Make a dictionary from two lists after applying a function

Question

I have two lists of tuples

keys = [(0,1), (2,1)]
values = [('a','b'), ('c','d')]

I want to make a dictionary dict that will apply a function f1 to each

dict.keys[i] = keys[i][0], keys[i][i]: f1(keys[i][0],keys[i][1])

And for the values of the dictionary, I would like to be tuples

dict.values[i] = (f2(values[i][0]), f2(values[i][1]))

What is the most efficient way of doing that in one pass in a pythonic way?

score 7 · Accepted Answer · edited May 23 '17 at 11:45

7

You can do this using a dictionary comprehension.

out = {f1(keys[i][0], keys[i][1]):(f2(values[i][0]),f2(values[i][1])) for i in range(len(keys))}

If you want to avoid using range you can also use zip to accomplish the same thing:

out = {f1(M[0],M[1]):(f2(N[0]), f2(N[1])) for M,N in zip(keys, values)}

And even briefer

out = {f1(*M):tuple(map(f2, N)) for M,N in zip(keys, values)}

If you're on Python 2.6 or earlier (prior to dictionary comprehensions), you can always use a list comprehension and convert to a dictionary explicitly.

out = dict([(f1(*M), tuple(map(f2, N))) for M,N in zip(keys, values)])

edited May 23 '17 at 11:45

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answered Mar 21 '16 at 23:07

Suever

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Thanks!What is this *M notation at the third solution? – curious Mar 21 '16 at 23:23
Is is possible in a single expression to create another dictionary based on the out one without the need to pass again the elements of out? In one loop? – curious Mar 21 '16 at 23:31
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@curious the * expands the elements in the tuple to be each a separate input to `f1` – Suever Mar 21 '16 at 23:37
@curious it may be possible depending on what you need but it is likely clearer to break it into two separate statements – Suever Mar 21 '16 at 23:38
Yes but would it be much faster than 2 loops of a huge list? – curious Mar 21 '16 at 23:39
@curious Maybe, but it depends what you are doing inside of the loop. – Suever Mar 22 '16 at 01:36

Make a dictionary from two lists after applying a function

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