34. Transforming data with map, filter, reduce

Favor map, filter, and reduce to transform data. Your code will be more concise and better understood, as well as faster.

34.1. Don’t do this

data = [i for i in range(10000000)]

x = []
for val in data:
    x.append(val * 2)

y = []
for val in x:
    if val % 2 == 0:
        y.append(val)

z = 0
for val in y:
    z = z + val

34.2. Do this

from functools import reduce

data = [i for i in range(10000000)]

x = map(lambda val: val * 2, data)
y = filter(lambda val: val % 2 == 0, x)
z = reduce(lambda val1, val2: val1 + val2, y)