The following is an example of python closure.

def outer_function(outter_arg):
    closure_var = outter_arg
    def inner_function(inner_arg):
        print("{} {}".format(closure_var, inner_arg))

    return inner_function

# Usage of a python closure
closure_func = outer_function("X")
closure_func("Y1") # print "Y1 X"
closure_funct("Y2") # output "Y2 X"

Variables and nested function

Python has two types of variables: local variable and global variable. Variables defined within a function has a local scope, while variables defined outside a function has global scope.

When a function is defined within another function, the function is called nested function. The nested function, however, can access the outer function’s variables. In the example above, outer function defined a variable called closure_var, and this variable would be initiated by the input variable. The inner function is able to access this variable and use this variable in its own function definition.

There are two steps of using a closure function: initiate the closure function and assign to a local variable, then call the variable with parameter to invoke the inner function.

When to use closure?

  • Reduce the use of global variables

Closure could hide variables inside the function, in this way we reduce the use of global variables.

  • Simplify single function classes.
    For example, the above example could be converted into a single function class:
class OuterClass:
    def __init__(outer_arg):
        self.arg = outer_arg

    def inner_function(self, inner_arg):
        print("{} {}".format(self.arg, inner_arg))

In general, closure runs faster than instance function calls.