一、_func單下劃線開頭 --口頭私有變量 1.1、在模塊中使用單下劃線開頭

在Python中，通過單下劃線_來實現模塊級別的私有化，變量除外。一般約定以單下劃線開頭的函數為模塊私有的，也就是說from moduleName import *將不會引入以單下劃線開頭的函數。模塊中使用單下劃線開頭定義函數、全局變量和類均適用，但可以用：from module import _func形式單獨導入。
實例如下:
lerarn_under_line.py

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


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


def _call_var():
    print "帶下劃線course:%s" % course
    print "帶下劃線_credit:%d" % _credit


def __call_var():
    print "帶雙下劃線course:%s" % course
    print "帶雙下劃線_credit:%d" % _credit

import lerarn_under_line

>>>import lerarn_under_line
>>>lerarn_under_line.call_var>>>lerarn_under_line.call_var()
course:math
_credit:4
>>>lerarn_under_line._call_var()   # 單下劃線可以調用
帶下劃線course:math
帶下劃線_credit:4
>>>>>>lerarn_under_line.__call_var()   # 雙下劃線不可調用
Traceback (most recent call last):
  File "", line 1, inAttributeError: '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 "", line 1, inNameError: name '_credit' is not defined
>>>call_var()
course:math
_credit:4
>>>_call_var()
Traceback (most recent call last):
  File "", line 1, inNameError: name '_call_var' is not defined
>>>__call_var()
Traceback (most recent call last):
  File "", line 1, inNameError: 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()
帶下劃線course:math
帶下劃線_credit:4
>>>from lerarn_under_line import __call_var
>>>__call_var()
帶雙下劃線course:math
帶雙下劃線_credit:4

1.2、在類中使用單下劃線開頭

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 為%d" % (self.name, 4)
        else:
            print "%s的credit 為%d" % (self.name, 2)

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

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

import lerarn_under_line

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

from lerarn_under_line import *

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

綜上，單下劃線開頭的函數表示是口頭實例私有變量，是可訪問的，但是也不要隨意訪問，即所謂防君子不防小人。

二、__func雙下劃線開頭的函數 --私有變量 2.1、在模塊中使用雙下劃線開頭

• 在實例中，帶雙下劃線的類變量、實例變量、方法不能被直接訪問。但有辦法間接訪問。如1.1中的from module import __func

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

>>>import lerarn_under_line
>>>lerarn_under_line.__call_var()   # 雙下劃線不可調用
Traceback (most recent call last):
  File "", line 1, inAttributeError: 'module' object has no attribute '__call_var'

>>>from lerarn_under_line import course
>>>from lerarn_under_line import __call_var
>>>__call_var()
帶雙下劃線course:math
帶雙下劃線_credit:4

2.2、在類中使用雙下劃線開頭

• 在class類的內部，帶雙下劃線的類變量、實例變量、方法具有正常訪問權限。
• 在繼承結構中，帶雙下劃線的基類的類變量和實例變量不能被子類直接訪問。

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')  # 無法實例化
Traceback (most recent call last):
  File "", line 1, inNameError: 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 "", line 1, inFile "lerarn_under_line.py", line 77, inb = 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 "", line 1, inFile "lerarn_under_line.py", line 77, inb = 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 "", line 1, inFile "lerarn_under_line.py", line 77, inb = 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 "", line 1, inFile "lerarn_under_line.py", line 77, inb = 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、前后都有雙下劃線 --特殊變量

Python保留了有雙前導和雙末尾下劃線的名稱，用于特殊用途。 這樣的例子有，init__對象構造函數，或__call — 它使得一個對象可以被調用。這些方法通常被稱為神奇方法，最好避免在自己的程序中使用以雙下劃線開頭和結尾的名稱，以避免與將來Python語言的變化產生沖突。
常見方法：

方法	含義
__str__	當將對象轉換成字符串時會執(zhí)行
__init__	初始化方法，為對象變量賦值
__new__	構造方法，創(chuàng)建一個對象
__call__	在對象后面加括號會執(zhí)行該方法
__getattr__	當使用對象.屬性時，若屬性不存在會調用該方法
__setattr__	當使用對象.屬性 = 值，會調用該方法
__iter__	類內部定義了該方法，對象就變成了可迭代對象
__add__	當兩個對象使用+號會調用該方法
__enter__和__exit__	上下文管理

參考文檔

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#sc=2340

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