How to convert a list to dict with duplicate keys?-CodePudding

I am a freshman in python programming. Now I have a list "lst" like : lst = [(1,'a'), (2,'b'), (2,'c'), (3,'d')] and I want to get a dict "d" from lst. "d" should be {1: ['a'], 2:['b','c'], 3:['d']}.

d = defaultdict(list)

for k,v in lst:
    d[k].append(v)

How can I achieve this using comprehension?

CodePudding user response：

You solution is pretty good. list/dictionary comprehensions are not always the best approach. But If you insist, you can do something like:

from itertools import groupby

lst = [(1, 'a'), (2, 'b'), (2, 'c'), (3, 'd')]

res = {k: [b for a, b in g] for k, g in groupby(lst, key=lambda x: x[0])}
print(res)

output:

{1: ['a'], 2: ['b', 'c'], 3: ['d']}

note: This solution would work if you pass a sorted list, If it's not, make sure you sort the list before using it in the dictionary comprehension (Thanks @Chepner)

sorted_lst = sorted(lst, key=lambda x: x[0])

CodePudding user response：

What you have is already quite efficient and not that long. It could be converted to a one-liner at the expense of legibility with little or no performance benefit:

like this:

dic = next(g  for g in [{}] if not any(g.setdefault(k,[]).append(v) for k,v in lst))

Or this:

dic = {}; dic.update((k,dic.get(k,[]) [v]) for k,v in lst)