error 'dictionary changed size during iteration'-CodePudding

I didn't understand when I remove list() from list_of_dictionaries , python gives this error 'dictionary changed size during iteration'

dissimilar_frag_dict=frag_dict
list_of_dictionary_elements=list(frag_dict.items())   #this is the problem
for s1_key,s1_value in list_of_dictionary_elements :
    for s2_key,s2_value in list_of_dictionary_elements :      
        similarity=get_similarity(s1_value,s2_value)    
        if similarity>=threshold and s1_key!=s2_key:
            max_lexicographic_order=max(s1_key,s2_key) 
            dissimilar_frag_dict.pop(max_lexicographic_order,'dne')    
print(f'number 0f dissimilar genes:{len(dissimilar_frag_dict)}')
return dissimilar_frag_dict

CodePudding user response：

As @john-gordon has said. This line

dissimilar_frag_dict=frag_dict

actually means gives the reference of frag_dict to dissimilar_frag_dict. They both point to the same content. list(frag_dict.items()) gives a list of item dicts that is iterated. In midst of iteration you run

dissimilar_frag_dict.pop(max_lexicographic_order,'dne')

that is exactly the same as

frag_dict.pop(max_lexicographic_order,'dne')

what comes from fact that dissimilar_frag_dict references frag_dict. Thus the contents of dictionary actually changes in midst of iteration. It is not allowed, because the content taken out from the dict could be the current iterated value and the next value would be invalid in these conditions, triggering the referred error to avoid potential issues.

What I suggest you do is replace

dissimilar_frag_dict=frag_dict

dissimilar_frag_dict={**frag_dict}

that creates a new dictionary with the copy of key/value pairs of the old one.