How do I resolve this unexpected output in Python code?

Question

I've written some code, but it does not output what I expected.

Here is the code:

query_words = ['probabilistic']
query_word_to_synonym_dict = {'probabilistic': ['probabilistic', 'algorithms']}

mail_ids = {83, 108}

big_ds = {}
empty_dict = {}
index = {'probabilistic':{(108, 1)}, 'algorithms':{(83, 1)}}

for query_word in query_words:
    empty_dict.update({query_word:[]})

for mail_id in mail_ids:
    big_ds.update({mail_id:empty_dict})


for query_word in query_words:
    syns = query_word_to_synonym_dict[query_word]

    for syn in syns:
        
        index_of_word = index[syn]
                
        tuple_first = []
        for tuples in index_of_word:
            tuple_first.append(tuples[0])
                
        for number in tuple_first:            
            (big_ds[number][query_word]).append(syn)

print(big_ds)

The expected final value of big_ds is:

{83: {'probabilistic': ['algorithms']}, 108: {'probabilistic': ['probabilistic']}}

But the code sets the value of big_ds to the following:

{83: {'probabilistic': ['probabilistic', 'algorithms']}, 108: {'probabilistic': ['probabilistic', 'algorithms']}}

I tried debugging, and I know that the error has something to do with this line:

(big_ds[number][query_word]).append(syn)

But I can't seem to figure out how to fix things. Any solution?