Question

I have two nested lists with strings (list_a and list_b), details below:

list_a = [
('shop1', 'stand1', 'shelf1', 'fruit1'),
('shop1', 'stand1', 'shelf2', 'fruit2'),
('shop1', 'stand1', 'shelf3', 'fruit3'),
('shop1', 'stand2', 'shelf1', 'fruit1'),
('shop1', 'stand2', 'shelf2', 'fruit2'),
('shop1', 'stand2', 'shelf3', 'fruit3'),
('shop2', 'stand3', 'shelf1', 'fruit1'),
('shop2', 'stand3', 'shelf2', 'fruit2'),
('shop2', 'stand3', 'shelf3', 'fruit3')
]
list_b = [
('shop1', 'stand1', 'shelf1', 'fruit1'),
('shop1', 'stand1', 'shelf2', 'fruit2'),
('shop1', 'stand1', 'shelf2', 'fruit2'),
('shop1', 'stand1', 'shelf3', 'fruit3'),
('shop1', 'stand1', 'shelf3', 'fruit3'),
('shop1', 'stand1', 'shelf3', 'fruit3'),
('shop1', 'stand2', 'shelf1', 'fruit1'),
('shop1', 'stand2', 'shelf1', 'fruit1'),
('shop1', 'stand2', 'shelf1', 'fruit1'),
('shop1', 'stand2', 'shelf2', 'fruit2'),
('shop1', 'stand2', 'shelf2', 'fruit2'),
('shop1', 'stand2', 'shelf2', 'fruit2'),
('shop1', 'stand2', 'shelf3', 'fruit3'),
('shop2', 'stand3', 'shelf1', 'fruit1'),
('shop2', 'stand3', 'shelf1', 'fruit1'),
('shop2', 'stand3', 'shelf2', 'fruit2'),
('shop2', 'stand3', 'shelf3', 'fruit3'),
('shop2', 'stand3', 'shelf3', 'fruit3'),
('shop2', 'stand3', 'shelf3', 'fruit3')
]

and I would like to find identical rows from list_b in list_a, count “duplicated” rows and merge list_a with one additional column (number of occurrences) as a new list, like this below:

result_list = [
('shop1', 'stand1', 'shelf1', 'fruit1', 1),
('shop1', 'stand1', 'shelf2', 'fruit2', 2),
('shop1', 'stand1', 'shelf3', 'fruit3', 3),
('shop1', 'stand2', 'shelf1', 'fruit1', 3),
('shop1', 'stand2', 'shelf2', 'fruit2', 3),
('shop1', 'stand2', 'shelf3', 'fruit3', 1),
('shop2', 'stand3', 'shelf1', 'fruit1', 2),
('shop2', 'stand3', 'shelf2', 'fruit2', 1),
('shop2', 'stand3', 'shelf3', 'fruit3', 3)
]

Is there any quick and efficient way to do this?

Answer 1

dict_a = {row: 0 for row in list_a}
for row in list_b:
    if row in dict_a:
        dict_a[row] += 1

result = [row + (dict_a[row],) for row in list_a]

On Python 2.6 use dict((row, 0) for row in list_a) instead of the dictionary comprehension.