.__call_var()   # 双下划线不可调用
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'module' object has no attribute '__call_var'

>>> from lerarn_under_line import course
>>> from lerarn_under_line import __call_var
>>> __call_var()
带双下划线course:math
带双下划线_credit:4

2.2、在类中使用双下划线开头

lerarn_under_line.py

class Course(object):
    def __init__(self, name, _teacher, __classroom):
        self.name = name
        self._teacher = _teacher
        self.__classroom = __classroom

    def call_var(self):
        print "name:%s" % self.name
        print "_teacher:%s" % self._teacher
        print "__classroom:%s" % self.__classroom   
>>> import lerarn_under_line
>>> a = Course('math', 'zhangyu', 'juyiting')  # 无法实例化
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'Course' is not defined
>>> from lerarn_under_line import *
>>> a = Course('math', 'zhangyu', 'juyiting')
>>> a.call_var()
name:math
_teacher:zhangyu
__classroom:juyiting

lerarn_under_line.py

class Course(object):
    def __init__(self, name, _teacher, __classroom):
        self.name = name
        self._teacher = _teacher
        self.__classroom = __classroom

    def call_var(self):
        print "name:%s" % self.name
        print "_teacher:%s" % self._teacher
        print "__classroom:%s" % self.__classroom


class SonCourse(Course):
    def __init__(self, name, _teacher, __classroom, time):
        super(Course, self).__init__()
        self.time = time
        self.name = name
        self.__classroom = self.__classroom
        self._teacher = self._teacher
        self.__classroom = self.__classroom

    def call_son_var(self):
        print "time:%s" % self.time
        print "name:%s" % self.name
        print "_teacher:%s" % self._teacher
        print "__classroom:%s" % self.__classroom
>>> import lerarn_under_line
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "lerarn_under_line.py", line 77, in <module>
    b = SonCourse('math', 'zhangyu', 'juyiting', "12：00")
  File "lerarn_under_line.py", line 63, in __init__
    self.__classroom = self.__classroom
AttributeError: 'SonCourse' object has no attribute '_SonCourse__classroom'

>>> from lerarn_under_line import *
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "lerarn_under_line.py", line 77, in <module>
    b = SonCourse('math', 'zhangyu', 'juyiting', "12：00")
  File "lerarn_under_line.py", line 63, in __init__
    self.__classroom = self.__classroom
AttributeError: 'SonCourse' object has no attribute '_SonCourse__classroom'

>>> from lerarn_under_line import Course
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "lerarn_under_line.py", line 77, in <module>
    b = SonCourse('math', 'zhangyu', 'juyiting', "12：00")
  File "lerarn_under_line.py", line 63, in __init__
    self.__classroom = self.__classroom
AttributeError: 'SonCourse' object has no attribute '_SonCourse__classroom'

>>> from lerarn_under_line import sonCourse
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "lerarn_under_line.py", line 77, in <module>
    b = SonCourse('math', 'zhangyu', 'juyiting', "12：00")
  File "lerarn_under_line.py", line 63, in __init__
    self.__classroom = self.__classroom
AttributeError: 'SonCourse' object has no attribute '_SonCourse__classroom'

三、前后都有双下划线 --特殊变量

Python保留了有双前导和双末尾下划线的名称，用于特殊用途。 这样的例子有，init__对象构造函数，或__call — 它使得一个对象可以被调用。这些方法通常被称为神奇方法，最好避免在自己的程序中使用以双下划线开头和结尾的名称，以避免与将来Python语言的变化产生冲突。

常见方法：

方法含义
__str__当将对象转换成字符串时会执行
__init__初始化方法，为对象变量赋值
__new__构造方法，创建一个对象
__call__在对象后面加括号会执行该方法
__getattr__当使用对象.属性时，若属性不存在会调用该方法
__setattr__当使用对象.属性 = 值，会调用该方法
__iter__类内部定义了该方法，对象就变成了可迭代对象
__add__当两个对象使用+号会调用该方法
__enter__和__exit__上下文管理

参考文档

1、https://blog.csdn.net/brucewong0516/article/details/79120841

2、http://t.zoukankan.com/one-tom-p-11749739.html

3、https://www.cnblogs.com/bryant24/p/11429653.html

4、https://blog.csdn.net/m0_58357932/article/details/121062461

5、https://www.likecs.com/show-308380836.html

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