How does reduce function work?

Question

As far as I understand, the reduce function takes a list l and a function f. Then, it calls the function f on first two elements of th

Answer 1

Beyond the trivial examples, here is one where I find reduce to be actually quite useful:

Imagine an iterable of ordered int values, often with some runs of contiguous values, and that we'd like to "summarize" it as a list of tuples representing ranges. (Note also that this iterable could be a generator of a very long sequence --another reason to use reduce and not some operation on an in-memory collection).

from functools import reduce

def rle(a, b):
    if a and a[-1][1] == b:
        return a[:-1] + [(a[-1][0], b + 1)]
    return a + [(b, b + 1)]

reduce(rle, [0, 1, 2, 5, 8, 9], [])
# [(0, 3), (5, 6), (8, 10)]

Notice the use of a proper initial value ([] here) for reduce.

Corner cases handled as well:

reduce(rle, [], [])
# []

reduce(rle, [0], [])
# [(0, 1)]