How to separate a list into dictionary based on first and second values

Question

I wasn't sure exactly how to word my question, so I'll go into more depth here.

What I'm trying to do is perform the Graph Coloring problem in Python using input of a list such as this:

[('A','B'),('A','C'),('A','D'),('B','C'),('C','D')]

This is meant to denote the "neighbors" of each edge of the graph, such that A is the neighbor of B C & D, B is the neighbor of C, and C is the neighbor of D

Now, what I'm trying to do is break these into keys in a dictionary like this:

neighbors = {}
neighbors['A'] = ['B', 'C', 'D']
neighbors['B'] = ['A', 'C']
neighbors['C'] = ['A', 'B', 'D']
neighbors['D'] = ['A', 'C']

The issue I'm having is breaking down the initial input into this multi value per key dictionary. So far I have this:

neighbours = {}
myList = [('A','B'),('A','C'),('A','D'),('B','C'),('C','D')]
for i in myList:
    neighbours[i[0]] = (i[1])

print(neighbours)

This provides the output:

{'A': 'D', 'C': 'D', 'B': 'C'}

But I'd like to have it look like this:

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

Any help is greatly appreciated! Thanks :)

Answer 1

The straight forward, EAFP approach:

adj = [('A','B'),('A','C'),('A','D'),('B','C'),('C','D')]
mat = {}
for (x, y) in adj:
    try:
        mat[x].append(y)
    except KeyError:
        mat[x] = [y]
    try:
        mat[y].append(x)
    except KeyError:
        mat[y] = [x]

>>> mat
{'A': ['B', 'C', 'D'], 'C': ['A', 'B', 'D'], 'B': ['A', 'C'], 'D': ['A', 'C']}

Or, if you prefer, the default dict:

from collections import defaultdict
default = defaultdict(list)
for (x, y) in adj:
    default[x].append(y)
    default[y].append(x)

>>> default
defaultdict(<type 'list'>, {'A': ['B', 'C', 'D'], 'C': ['A', 'B', 'D'], 'B': ['A', 'C'], 'D': ['A', 'C']})

Which is 10-20% faster, if you're interested in performance. (see this repl.it comparison)