How to group similar items in a list?

Question

I am looking to group similar items in a list based on the first three characters in the string. For example:

test = [\'abc_1_2\', \'abc_2_2\', \'hij_1_1\',         


        

Answer 1

The .split("_")[0] part should be inside a single-argument function that you pass as the second argument to itertools.groupby.

>>> import os, itertools
>>> test = ['abc_1_2', 'abc_2_2', 'hij_1_1', 'xyz_1_2', 'xyz_2_2']
>>> [list(g) for _, g in itertools.groupby(test, lambda x: x.split('_')[0])]
[['abc_1_2', 'abc_2_2'], ['hij_1_1'], ['xyz_1_2', 'xyz_2_2']]
>>>

Having it in the for ... part does nothing since the result is immediately discarded.

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Also, it would be slightly more efficient to use str.partition when you only want a single split:

[list(g) for _, g in itertools.groupby(test, lambda x: x.partition('_')[0])]

Demo:

>>> from timeit import timeit
>>> timeit("'hij_1_1'.split('_')")
1.3149855638076913
>>> timeit("'hij_1_1'.partition('_')")
0.7576401470019234
>>>

This isn't a major concern as both methods are pretty fast on small strings, but I figured I'd mention it.