Adding Multiple Values to Single Key in Python Dictionary Comprehension

Question

I wanted to learn how to use dictionary comprehension and decided to use one for the previously solved task. I need to assign multiple values to the same key. I was wondering if there's a better way to achieve what I'm trying to do than with the code I've written so far.

graph = {(x1,y1): [(c,d) for a,b,c,d in data if a == x1 and b == y1] for x1 ,y1, x2, y2 in data}

For example I have this:

data = {(1,2,1,5),(1,2,7,2),(1,5,4,7),(4,7,7,5)}

The first two values should create a key and the remaining two should be added as a value of a key. With the given example I would like to return:

{(1, 2): [(1, 5), (7, 2)], (1, 5): [(4, 7)], (4, 7): [(7, 5)]}

Is there an easier way to do it than iterate through the entire data just to find the matching values?

Answer 1

Using this dict comprehension isn’t an efficient way here. It loops over the same input data repeatedly.

It's more Pythonic to just use a simple for loop, iterating the data only once:

from collections import defaultdict

data = {(1,2,1,5),(1,2,7,2),(1,5,4,7),(4,7,7,5)}
output = defaultdict(list)

for a, b, c, d in data:
    output[a, b].append((c, d))