Python's with

Consider:

    Set up
    try:
        Do something
    finally:
        Tear down

“Set up” could be opening a file and “Tear down” is closing the file. The try-finally construct guarantees that the “Tear down” part is always executed, even if the code that does the work doesn't finish.

If you do this a lot, it would be quite convenient if you could put the “Set up” and “Tear down” code in a library function, to make it easy to reuse. You can of course do something like

    def controlled_execution(callback):
        set things up
        try:
            callback(thing)
        finally:
            tear things down

    def my_function(thing):
        do something

    controlled_execution(my_function)

But that’s a bit verbose, especially if you need to modify local variables. Another approach is to use a one-shot generator, and use the for-in statement to “wrap” the code:

    def controlled_execution():
        set things up
        try:
            yield thing
        finally:
            tear things down

    for thing in controlled_execution():
        do something with thing

But yield isn't even allowed inside a try-finally in 2.4 and earlier. And while that could be fixed (and it has been fixed in 2.5), it’s still a bit weird to use a loop construct when you know that you only want to execute something once.

So after contemplating a number of alternatives,  the python-dev team finally came up with a generalization of the latter, using an object instead of a generator to control the behavior of an external piece of code:

    class controlled_execution:
        def __enter__(self):
            set things up
            return thing
        def __exit__(self, type, value, traceback):
            tear things down

    with controlled_execution() as thing:
         some code

Now, when the “with” statement is executed, Python evaluates the expression, calls the __enter__ method on the resulting value (which is called a “context guard”), and assigns whatever __enter__ returns to the variable given by as. Python will then execute the code body, and no matter what happens in that code, call the guard object’s __exit__ method.

As an extra bonus, the __exit__ method can look at the exception, if any, and suppress it or act on it as necessary. To suppress the exception, just return a true value. 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)

In Python 2.5, the file object has been equipped with __enter__ and__exit__ methods; the former simply returns the file object itself, and the latter closes the file:

    >>> f = open("x.txt")
    >>> f
    'x.txt'

, mode 'r' at 0x00AE82F0> >>> f.__enter__() 'x.txt', mode 'r' at 0x00AE82F0> >>> f.read(1) 'X' >>> f.__exit__(None, None, None) >>> f.read(1) Traceback (most recent call last): File "", line 1, in ValueError: I/O operation on closed file

so to open a file, process its contents, and make sure to close it, you can simply do:

with open("x.txt") as f:
    data = f.read()
    do something with data