Explaining Python s enter and exit

Question

I saw this in someone s code  What does it mean       def   enter   self           return self      def   exit   self  type  value  tb           self stream close       from   future   import with statement for python2 5   class a object       def   enter   self           print  sss          return  sss111      def   exit   self  type  value  traceback           print  ok          return False  with a   as s      print s   print s

User · Accepted Answer

Using these magic methods    enter      exit    allows you to implement objects which can be used easily with the with statement    The idea is that it makes it easy to build code which needs some  cleandown  code executed  think of it as a try-finally block   Some more explanation here   A useful example could be a database connection object  which then automagically closes the connection once the corresponding  with -statement goes out of scope    class DatabaseConnection object        def   enter   self             make a database connection and return it                     return self dbconn      def   exit   self  exc type  exc val  exc tb             make sure the dbconnection gets closed         self dbconn close                 As explained above  use this object with the with statement  you may need to do from   future   import with statement at the top of the file if you re on Python 2 5    with DatabaseConnection   as mydbconn        do stuff   PEP343 -- The  with  statement  has a nice writeup as well

User · Answer

I found it strangely difficult to locate the python docs for   enter   and   exit   methods by Googling  so to help others here is the link   https   docs python org 2 reference datamodel html with-statement-context-managers https   docs python org 3 reference datamodel html with-statement-context-managers  detail is the same for both versions      object   enter   self    Enter the runtime context related to this object  The with statement will bind this method   s return value to the target s  specified in the as clause of the statement  if any       object   exit   self  exc type  exc value  traceback    Exit the runtime context related to this object  The parameters describe the exception that caused the context to be exited  If the context was exited without an exception  all three arguments will be None       If an exception is supplied  and the method wishes to suppress the exception  i e   prevent it from being propagated   it should return a true value  Otherwise  the exception will be processed normally upon exit from this method       Note that   exit     methods should not reraise the passed-in exception  this is the caller   s responsibility    I was hoping for a clear description of the   exit   method arguments  This is lacking but we can deduce them     Presumably exc type is the class of the exception   It says you should not re-raise the passed-in exception  This suggests to us that one of the arguments might be an actual Exception instance    or maybe you re supposed to instantiate it yourself from the type and value   We can answer by looking at this article  http   effbot org zone python-with-statement htm     For example  the following   exit   method swallows any TypeError  but lets all other exceptions through    def   exit   self  type  value  traceback       return isinstance value  TypeError       so clearly value is an Exception instance   And presumably traceback is a Python traceback object

User · Answer

This is called context manager and I just want to add that similar approaches exist for other programming languages  Comparing them could be helpful in understanding the context manager in python  Basically  a context manager is used when we are dealing with some resources  file  network  database  that need to be initialized and at some point  tear downed  disposed   In Java 7 and above we have automatic resource management that takes the form of     Java code try  Session session   new Session           do stuff     Note that Session needs to implement AutoClosable or one of its  many  sub-interfaces   In C   we have using statements for managing resources that takes the form of     C  code using Session session   new Session            do stuff      In which Session should implement IDisposable    In python  the class that we use should implement   enter   and   exit    So it takes the form of    Python code with Session   as session       do stuff   And as others pointed out  you can always use try finally statement in all the languages to implement the same mechanism  This is just syntactic sugar

User · Answer

Python calls   enter   when execution enters the context of the with statement and it   s time to acquire the resource  When execution leaves the context again  Python calls   exit   to free up the resource Let s consider Context Managers and the    with    Statement in Python  Context Manager is a simple    protocol     or interface  that your object needs to follow so it can be used with the with statement  Basically all you need to do is add enter and exit methods to an object if you want it to function as a context manager  Python will call these two methods at the appropriate times in the resource management cycle  Let   s take a look at what this would look like in practical terms  Here   s how a simple implementation of the open   context manager might look like  class ManagedFile      def   init   self  name           self name   name      def   enter   self           self file   open self name   w           return self file      def   exit   self  exc type  exc val  exc tb           if self file              self file close    Our ManagedFile class follows the context manager protocol and now supports the with statement   gt  gt  gt  with ManagedFile  hello txt   as f         f write  hello  world           f write  bye now   enter code here   Python calls enter when execution enters the context of the with statement and it   s time to acquire the resource  When execution leaves the context again  Python calls exit to free up the resource  Writing a class-based context manager isn   t the only way to support the with statement in Python  The contextlib utility module in the standard library provides a few more abstractions built on top of the basic context manager protocol  This can make your life a little easier if your use cases matches what   s offered by contextlib

User · Answer

If you know what context managers are then you need nothing more to understand   enter   and   exit   magic methods  Lets see a very simple example   In this example I am opening myfile txt with help of open function  The try finally block ensures that even if an unexpected exception occurs myfile txt will be closed   fp open r C  Users SharpEl Desktop myfile txt   try      for line in fp          print line  finally      fp close     Now I am opening same file with with statement   with open r C  Users SharpEl Desktop myfile txt   as fp      for line in fp          print line     If you look at the code  I  didn t close the file  amp  there is no try finally block  Because with statement automatically closes  myfile txt   You can even check it by calling print fp closed  attribute -- which returns True   This is because the file objects  fp in my example  returned by open function has two built-in methods   enter   and   exit    It is  also known as context manager    enter   method is called at the start of with block and   exit    method is called at the end   Note  with statement only works with objects that support the context mamangement protocol i e  they have   enter   and   exit   methods  A class which implement both methods is known as context manager class   Now lets define our own context manager class    class Log      def   init   self filename           self filename filename         self fp None         def logging self text           self fp write text   n       def   enter   self           print    enter             self fp open self filename  a            return self         def   exit   self  exc type  exc val  exc tb           print    exit             self fp close    with Log r C  Users SharpEl Desktop myfile txt   as logfile      print  Main       logfile logging  Test1       logfile logging  Test2     I hope now you have basic understanding of both   enter   and   exit   magic methods

User · Answer

In addition to the above answers to exemplify invocation order  a simple run example   class myclass      def   init   self           print    init          def   enter   self            print    enter          def   exit   self  type  value  traceback           print    exit          def   del   self           print    del      with myclass         print  body     Produces the output      init     enter   body   exit     del     A reminder  when using the syntax with myclass   as mc  variable mc gets the value returned by   enter      in the above case None  For such use  need to define return value  such as    def   enter   self        print    enter         return self

User · Answer

try adding my answers  my thought of learning       enter   and    exit    both are methods that are invoked on entry to and exit from the body of  the with statement   PEP 343  and implementation of both is called context manager   the with statement is intend to hiding flow control of try finally clause and make the code inscrutable   the syntax of the with statement is    with EXPR as VAR      BLOCK   which translate to  as mention in PEP 343     mgr    EXPR  exit   type mgr    exit      Not calling it yet value   type mgr    enter   mgr  exc   True try      try          VAR   value    Only if  as VAR  is present         BLOCK     except            The exceptional case is handled here         exc   False         if not exit mgr   sys exc info                 raise           The exception is swallowed if exit   returns true finally        The normal and non-local-goto cases are handled here     if exc          exit mgr  None  None  None    try some code    gt  gt  gt  import logging  gt  gt  gt  import socket  gt  gt  gt  import sys   server socket on another terminal   python interpreter  gt  gt  gt  s   socket socket socket AF INET  socket SOCK STREAM   gt  gt  gt  s listen 5   gt  gt  gt  s bind  socket gethostname    999    gt  gt  gt  while True   gt  gt  gt      clientsocket  addr    s accept    gt  gt  gt     print  get connection from  r    addr 0    gt  gt  gt     msg   clientsocket recv 1024   gt  gt  gt     print  received  r    msg   gt  gt  gt     clientsocket send b connected    gt  gt  gt     continue   the client side  gt  gt  gt  class MyConnectionManager   gt  gt  gt      def   init   self  sock  addrs    gt  gt  gt          logging basicConfig level logging DEBUG  format    asctime s    gt  gt  gt              levelname s -- gt    message s    gt  gt  gt          logging info  Initiating My connection    gt  gt  gt          self sock   sock  gt  gt  gt          self addrs   addrs  gt  gt  gt      def   enter   self    gt  gt  gt          try   gt  gt  gt              self sock connect addrs   gt  gt  gt              logging info  connection success    gt  gt  gt              return self sock  gt  gt  gt          except   gt  gt  gt              logging warning  Connection refused    gt  gt  gt              raise  gt  gt  gt      def   exit   self  type  value  tb    gt  gt  gt              logging info  CM suppress exception    gt  gt  gt              return False  gt  gt  gt  addrs    socket gethostname     gt  gt  gt  s   socket socket socket AF INET  socket SOCK STREAM   gt  gt  gt  with MyConnectionManager s  addrs  as CM   gt  gt  gt      try   gt  gt  gt          CM send b establishing connection    gt  gt  gt          msg   CM recv 1024   gt  gt  gt          print msg   gt  gt  gt      except   gt  gt  gt          raise  will result  client side    2018-12-18 14 44 05 863           INFO -- gt  Initiating My connection 2018-12-18 14 44 05 863           INFO -- gt  connection success b connected  2018-12-18 14 44 05 864           INFO -- gt  CM suppress exception   result of server side get connection from  127 0 0 1  received b establishing connection    and now try manually  following translate syntax     gt  gt  gt  s   socket socket socket AF INET  socket SOCK STREAM   make new socket object  gt  gt  gt  mgr   MyConnection s  addrs  2018-12-18 14 53 19 331           INFO -- gt  Initiating My connection  gt  gt  gt  ext   mgr   exit    gt  gt  gt  value   mgr   enter     2018-12-18 14 55 55 491           INFO -- gt  connection success  gt  gt  gt  exc   True  gt  gt  gt  try   gt  gt  gt      try   gt  gt  gt          VAR   value  gt  gt  gt          VAR send b establishing connection    gt  gt  gt          msg   VAR recv 1024   gt  gt  gt          print msg   gt  gt  gt      except   gt  gt  gt          exc   False  gt  gt  gt          if not ext  sys exc info      gt  gt  gt              raise  gt  gt  gt  finally   gt  gt  gt      if exc   gt  gt  gt          ext None  None  None   the result  b connected  2018-12-18 15 01 54 208           INFO -- gt  CM suppress exception   the result of the server side same as before  sorry for my bad english and my unclear explanations  thank you

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