Is there a way to create a dictionary with consecutive numbers for values?

Question

I'm working on a classification task and need to turn a list of labels into one hot output for my model. For this, I'm trying to find some neat one-liner to turn a list of potential labels into a dictionary with values that consecutively ascend. So a simple list like: ['a', 'b', 'c'] would turn into a dictionary {'a':0, 'b':1, 'c':2}. This is easier for the one-hot encoding. So far, I've tried using the dictionary's length to do this, but I think in line loops get performed before a function because when I try categories = {i:len(categories) for i in labels}, it yields all values as 0. I can do it using bigger loops and such, but I feel I'm missing some really obvious trick. Thanks in advance.