Converting list of lists to a dictionary with multiple values for a key

Question

I need to write a function that accepts a list of lists representing friends for each person and need to convert it into a dictionary. so an input of [['A','B'],['A','C'],['A','D'],['B','A'],['C','B'],['C','D'],['D','B'],['E']] should return {A:[B,C,D],B:[A],C:[B,D],D:[B],E:None}

Input:

[['A','B'],['A','C'],['A','D'],['B','A'],['C','B'],['C','D'],['D','B'],['E']]

Expected Output:

{A:[B,C,D],B:[A],C:[B,D],D:[B],E:None}

Currently I am trying the following:

s=[['A','B'],['A','C'],['A','D'],['B','A'],['C','B'],['C','D'],['D','B'],['E']]

output=dict.fromkeys((set([x[0] for x in s])),[ ])

for x in s:
    if len(x)>1:
        output[x[0]].append(x[1])
    else:
        output[x[0]].append(None)

But the output is giving me all values for every key rather than returning only the corresponding values

The output i am getting is:

{
'A': ['B', 'C', 'D', 'A', 'B', 'D', 'B', None],

 'B': ['B', 'C', 'D', 'A', 'B', 'D', 'B', None],

 'C': ['B', 'C', 'D', 'A', 'B', 'D', 'B', None],

 'D': ['B', 'C', 'D', 'A', 'B', 'D', 'B', None],

 'E': ['B', 'C', 'D', 'A', 'B', 'D', 'B', None]
}

Answer 1

You can iterate through the key-value pairs in the list of lists, but unpack the value as a list to accommodate the possible lack of a value:

s = [['A','B'],['A','C'],['A','D'],['B','A'],['C','B'],['C','D'],['D','B'],['E']]
output = {}
for k, *v in s:
    if v:
        output.setdefault(k, []).extend(v)
    else:
        output[k] = None

output becomes:

{'A': ['B', 'C', 'D'], 'B': ['A'], 'C': ['B', 'D'], 'D': ['B'], 'E': None}

Or if you don't mind that keys without a value get an empty list instead of None, you can simply do:

output = {}
for k, *v in s:
    output.setdefault(k, []).extend(v)

output would then become:

{'A': ['B', 'C', 'D'], 'B': ['A'], 'C': ['B', 'D'], 'D': ['B'], 'E': []}