Why are useful

A closure is a way to define a variable statically into a function. They are good to:

Some examples

Define a function that return another function:

def print_something(msg):
    def _print(var):
        print msg % var
    return _print

Now we can use to instantiate the function and define the string formatting statically and then call the returned function.

>>> p = print_something('Hi %s!')
>>> p('Pietro')
Hi Pietro!

Now a more interesting example for the grass users:

from grass.pygrass.modules import Module


def get_module(cmd):
    mod = Module(cmd)

    def do(**kwargs):
        mod(**kwargs)
    return do

If we test the speed of a GRASS module, using ipython I got:

>>> %timeit Module('g.region', flags='p')
10 loops, best of 3: 30.8 ms per loop

Instead, using the python closure, I got:

>>> greg = get_module('g.region')
>>> %timeit greg(flags='p')
100 loops, best of 3: 12.8 ms per loop

Note: This example doesn't work with multiprocessing.