Group tuples inside a list by matching positions of two of its sub-elements

Question

I have a list of tuples as below. The tuple in itself is a nested tuple with 3 sub-elements (tuples) inside it.

[(('a', 'apple'), ('b', 'mango'), ('c', 'grapes')),
 (('a', 'apple'), ('b', 'mango'), ('c', 'grapes')),
 (('e', 'apple'), ('b', 'mango'), ('c', 'grapes')),
 (('a', 'apple'), ('d', 'mango'), ('c', 'peach')),
 (('e', 'apple'), ('d', 'mango'), ('f', 'grapes')),
 (('f', 'grapes'), ('e', 'apple'), ('d', 'mango')),
 (('f', 'peach'), ('e', 'apple'), ('e', 'mango')),
 (('f', 'grapes'), ('c', 'apple'), ('d', 'mango')), 
 (('e', 'apple'), ('f', 'grapes'), ('d', 'mango')),
 (('a', 'apple'), ('c', 'grapes'), ('b', 'mango')),
 ]

I want to group these tuples by matching the positions of two of its elements viz. apple and mango (which is fixed and known beforehand) inside the tuples!

Desired output:

[
# apple and mango at positions 1 and 2.
[(('a', 'apple'), ('b', 'mango'), ('c', 'grapes')),
 (('a', 'apple'), ('b', 'mango'), ('c', 'grapes')),
 (('e', 'apple'), ('b', 'mango'), ('c', 'grapes')),
 (('a', 'apple'), ('d', 'mango'), ('c', 'peach')),
 (('e', 'apple'), ('d', 'mango'), ('f', 'grapes'))],

# apple and mango at positions 2 and 3.
 [(('f', 'grapes'), ('e', 'apple'), ('d', 'mango')),
 (('f', 'peach'), ('e', 'apple'), ('e', 'mango')),
 (('f', 'grapes'), ('c', 'apple'), ('d', 'mango'))], 

# apple and mango at positions 1 and 3.
 [(('e', 'apple'), ('f', 'grapes'), ('d', 'mango')),
 (('a', 'apple'), ('c', 'grapes'), ('b', 'mango'))]
 ]

I tried using Counter and also checked some other examples but couldn't succeed in coming close the desired output. As such, any help or pointers would be really appreciated.

Answer 1

My go-to solution for grouping tasks like this is collections.defaultdict. I've written a lengthy answer about grouping things, which you can read here. Picking out the relevant snippets from that answer gives us this piece of code:

import collections

groupdict = collections.defaultdict(list)
for value in your_list_of_tuples:  # input
    group = ???  # group identifier
    groupdict[group].append(value)

result = list(groupdict.values())  # output

Where all that's left is to find a way to uniquely represent each group with a hashable value (that is, we need to fill in the group = ??? line).

The easiest solution is probably to extract the apple and mango values from the nested tuples and replace all other values with None:

>>> tup = (('a', 'apple'), ('c', 'grapes'), ('b', 'mango'))
>>> tuple((t[1] if t[1] in {'apple','mango'} else None) for t in tup)
('apple', None, 'mango')

Add that in and we're done:

import collections

groupdict = collections.defaultdict(list)
for value in your_list_of_tuples:
    group = tuple((t[1] if t[1] in {'apple','mango'} else None) for t in value)
    groupdict[group].append(value)

result = list(groupdict.values())

# result:
# [[(('a', 'apple'), ('b', 'mango'), ('c', 'grapes')),
#   (('a', 'apple'), ('b', 'mango'), ('c', 'grapes')),
#   (('e', 'apple'), ('b', 'mango'), ('c', 'grapes')),
#   (('a', 'apple'), ('d', 'mango'), ('c', 'peach')),
#   (('e', 'apple'), ('d', 'mango'), ('f', 'grapes'))],
#  [(('f', 'grapes'), ('e', 'apple'), ('d', 'mango')),
#   (('f', 'peach'), ('e', 'apple'), ('e', 'mango')),
#   (('f', 'grapes'), ('c', 'apple'), ('d', 'mango'))],
#  [(('e', 'apple'), ('f', 'grapes'), ('d', 'mango')),
#   (('a', 'apple'),('c', 'grapes'), ('b', 'mango'))]]