[python] Understanding the map function

Python3 - map(func, iterable)

One thing that wasn't mentioned completely (although @BlooB kinda mentioned it) is that map returns a map object NOT a list. This is a big difference when it comes to time performance on initialization and iteration. Consider these two tests.

import time
def test1(iterable):
    a = time.clock()
    map(str, iterable)
    a = time.clock() - a

    b = time.clock()
    [ str(x) for x in iterable ]
    b = time.clock() - b

    print(a,b)


def test2(iterable):
    a = time.clock()
    [ x for x in map(str, iterable)]
    a = time.clock() - a

    b = time.clock()
    [ str(x) for x in iterable ]
    b = time.clock() - b

    print(a,b)


test1(range(2000000))  # Prints ~1.7e-5s   ~8s
test2(range(2000000))  # Prints ~9s        ~8s

As you can see initializing the map function takes almost no time at all. However iterating through the map object takes longer than simply iterating through the iterable. This means that the function passed to map() is not applied to each element until the element is reached in the iteration. If you want a list use list comprehension. If you plan to iterate through in a for loop and will break at some point, then use map.