How To Group Sublist's With Matching Values

I have a list with sublist's in it and want to group them based on a common value. For example: my_list = [['John', 1], ['Mark', 3], ['Peter', 5], ['Mark', 6], ['Mark', 33], ['John

Solution 1:

Use itertools.groupby.

Since you want to only group by the first element of each sublist, you must supply a custom key.

import itertools

[list(g) for _, g in itertools.groupby(sorted(my_list), lambda x: x[0])]

Output:

[[['John', 1], ['John', 2]],
 [['Mark', 3], ['Mark', 6], ['Mark', 33]],
 [['Peter', 5]]]

Solution 2:

Nice question. I prefer the answer by @user3483203 using itertools.groupby but here is a solution with no imports;

result = []
name_to_index = {}

for l in my_list:
    if l[0] in name_to_index:
        result[name_to_index.get(l[0])].append(l)
    else:
        result.append([l])
        name_to_index[l[0]] = len(result) - 1

Output:

[[['John', 1], ['John', 2]], [['Mark', 3], ['Mark', 6], ['Mark', 33]], [['Peter', 5]]]