Nested list to dict

I think one line solution is a bit confusion. I would write code like below

groups = [['Group1', 'A', 'B'], ['Group2', 'C', 'D']]

result = {}
for group in groups:
    for item in group[1:]:
        result[item] = group[0]
print result

What about:

d = {k:row[0] for row in groups for k in row[1:]}

This gives:

>>> {k:row[0] for row in groups for k in row[1:]}
{'D': 'Group2', 'B': 'Group1', 'C': 'Group2', 'A': 'Group1'}

So you iterate over every row in the groups. The first element of the row is taken as value (row[0]) and you iterate over row[1:] to obtain all the keys k.

Weird as it might seem, this expression also works when you give it an empty row (like groups = [[],['A','B']]). That is because row[1:] will be empty and thus the row[0] part is never evaluated:

>>> groups = [[],['A','B']]
>>> {k:row[0] for row in groups for k in row[1:]}
{'B': 'A'}