closures_overview.py

#!/usr/bin/python -Wall


################################################################################
#
# Lambda Expressions, and closures
#
################################################################################


# example of a simple anonymous function in python


add2 = lambda x: x + 2
print(add2(2))

#
# another way of generating functions with a function
# notice this will create a closure and bind the lambda around a which is
# scoped and bound in the outer function before the lambda is constructed.
#


def create_adder(a):
    return lambda b: a + b


add4 = create_adder(4)
add3 = create_adder(3)

print(add4(4))
print(add3(4))

# another way of creating closures (without lambdas)


def create_adder_old_way(a):
    def adder(b):
        return a + b
    return adder


add5 = create_adder_old_way(5)

print(add5(6))

# example of function that creates functions


def create_greeter():
    """
    Example of creating a function that creates function with
    inner(scoped) function definitions
    """
    def greeter(x):
        print(x)

    return greeter

a = create_greeter()
b = create_greeter()

a('a is called')
b('b is called')

# Gotcha about closures - Python binds variables in closures by "name" not by "value"
# which causes the following code not to behave as intended
adders = []
for x in range(5):
    adders.append(lambda y: x + y)

alist = [adder(10) for adder in adders]
blist = [adder(20) for adder in adders]

# simple fix: use actual inner functions instead of anonymous functions


def make_adder(x):
    def add(y):
        return x + y
    return add


badders = []
for x in range(5):
    badders.append(make_adder(x))

alist = [adder(10) for adder in badders]
blist = [adder(20) for adder in badders]