itertools builds lazy iterators that combine like pipes. Nothing is computed until something loops over the result.

from itertools import (
    accumulate, batched, chain,
    groupby, islice, pairwise)

print(list(chain([1, 2], [3])))
print(list(accumulate([1, 2, 3])))
print(list(pairwise("abc")))
print(list(batched(range(5), 2)))
print(list(islice("abcdef", 1, 4)))
for key, grp in groupby("aabccc"):
    print(key, len(list(grp)))

• chain joins iterables, and islice slices any iterator, even an endless one such as count().
• accumulate yields running totals, or running results of any two-argument function, such as max.
• pairwise yields neighbouring pairs, and batched yields tuples of n items with a shorter last one.
• groupby groups only consecutive items with equal keys. Sort by the same key first if you want one group per key.
• product, permutations and combinations replace nested loops over choices.

These functions accept any iterable and never need len or indexing, so they also work on generators and files.

Using itertools, write runs(text) returning (char, count) pairs for each run of repeated characters, increases(nums) counting the neighbouring pairs where the second number is larger, peaks(nums) returning the running maximum as a list, and pairings(names) returning every unordered pair as a list of tuples in input order. Each must accept any iterable.

The explanation opens here once every check passes.