python itertools

The `itertools` module is a standard library module in Python that provides a set of tools for working with iterators. Iterators are objects that can be iterated (looped) upon, and the `itertools` module provides functions that allow us to create and manipulate iterators in a variety of ways.

Here are some of the functions provided by the `itertools` module:

1. `count(start=0, step=1)` - This function returns an iterator that produces an infinite sequence of numbers, starting from `start` and incrementing by `step`.

2. `cycle(iterable)` - This function returns an iterator that repeats the elements of the `iterable` indefinitely.

3. `repeat(elem, n=None)` - This function returns an iterator that produces `elem` indefinitely, or up to `n` times if `n` is specified.

4. `chain(*iterables)` - This function returns an iterator that produces the elements of each iterable in sequence.

5. `zip_longest(*iterables, fillvalue=None)` - This function returns an iterator that aggregates the elements from each of the iterables. If the iterables are not of equal length, missing values are filled with the `fillvalue`.

6. `combinations(iterable, r)` - This function returns an iterator that produces all possible combinations of `r` elements from the `iterable`.

7. `permutations(iterable, r=None)` - This function returns an iterator that produces all possible permutations of the elements in the `iterable`, with a length of `r` if specified.

8. `product(*iterables, repeat=1)` - This function returns an iterator that produces the Cartesian product of the input iterables. It is equivalent to nested for-loops.