Splitting a list of tuples to several lists by the same tuple items

This can be done relatively efficiently with a supporting dict:

def split_by_idx(items, idx=1):
    result = {}
    for item in items:
        key = item[idx]
        if key not in result:
            result[key] = []
        result[key].append(item)
    return result

and the lists can be collected from result with dict.values():

lst = [("hello", "Blue"), ("hi", "Red"), ("hey", "Blue"), ("yo", "Green")]


d = split_by_idx(lst)
print(list(d.values()))
# [[('hello', 'Blue'), ('hey', 'Blue')], [('hi', 'Red')], [('yo', 'Green')]]

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This could be implemented also with dict.setdefault() or a defaultdict which are fundamentally the same except that you do not explicitly have to handle the "key not present" case:

def split_by_idx_sd(items, idx=1):
    result = {}
    for item in items:
        result.setdefault(item[idx], []).append(item)
    return result

import collections


def split_by_idx_dd(items, idx=1):
    result = collections.defaultdict(list)
    for item in items:
        result[item[idx]].append(item)
    return result

---

Timewise, the dict-based solution is the fastest for your input:

%timeit split_by_idx(lst)
# 1000000 loops, best of 3: 776 ns per loop
%timeit split_by_idx_sd(lst)
# 1000000 loops, best of 3: 866 ns per loop
%timeit split_by_idx_dd(lst)
# 1000000 loops, best of 3: 1.16 µs per loop

but you would get different timings depending on the "collision rate" of your input. In general, you should expect split_by_idx() to be the fastest with low collision rate (i.e. most of the entries create a new element of the dict), while split_by_idx_dd() should be fastest for high collision rate (i.e. most of the entries get appended to existing defaultdict key).

You could use a collections.defaultdict to group by colour:

from collections import defaultdict

lst = [("hello", "Blue"), ("hi", "Red"), ("hey", "Blue"), ("yo", "Green")]

colours = defaultdict(list)
for word, colour in lst:
    colours[colour].append((word, colour))

print(colours)
# defaultdict(<class 'list'>, {'Blue': [('hello', 'Blue'), ('hey', 'Blue')], 'Red': [('hi', 'Red')], 'Green': [('yo', 'Green')]})

Or if you prefer using no libraries, dict.setdefault is an option:

colours = {}
for word, colour in lst:
      colours.setdefault(colour, []).append((word, colour))

print(colours)
# {'Blue': [('hello', 'Blue'), ('hey', 'Blue')], 'Red': [('hi', 'Red')], 'Green': [('yo', 'Green')]}

If you just want the colour tuples separated into nested lists of tuples, print the values() as a list:

print(list(colours.values()))
# [[('hello', 'Blue'), ('hey', 'Blue')], [('hi', 'Red')], [('yo', 'Green')]]

Benefit of the above approaches is they automatically initialize empty lists for new keys as you add them, so you don't have to do that yourself.