1. Single leading underline

• A single underscore is a python naming convention that indicates that the name is for internal use. It is not usually enforced by the Python interpreter, just as a hint to the programmer.

• If you use wildcards to import all names from a module (from unittest import *), Python will not import names with leading underscores (unless the module defines a _all _listthat overrides this behavior)

• Regular import (eg: from unittest import _fun) is not affected by the naming convention of leading single underscores.

2. Single end underline

• A single end underscore (suffix) is a convention to avoid naming conflicts with Python keywords.

3. Double leading underline

• Double underscore prefixes cause the Python interpreter to override attribute names to avoid naming conflicts in subclasses.

class Test:
   def __init__(self):
       self.foo = 11
       self._bar = 23
       self.__baz = 23
       

>>> t = Test()
>>> dir(t)
['_Test__baz', '__class__', '__delattr__', '__dict__', '__dir__',
'__doc__', '__eq__', '__format__', '__ge__', '__getattribute__',
'__gt__', '__hash__', '__init__', '__le__', '__lt__', '__module__',
'__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__',
'__setattr__', '__sizeof__', '__str__', '__subclasshook__',
'__weakref__', '_bar', 'foo']

Explanation:__baz Rewritten by interpreter as'_Test__baz'. This is done to prevent variables from being overridden in subclasses


# Create another class that extends the Test class and try to override the existing properties added in the constructor
class ExtendedTest(Test):
   def __init__(self):
       super().__init__()
       self.foo = 'overridden'
       self._bar = 'overridden'
       self.__baz = 'overridden'


>>> t2 = ExtendedTest()
>>> t2.foo
'overridden'
>>> t2._bar
'overridden'
>>> t2.__baz
AttributeError: "'ExtendedTest' object has no attribute '__baz'"

>>> dir(t2)
['_ExtendedTest__baz', '_Test__baz', '__class__', '__delattr__',
'__dict__', '__dir__', '__doc__', '__eq__', '__format__', '__ge__',
'__getattribute__', '__gt__', '__hash__', '__init__', '__le__',
'__lt__', '__module__', '__ne__', '__new__', '__reduce__',
'__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__',
'__subclasshook__', '__weakref__', '_bar', 'foo', 'get_vars']

>>> t2._ExtendedTest__baz
'overridden'

Explanation:__baz become_ExtendedTest__baz To prevent accidental modification;

4. Double leading and double ending underline

• python comes with special functions, such as

  __init__Object constructor, or__call__ --- It enables an object to be called.
  

• It is best to avoid using names that begin and end with double underscores ("dunders") in your own programs to avoid conflicts with future changes in the Python language.

5. Single underline

• "_" can be used as a placeholder variable (temporary variable);

• Represents the result of the most recent expression evaluated by the interpreter.

  >>> 20 + 3
  23
  >>> _
  23
  >>> print(_)
  23
   
  >>> list()
  []
  >>> _.append(1)
  >>> _.append(2)
  >>> _.append(3)
  >>> _
  [1, 2, 3]
  

6. Summary

7. Reference links

https://blog.csdn.net/tcx1992/article/details/80105645