create a dictionary with incrementing values

Question

I have a list and I want to generate a dictionary d taking out duplicates and excluding a single item, such that the first key has value 0, the second has value 1, and so on.

I have written the following code:

d = {}
i = 0
for l in a_list:
    if (l not in d) and (l != '<'):
        d[l] = i
        i += 1

If a_list = ['a', 'b', '<', 'c', 'b', 'd'], after running the code d contains {'a': 0, 'b': 1, 'c': 2, 'd':3}. Order is not important. Is there a more elegant way to obtain the same result?

Answer 1

Use dict.fromkeys to get your unique occurrences (minus values you don't want), then .update it to apply the sequence, eg:

a_list = ['a', 'b', '<', 'c', 'b', 'd']

d = dict.fromkeys(el for el in a_list if el != '<')
d.update((k, i) for i, k in enumerate(d))

Gives you:

{'a': 0, 'b': 1, 'd': 2, 'c': 3}

If order is important, then use collections.OrderedDict.fromkeys to retain the ordering of the original values, or sort the unique values if they should be alphabetical instead.

Answer 2

{b: a for a, b in enumerate(set(a_list) - {'<'})}

set(a_list) creates a set from a_list. That effectively strips duplicate numbers in a_list, as a set can only contain unique values.