The most simple example of using closures is in something called currying. Basically, let's assume we have a function f() which, when called with two arguments a and b, adds them together. So, in Python, we have:
def f(a, b):
return a + b
But let's say, for the sake of argument, that we only want to call f() with one argument at a time. So, instead of f(2, 3), we want f(2)(3). This can be done like so:
def f(a):
def g(b): # Function-within-a-function
return a + b # The value of a is present in the scope of g()
return g # f() returns a one-argument function g()
Now, when we call f(2), we get a new function, g(); this new function carries with it variables from the scope of f(), and so it is said to close over those variables, hence the term closure. When we call g(3), the variable a (which is bound by the definition of f) is accessed by g(), returning 2 + 3 => 5
This is useful in several scenarios. For example, if I had a function which accepted a large number of arguments, but only a few of them were useful to me, I could write a generic function like so:
def many_arguments(a, b, c, d, e, f, g, h, i):
return # SOMETHING
def curry(function, **curry_args):
# call is a closure which closes over the environment of curry.
def call(*call_args):
# Call the function with both the curry args and the call args, returning
# the result.
return function(*call_args, **curry_args)
# Return the closure.
return call
useful_function = curry(many_arguments, a=1, b=2, c=3, d=4, e=5, f=6)
useful_function is now a function which only needs 3 arguments, instead of 9. I avoid having to repeat myself, and also have created a generic solution; if I write another many-argument function, I can use the curry tool again.
