Modern Python Standard Library Cookbook

Flattening a list of lists

When you have multiple nested lists, you often need to just iterate over all the items contained in the lists without much interest in the depth at which they are actually stored.

Say you have this list:

values = [['a', 'b', 'c'],
          [1, 2, 3],
          ['X', 'Y', 'Z']]

If you just want to grab all the items within it, you really don't want to iterate over the lists within the list and then on the items of each one of them. We just want the leaf items and we don't care at all that they are in a list within a list.