with python compound statement in depth [ADVANCE]:

with is ported into python in version 2.5.

with is used to wrap the execution of a code block using methods defined by a context manager.

The process of with statement:
- The expression given in front of with statement is evaluated to obtain a context manager.
- The context manager’s __exit__() is loaded (it is not executed, it is just loaded for later use).
- The context manager’s __enter__() method is invoked.
- If a target was included in the with statement, the return value from __enter__() is assigned to it (like opening a file)
- The suite is executed (your code block)
- The context manager’s __exit__() method is invoked.

Now consider the example below:

with VAR = EXPR:
     BLOCK

Would roughly translates to:

VAR = EXPR
 VAR.__enter__()
 try:
     BLOCK
 finally:
     VAR.__exit__()

The moral of the story is that when you open a file it will close the file automatically at the end:

@contextmanager
 def opening(filename):
     f = open(filename)
     try:
         yield f
     finally:
         f.close()

At the finally stage, the file will always be closed, and you don't need to manage it.

Another example:

f = open(filename)
 with f:
     BLOCK1
 with f:
     BLOCK2

NOTE: The above code does not do what one might think (f is closed before BLOCK2 is entered!).

List of types which can be as a context manager (can be used in `with`):

- file
 - thread.LockType
 - threading.Lock
 - threading.RLock
 - threading.Condition
 - threading.Semaphore
 - threading.BoundedSemaphore

What is a context manager: it is an object that consists of __enter__() and __exit__() methods.

What is a context expression: the expression immediately following the with keyword in the statement is a context expression.

There is more about with to talk about, but I think it is more advanced and not needed for the time being :)

#python #with #context_manager #with_statement #advance