I have a list of objects (strings in this example) which I want to categorize according to a certain characteristic returned by a function.

For example, consider the following list:

['sky', 'ocean', 'grass', 'tomato', 'leaf']

and a function color(item) which returns the color of the string passed to it, e.g. color('sky') returns 'blue'. I now want to transform the list into a dictionary or a list of lists which groups the items according to their color/the value returned by the function. A possible result would look like this:

{
    'blue': ['sky', 'ocean'],
    'green': ['grass', 'leaf'],
    'red': ['tomato']
}

I do not care for the key itself, only that the items are grouped accordingly, so nested lists would be fine too. Just trying to do this in a pythonic way :)

I think I'd go about this question this way:

from collections import defaultdict

D = defaultdict(list)

a = ['sky', 'ocean', 'grass', 'tomato', 'leaf']

for item in a:
  D[color(item)].append(item)

That gives you a dictionary of lists, keyed by colors, that contains items for that category.

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