functools.md

Map-Filter-Reduce

Python offers many functions that can be used to implement patterns from functional programming.

Functional Programming is a programming philosophy where a program is defined by functions. These functions are stateless (i.e. there are no global variables). This paradigm helps designing distributed and concurrent algorithms.

Although one would rarely implement these in pure Python, it is a good exercise. Functional code often comes out very clean and is easy to debug and maintain.

A side effect is that it helps you to avoid class-based implementations, which often are more wordy.

Map

The map() function applies a function to all elements of an iterable. It is returning a generator. map() is an alternative to a comprehension.

:::python
def square(x):
    return x ** 2

squares = map(square, numbers)
print(squares)
print(list(squares))

---

Filter

The filter() function returns all elements of an iterable for which a function returns True. It is the functional equivalent of the if clause in a comprehension.

:::python
def odd(x):
    return x % 2 != 0

odd_numbers = filter(odd, range(10))
print(odd_numbers)
print(list(odd_numbers))

---

Reduce

The reduce function aggregates data by recursively calling the same function. For instance, you could use it to add numbers and concatenate lists:

:::python
from functools import reduce

numbers = [1, 2, 3, 4, 5, 6, 7, 8]


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

print(reduce(add, numbers))
print(reduce(add, [[1, 2, 3], [4, 5, 6]]))

reduce allows for quite sophisticated patterns:

:::python
from functools import reduce

words = [
    (455, 'sea'),
    (336, 'boat'),
    (281, 'white'),
    (1226, 'whale'),
    (329, 'captain'),
    (510, 'Ahab')
]


def newlines(a, b):
    return a + '\n' + b


def wordformat(x):
    return "{:>10s}:{:5d}".format(x[1], x[0])


print(reduce(newlines, map(wordformat, words)))

---

Partial

The functools module contains a couple of ways to manipulate functions. One of them is partial that fills in a part of the parameters, resulting in a new function:

:::python
from functools import partial

add3 = partial(add, 3)   # add3 is a function
print(add3(5))           # results in 8

add5 = partial(add, 5)   # can be done more than once
print(add5(8))           # results in 13