Question

How do I build a dict using list comprehension?

I have two lists.

 series = [1,2,3,4,5]
 categories = ['A', 'B', 'A', 'C','B']

I want to build a dict where the categories are the keys.

Thanks for your answers I’m looking to produce:

 {'A' : [1, 3], 'B' : [2, 5], 'C' : [4]}

Because the keys can’t exist twice

Answer 1

You have to have a list of tuples. The tuples are key/value pairs. You don’t need a comprehension in this case, just zip:

dict(zip(categories, series))

Produces {'A': 3, 'B': 5, 'C': 4} (as pointed out by comments)

Edit: After looking at the keys, note that you can’t have duplicate keys in a dictionary. So without further clarifying what you want, I’m not sure what solution you’re looking for.

Edit: To get what you want, it’s probably easiest to just do a for loop with either setdefault or a defaultdict.

categoriesMap = {}
for k, v in zip(categories, series):
    categoriesMap.setdefault(k, []).append(v)

That should produce {'A': [1, 3], 'B': [2, 5], 'C': [3]}