Question

I would like to reshape the following list :

wide_list = [[1,['a','b','c']],[2,['d','e']],[3,'f']]

in a “long format”:

long_list = [[1,'a'],[1,'b'],[1,'c'],[2,'d'],[2,'e'],[3,'f']]

How can this be achieved efficiently in Python?

Answer 1

Try a nested list comprehension:

>>> wide_list = [[1,['a','b','c']],[2,['d','e']],[3, ['f']]]
>>> long_list = [[k, v] for k, sublist in wide_list for v in sublist]
>>> long_list
[[1, 'a'], [1, 'b'], [1, 'c'], [2, 'd'], [2, 'e'], [3, 'f']]

Note, the last group had to be changed to match the pattern of the first two groups. Instead of [3, 'f'], use [3, ['f']] instead. Otherwise, you’ll need special case logic for groups that don’t follow the pattern.