What's the fastest way to split it into a string-type key into tuple and append to last item in the tuple?

Given a dictionary of string key and integer values, what's the fastest way to

1. split it into a string-type key into tuple
2. then append a special substring </w> to last item in the tuple

Given:

counter = {'The': 6149,
     'Project': 205,
     'Gutenberg': 78,
     'EBook': 5,
     'of': 39169,
     'Adventures': 2,
     'Sherlock': 95,
     'Holmes': 198,
     'by': 6384,
     'Sir': 30,
     'Arthur': 18,
     'Conan': 3,
     'Doyle': 2,}

The goal is to achieve:

counter = {('T', 'h', 'e</w>'): 6149,
 ('P', 'r', 'o', 'j', 'e', 'c', 't</w>'): 205,
 ('G', 'u', 't', 'e', 'n', 'b', 'e', 'r', 'g</w>'): 78,
 ('E', 'B', 'o', 'o', 'k</w>'): 5,
 ('o', 'f</w>'): 39169,
 ('A', 'd', 'v', 'e', 'n', 't', 'u', 'r', 'e', 's</w>'): 2,
 ('S', 'h', 'e', 'r', 'l', 'o', 'c', 'k</w>'): 95,
 ('H', 'o', 'l', 'm', 'e', 's</w>'): 198,
 ('b', 'y</w>'): 6384,
 ('S', 'i', 'r</w>'): 30,
 ('A', 'r', 't', 'h', 'u', 'r</w>'): 18,
 ('C', 'o', 'n', 'a', 'n</w>'): 3,
 ('D', 'o', 'y', 'l', 'e</w>'): 2,}

One way to do it is to

• iterate through the counter and
• converting all but the last character to tuple
• add to the tuple and create an outer tuple
• and assign the tuple key to the count

I've tried

{(tuple(k[:-1])+(k[-1]+'</w>',) ,v) for k,v in counter.items()}

In more verbose form:

new_counter = {}
for k, v in counter.items():
    left = tuple(k[:-1])
    right = tuple(k[-1]+'w',)
    new_k = (left + right,)
    new_counter[new_k] = v

Is there a better way to do this?

Regarding the adding tuple and casting it to an outer tuple. Why is this allowed? Isn't tuple supposed to be immutable?

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