Proper way of Initializing dictionary with list?

I am trying to construct a dictionary whose values are lists in the example code below why all the values are not coming for US & UK

cities = {'San Francisco': 'US', 'London': 'UK',
          'Manchester': 'UK', 'Paris': 'France',
          'Los Angeles': 'US', 'Seoul': 'Korea'}

a = {}

for item, key in cities.items():
    a[key] = [item]

print(a)

Current Output

{'US': ['Los Angeles'], 'UK': ['Manchester'], 'France': ['Paris'], 'Korea': ['Seoul']}

Expected Output

{'US': ['San Francisco', 'Los Angeles'], 'UK': ['London', 'Manchester'], 'France': ['Paris'], 'Korea': ['Seoul']}

>Solution :

You might use .setdefault method of dict for this task as follows

cities = {'San Francisco': 'US', 'London': 'UK',
          'Manchester': 'UK', 'Paris': 'France',
          'Los Angeles': 'US', 'Seoul': 'Korea'}

a = {}

for item, key in cities.items():
    a.setdefault(key,[]).append(item)

print(a)

output

{'US': ['San Francisco', 'Los Angeles'], 'UK': ['London', 'Manchester'], 'France': ['Paris'], 'Korea': ['Seoul']}

Explanation: If a does not already have key then put [] (empty list). Add item at end of list which is under key.