Insert element in Python list after every nth element

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I’ve got two one liners.

Given:

>>> letters = ['a','b','c','d','e','f','g','h','i','j']

  1. Use enumerate to get index, add 'x' every 3rd letter, eg: mod(n, 3) == 2, then concatenate into string and list() it.

    >>> list(''.join(l + 'x' * (n % 3 == 2) for n, l in enumerate(letters)))

['a', 'b', 'c', 'x', 'd', 'e', 'f', 'x', 'g', 'h', 'i', 'x', 'j']

    But as @sancho.s points out this doesn’t work if any of the elements have more than one letter.

  2. Use nested comprehensions to flatten a list of lists(a), sliced in groups of 3 with 'x' added if less than 3 from end of list.

    >>> [x for y in (letters[i:i+3] + ['x'] * (i < len(letters) - 2) for
     i in xrange(0, len(letters), 3)) for x in y]

['a', 'b', 'c', 'x', 'd', 'e', 'f', 'x', 'g', 'h', 'i', 'x', 'j']

(a) [item for subgroup in groups for item in subgroup] flattens a jagged list of lists.

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