Python – Map elements to a list of unique indexes

Let’s say I have a list of elements:

my_list = ['CatA', 'CatB', 'CatC', 'CatA', 'CatA', 'CatC']

I want to convert this list to a unique element indexed list.

So CatA is assigned to index 0, CatB is assigned to index

1, and CatC is assigned to index 2.

The result I want is:

result = [0, 1, 2, 0, 0, 2]

Currently, I’m creating the final indexed list by creating a dictionary that assigns a unique id to each element and then using list understanding:

unique_classes = np.unique(my_list)
conversion_dict = dict(unique_classes, range(len(unique_classes))
result = [conversion_dict[i] for i in my_list]

My question is: is there an easier and more straightforward way to do this?

I’m thinking about having a large list of categories, so it needs to be efficient but prevents me from manually creating unique lists, dictionaries, and list understanding.

That’s it:

my_list = ['CatA', 'CatB', 'CatC', 'CatA', 'CatA', 'CatC']
first_occurances = dict()
result = []

for i, v in enumerate(my_list):
    try:
        index = first_occurances[v]
    except KeyError:
        index = i
        first_occurances[v] = i
    result.append(index)

The complexity will be O(n).

Basically, what you do is store the index where the first value appears in dict. If first_occurances does not have the value v, then we save the current index i.