An input-dependent iterator will terminate based on the length of one or more input values. They are great for working with and modifying existing iterators.

A useful itertool that is an input-dependent iterator is the chain() itertool. chain() takes in one or more iterables and combine them into a single iterator. Here is what the base syntax looks like:

chain(*iterables)

The input value of chain() is one or more iterables of the same or varying iterable types. For example, we could use the chain() itertool to combine a list and a set into one iterator.

To show how it’s used in a scenario, suppose we want to combine a list containing odd numbers and a set containing even numbers:

import itertools

odd = [5, 7, 9]
even = {6, 8, 10}

all_numbers = itertools.chain(odd, even)

for number in all_numbers:
  print(number)

The above example:

• Imports the itertools module.
• Sets all_numbers to the iterator returned by the itertool chain().
• Uses the list iterable odd and the set iterable even as the arguments to chain().
• Implements a for loop using the iterator in all_numbers
• Prints the results, which will be:

  5
  7
  9
  8
  10
  6

The output is finite since the input iterables, odd and even are also finite. Note that Python sets are not ordered so the last 3 numbers in this example’s output will not always be in the initialized order.

Let’s use chain() to work with SKU iterables in our pet store!