Re: [Python-ideas] Avoiding nested for try..finally: atexit for functions?

Nick Coghlan ncoghlan-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org writes:

On Wed, Oct 19, 2011 at 1:14 PM, Chris Rebert pyideas-QkDgq5C4a+JWk0Htik3J/w@public.gmane.org wrote:

...

On Tue, Oct 18, 2011 at 7:14 PM, Nikolaus Rath Nikolaus-BTH8mxji4b0@public.gmane.org wrote:

...

Hello,

I often have code of the form:

def my_fun():    allocate_res1()    try:       # do stuff       allocate_res2()       try:           # do stuff           allocate_res3()           try:               # do stuff           finally:               cleanup_res3()       finally:           cleanup_res2()    finally:        cleanup_res1()

return

With increasing number of managed resources, the indentation becomes really annoying, there is lots of line noise, and I don't like the fact that the cleanup is so far away from the allocation.

Use the `with` statement and context managers. They were added for this exact situation. See http://www.python.org/dev/peps/pep-0343/

Resulting code will resemble:

def func():    with alloc() as res1, alloc() as res2, alloc() as res3:        # do stuff

Or, to more closely mirror the original example:

# Define these wherever the current resources are defined
@contextlib.contextmanager
def cm1():
    res1 = allocate_res1()
    try:
        yield res1
    finally:
        cleanup_res1()

@contextlib.contextmanager
def cm2():
    res2 = allocate_res2()
    try:
        yield res2
    finally:
        cleanup_res2()

@contextlib.contextmanager
def cm3():
    res3 = allocate_res3()
    try:
        yield res2
    finally:
        cleanup_res3()

def func():
    with cm1() as res1:
        # do stuff
        with cm2() as res2:
            # do stuff
            with cm3() as res3:
                # do stuff

Indeed, that works. But I do you really consider this code nicer than the original one? I think a simple line count answers the question :-).

Best,

-Nikolaus