This Python code gives unwanted output when query_words if of size greater than 1

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

Here is the code:

query_words = ['dollar', 'probabilistic']
query_word_to_synonym_dict = {'probabilistic': ['probabilistic'], 'dollar' : ['currency']}

mail_ids = {123, 108}

big_ds = {}
empty_dict = {}
index = {'probabilistic':{(108, 1)}, 'currency':{(123, 1)}}

for mail_id in mail_ids:
    empty_dict = dict.fromkeys(query_words, [])
    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:

{123: {'dollar': ['currency'], 'probabilistic': []}, 108: {'dollar': [], 'probabilistic': ['probabilistic']}}

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

{123: {'dollar': ['currency'], 'probabilistic': ['currency']}, 108: {'dollar': ['probabilistic'], 'probabilistic': ['probabilistic']}}

I asked a similar question a while back: How do I resolve this unexpected output in Python code? and was able to solve the issue for that use case. But that code I wrote fails when query_words has a size>1.

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

Answer

It’s because:

dict.fromkeys(query_words, [])

…the keys in each mail_id sub-dict each share the same list instance.

See:

• “Least Astonishment” and the Mutable Default Argument
• Dictionary creation with fromkeys and mutable objects. A surprise

Try this instead:

query_words = ['dollar', 'probabilistic']
query_word_to_synonym_dict = {'probabilistic': ['probabilistic'], 'dollar' : ['currency']}

mail_ids = {123, 108}

big_ds = {}
index = {'probabilistic':{(108, 1)}, 'currency':{(123, 1)}}

for mail_id in mail_ids:
    big_ds[mail_id] = {word: [] for word in query_words}


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)