How to merge lists of dictionaries

Question

With lists of dictionaries such as the following:

user_course_score = [
    {'course_id': 1456, 'score': 56}, 
    {'course_id': 316, 'score': 71}
]
courses = [
    {'course_id': 1456, 'name': 'History'}, 
    {'course_id': 316, 'name': 'Science'}, 
    {'course_id': 926, 'name': 'Geography'}
]

What is the best way to combine them into the following list of dictionaries:

user_course_information = [
    {'course_id': 1456, 'score': 56, 'name': 'History'}, 
    {'course_id': 316, 'score': 71, 'name': 'Science'}, 
    {'course_id': 926, 'name': 'Geography'} # Note: the student did not take this test
]

Or would it be better to store the data differently, such as:

courses = {
    '1456': 'History',
    '316': 'Science',
    '926': 'Geography'
}

Thanks for your help.

I have accepted Adam's answer as it does exactly what I require. The other answers - suggesting I restructure the data as dictionaries - work fine, but for my project I would rather not use ids as keys. — Chris, Aug 06 '10 at 09:48

Adam · Accepted Answer · 2016-07-05T08:39:25.383

24

Here's a possible solution:

def merge_lists(l1, l2, key):
    merged = {}
    for item in l1+l2:
        if item[key] in merged:
            merged[item[key]].update(item)
        else:
            merged[item[key]] = item
    return merged.values()

courses = merge_lists(user_course_score, courses, 'course_id')

Produces:

[{'course_id': 1456, 'name': 'History', 'score': 56},
 {'course_id': 316, 'name': 'Science', 'score': 71},
 {'course_id': 926, 'name': 'Geography'}]

As you can see, I used a dictionary ('merged') as a halfway point. Of course, you can skip a step by storing your data differently, but it depends also on the other uses you may have for those variables.

All the best.

edited Jul 05 '16 at 08:39

answered Aug 06 '10 at 08:56

Adam

2,110
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1

Hey Adam. I'm using this method to accomplish the same but my dicts and lists are huge. I'm wondering if you'd know a faster way to do this. Thanks. – Mridang Agarwalla Dec 05 '11 at 12:00
2

@MridangAgarwalla did you find any faster way ?? – Thameem Mar 11 '18 at 08:27

score 3 · Answer 2 · answered Aug 06 '10 at 08:00

dictionary is basically a list of (key, value) pairs.

in your case,

user_course_score can just be a dictionary of (course_id, score) rather than a list of dictionaries (you are just complicating it unnecessarily)

similarly, course can just be a dictionary of (course_id, name)

what you have suggested in the end is the right way :)

score 2 · Answer 3 · answered Aug 06 '10 at 08:34

Rahul is correct; a list of dictionaries is not the right way to do this. Think about it like this: dictionaries are mappings between pieces of data. Your final example, courses, is the right way to store the data; you could then do something like this to store the per-user data:

courses = {
    1456: 'History',
    316: 'Science',
    926: 'Geography'
} # Note the lack of quotes

test_scores = {
    1456: { <user-id>: <score on History test> },
    316: { <user-id>: <score on History test> },
    926: { <user-id>: <score on History test> }
}

score 1 · Answer 4 · answered Jun 21 '12 at 04:40

1

You could also try:

[
    course.update(score) for course 
    in courses for score in user_course_score 
    if course['course_id'] == score['course_id']
]

:)

answered Jun 21 '12 at 04:40

David Lemphers

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How to merge lists of dictionaries

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