Python compare dictonaries for different values

Question

I have 2 lists of dictonaries and want to return items which have the same id but different title. i.e.

list1 = [{'id': 1, 'title': 'title1'}, {'id': 2, 'title': 'title2'}, {'id': 3, 'title': 'title3'}]

list2 = [{'id': 1, 'title': 'title1'}, {'id': 2, 'title': 'title3'}, {'id': 3, 'title': 'title4'}]

Would return [{'id': 2, 'title': 'title2'}, {'id': 3, 'title': 'title3'}] as the titles are different in list2 to list1.

Answer 1

Different dictionaries are equal if their contents are equal. So you can just do:

for i in list1:
    if i not in list2:
        result.append(i)

Answer 2

I propose that you refactor your design to not be a list of dictionaries, but 2 dictionaries of id: title pairs. The algorithm is trivial at that point and the performance is better.

Code example (edited to reflect SilentGhost's correct assertion):

titles1 = {1: "title1", 2: "title2", 3: "title3"}
titles2 = {1: "title1", 2: "not_title2", 3: "title3"}
for id, title in titles1.iteritems():
    # verify the key is in titles2, compare title to titles2[id]

Code example to convert list of dictionary to dictionary with id as key:

titles1 = dict([(x["id"], x) for x in list1])

Answer 3

[dc for dc in list1 if dc['id'] in [d["id"] for d in list2] and dc not in list2]

Answer 4

If you refactor your data structure (assuming id is unique within one dictionary), a comparison could be implemented more efficiently (namely in O(n). Dictionary lookups are O(1). Example:

#!/usr/bin/env python

d1 = {
    1 : {"title" : "title1"},
    2 : {"title" : "title2"},
    3 : {"title" : "title3"},
}

d2 = {
    1 : {"title" : "title1"},
    2 : {"title" : "title3"},
    3 : {"title" : "title4"},
}

for key, value in d1.items():
    if not value == d2[key]:
        print "@", key, "values differ:", d1[key], "vs", d2[key]

# @ 2 values differ: {'title': 'title2'} vs {'title': 'title3'}
# @ 3 values differ: {'title': 'title3'} vs {'title': 'title4'}

Or shorter:

print [ (k, (d1[k], d2[k])) for k in d1 if not d2[k] == d1[k] ]
# [(2, ({'title': 'title2'}, {'title': 'title3'})), \ 
#    (3, ({'title': 'title3'}, {'title': 'title4'}))]