Why do some functions have underscores before and after the function name

Question

This  underscoring  seems to occur a lot  and I was wondering if this was a requirement in the Python language  or merely a matter of convention   Also  could someone name and explain which functions tend to have the underscores  and why    init    for instance

User · Answer

This convention is used for special variables or methods (so-called “magic method”) such as __init__ and __len__. These methods provides special syntactic features or do special things.

For example, __file__ indicates the location of Python file, __eq__ is executed when a == b expression is executed.

A user of course can make a custom special method, which is a very rare case, but often might modify some of the built-in special methods (e.g. you should initialize the class with __init__ that will be executed at first when an instance of a class is created).

class A:
    def __init__(self, a):  # use special method '__init__' for initializing
        self.a = a
    def __custom__(self):  # custom special method. you might almost do not use it
        pass

User · Answer

Actually I use   method names when I need to differ between parent and child class names  I ve read some codes that used this way of creating parent-child classes  As an example I can provide this code    class ThreadableMixin     def start worker self          threading Thread target self worker  start       def worker self         try          self  worker       except tornado web HTTPError  e          self set status e status code      except          logging error   worker problem   exc info True          self set status 500      tornado ioloop IOLoop instance   add callback self async callback self results          and the child that have a  worker method   class Handler tornado web RequestHandler  ThreadableMixin      def  worker self         self res   self render string  template html           title      Title            data   self application db query  select     where object id  s   self object id

User · Answer

The other respondents are correct in describing the double leading and trailing underscores as a naming convention for  special  or  magic  methods   While you can call these methods directly   10  20    len     for example   the presence of the underscores is a hint that these methods are intended to be invoked indirectly  len  10  20   for example    Most python operators have an associated  magic  method  for example  a x  is the usual way of invoking a   getitem   x

User · Answer

Names surrounded by double underscores are  special  to Python  They re listed in the Python Language Reference  section 3   Data model

User · Answer

Added an example to understand the use of    in python  Here is the list of All     https   docs python org 3 genindex-all html       Certain classes of identifiers  besides keywords  have special   meanings  Any use of   names  in any other context  that does not   follow explicitly documented use  is subject to breakage without   warning   Access restriction using         Identifiers  -  Contain only  A-z  0-9  and     -  Start with a lowercase letter or    -  Single leading      private -  Double leading       strong private -  Start  amp  End       Language defined Special Name of Object  Method -  Class names start with an uppercase letter  -        class BankAccount object       def   init   self  name  money  password           self name   name              Public         self  money   money           Private   Package Level         self   password   password    Super Private      def earn money self  amount           self  money    amount         print  Salary Received     amount    Updated Balance is     self  money       def withdraw money self  amount           self  money -  amount         print  Money Withdraw     amount    Updated Balance is     self  money       def show balance self           print   Current Balance is     self  money    account   BankAccount  Hitesh   1000   PWD      Object Initalization    Method Call account earn money 100     Show Balance print account show balance     print  PUBLIC ACCESS    account name     Public Access    account  money is accessible because it is only hidden by convention print  PROTECTED ACCESS    account  money     Protected Access    account   password will throw error but account  BankAccount  password will not   because   password is super private print  PRIVATE ACCESS    account  BankAccount  password     Method Call account withdraw money 200     Show Balance print account show balance       account  money is accessible because it is only hidden by convention print account  money     Protected Access

User · Answer

From the Python PEP 8 -- Style Guide for Python Code      Descriptive  Naming Styles      The following special forms using leading or trailing underscores are   recognized  these can generally be combined with any case convention            single leading underscore  weak  internal use  indicator  E g  from M import   does not import objects whose name starts with an underscore    single trailing underscore   used by convention to avoid conflicts with Python keyword  e g       Tkinter Toplevel master  class   ClassName       double leading underscore  when naming a class attribute  invokes name mangling  inside class FooBar    boo becomes  FooBar  boo  see below         double leading and trailing underscore     magic  objects or attributes that live in user-controlled namespaces   E g    init        import   or   file     Never invent such names  only use them as documented       Note that names with double leading and trailing underscores are essentially reserved for Python itself   Never invent such names  only use them as documented

[python] Why do some functions have underscores "__" before and after the function name?

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