List of tuples to dictionary with duplicates keys via list comprehension?

Question

I have a list consisting of tuples with duplicates and I've converted them to a dictionary using this code I found here:

mylist = [(a,1),(a,2),(b,3)]    

result = {}

for i in mylist:  

   result.setdefault(i[0],[]).append(i[1])

print(result)

>>> result = {a:[1,2], b:[3]}

I recall learning that most for loops can be re-written as comprehensions so I wanted to practice but I've failed for the past hour to make one work.

Answer 1

Comprehension means to map the items in a sequence-independent of each other which is not suitable for aggregations. In your question, where the sub-list an item appends to depends on a sub-list with a previous item appends to.

You can produce the desired output with a nested comprehension if you must, but it would turn what would've been solved in O(n) time complexity with a loop into one that takes O(n ^ 2) instead:

{k: [v for s, v in mylist if s == k] for k, _ in mylist}

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