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Python Functional Python Functional Workhorses Filter

Posted by Mark Chesney
Mark Chesney
11,747 Points

usefulness of <filter object> and <map object>

Hi. Out of curiosity, is there any usefulness to a map object, or a filter object, without having list() around it? Examples:

a = [1, 2, 3, 4]

def get_square(x):
    return x*x

map1 = map(get_square, a)

def is_even_num(x):
    return x%2==0

filt1 = filter(is_even_num, a)

print(list(map1))
print(list(filt1))

The reason I ask is because when I type the following, only the dunders are returned:

dir(map1)
dir(filt1)

Hopefully this question doesn't take me down a rabbit hole of impractical curiosity. Would list(map(...)) and list(filter(...))` be typed out 90% of the time?

Thanks

1 Answer

Jeff Muday
MOD
Jeff Muday
Treehouse Moderator 28,716 Points
Jeff Muday
Jeff Muday
Treehouse Moderator 28,716 Points

I agree with you, 90% of the time, you're going to be immediately instantiating a list.

Guido van Rossum is not a fan of map(), filter(), and reduce(). He prefers comprehensions. He disliked reduce() so much that it was removed from Python 3. (Though it's retained in functools.reduce)

https://www.artima.com/weblogs/viewpost.jsp?thread=98196

https://www.python-course.eu/python3_lambda.php

My take on it is this-- by definition, an iterable is simply any object that can return an iterator. Iterables can range from complex data structures, to functional "infinite" structures like a cycles-- where a map() and filter() might be performed on sub-slices, to strings, to lists of integers, lists of floats, lists of strings, etc. The generic nature of the map() and filter() does not impose any kind of "opinionated" extra functionality ("opinionated" is a term often associated with forced static/dynamic typing). It is up to the programmer to instantiate their choice from the map() and filter() result.

I think Vincent Driesen's blog post is worth a read (I like how he presents iterators).

https://nvie.com/posts/iterators-vs-generators/

Mark Chesney
Mark Chesney
11,747 Points

Thank you much Jeff! That second link you post is a great one!