[python] Explaining Python's '__enter__' and '__exit__'

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

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:

>>> import logging
>>> import socket
>>> import sys

#server socket on another terminal / python interpreter
>>> s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
>>> s.listen(5)
>>> s.bind((socket.gethostname(), 999))
>>> while True:
>>>    (clientsocket, addr) = s.accept()
>>>    print('get connection from %r' % addr[0])
>>>    msg = clientsocket.recv(1024)
>>>    print('received %r' % msg)
>>>    clientsocket.send(b'connected')
>>>    continue

#the client side
>>> class MyConnectionManager:
>>>     def __init__(self, sock, addrs):
>>>         logging.basicConfig(level=logging.DEBUG, format='%(asctime)s \
>>>         : %(levelname)s --> %(message)s')
>>>         logging.info('Initiating My connection')
>>>         self.sock = sock
>>>         self.addrs = addrs
>>>     def __enter__(self):
>>>         try:
>>>             self.sock.connect(addrs)
>>>             logging.info('connection success')
>>>             return self.sock
>>>         except:
>>>             logging.warning('Connection refused')
>>>             raise
>>>     def __exit__(self, type, value, tb):
>>>             logging.info('CM suppress exception')
>>>             return False
>>> addrs = (socket.gethostname())
>>> s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
>>> with MyConnectionManager(s, addrs) as CM:
>>>     try:
>>>         CM.send(b'establishing connection')
>>>         msg = CM.recv(1024)
>>>         print(msg)
>>>     except:
>>>         raise
#will result (client side) :
2018-12-18 14:44:05,863         : INFO --> Initiating My connection
2018-12-18 14:44:05,863         : INFO --> connection success
b'connected'
2018-12-18 14:44:05,864         : INFO --> 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):

>>> s = socket.socket(socket.AF_INET, socket.SOCK_STREAM) #make new socket object
>>> mgr = MyConnection(s, addrs)
2018-12-18 14:53:19,331         : INFO --> Initiating My connection
>>> ext = mgr.__exit__
>>> value = mgr.__enter__()
2018-12-18 14:55:55,491         : INFO --> connection success
>>> exc = True
>>> try:
>>>     try:
>>>         VAR = value
>>>         VAR.send(b'establishing connection')
>>>         msg = VAR.recv(1024)
>>>         print(msg)
>>>     except:
>>>         exc = False
>>>         if not ext(*sys.exc_info()):
>>>             raise
>>> finally:
>>>     if exc:
>>>         ext(None, None, None)
#the result:
b'connected'
2018-12-18 15:01:54,208         : INFO --> CM suppress exception

the result of the server side same as before

sorry for my bad english and my unclear explanations, thank you....

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.

>>> 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.

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

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.

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 & 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.

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.