将配对列表值转换为字典

Question

I have a list of pairs like the following one:

[['A', 'B'],
 ['C', 'E'],
 ['F', 'D'],
 ['C', 'D'], 
 ['D', 'E'],
 ['G', 'E'],
 ['B', 'A'],
 ['H', 'G'],
 ['A', 'F'],
 ['E', 'A']]

I want to generate a dictionary with the first value in each pair as keys and the second one as value like the following:

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

I used the following code, but didn't reached the desired outcome:

keys = ['A', 'B', 'C', 'D', 'E', 'F', 'G', 'H']

graph = {}
graph = graph.fromkeys(keys, [])

for row in pairs:
(graph[row[0]]).append(row[1])

Answer 1

dict.fromkeys will set all your values to the same list object! Usedict.setdefault instead which will create a new list for each unknown key:

graph = {}

for row in pairs:
    graph.setdefault(row[0], []).append(row[1])

Alternatively, use a collection.defaultdict

from collections import defaultdict

graph = defaultdict(list)        
for row in pairs:
    graph[row[0]].append(row[1])

Or, more elegantly:

for v1, v2 in in pairs:
    graph[v1].append(v2)