Python: Best way to check if a key value match exists between two dictionaries?

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edit I have 2 list of dictionaries and I am trying to compare the value of a key in the dictionaries list_1 to see if it that value exists in the value of a different key in the dictionaries list_2 as I iterate through list_2.

Lists of Dictionaries

list_1: {'Video ID': 12894, 'Title': 'Title 1'...}, {'Video ID': 128993, 'Title': 'Title 3'...}, {...}
list_2: ('post_id': 12894, 'title': 'Title 2'...,} {'post_id': 128993, 'title': 'Title 5'...,}, {...}

I would like to see if value of key Video ID in the dictionaries of list_1 match the value of the key post_id in the dictionaries of list_2.

I am trying the code below from a post here, but I am getting confused on using the keys to access the correct value.

Code

for s in dictionary_2:
    s = list(s.values())
    for f in dictionary_1:
        f = list(f.values())
        if f['Video Id'] in s and f['post_id'] in s:
            print(s)
key-value compare python-3.x dictionary
1 Answers

The Best way depends on what you're trying to achieve.

If you are looking for the most efficient solution given your context then Solution 1 is probably the way to go since you will probably have a lot of Video ID's and post_id's.

However I'm putting my original solution as Solution 2 as it could be helpful in understanding what's happening. I'm also putting Solution 3 if you want to better understand how to adapt this answer

Here is the example data that I'm using for both solutions:

list_1 = [
   {'Video ID': 12894, 'Title': 'Title 1'}, 
   {'Video ID': 128993, 'Title': 'Title 3'}, 
   {'Video ID': 1233, 'Title': 'Title 4'}
]
list_2 = [
   {'post_id': 12894, 'title': 'Title 2'}, 
   {'post_id': 128993, 'title': 'Title 5'}, 
   {'post_id': 1235, 'title': 'Title 6'}
]

Solution 1 (Creating a lookup table)

Credits to @JonClements

As @JonClements correctly pointed out the best solution (given your context) might be to create a lookup table by converting the post_id's in list_2 to a dictionary with all the post_id's as keys.

  • Create the lookup table by adding each post_id as the key and each dictionary in list_2 as the value.
  • Iterate over each video dictionary in list_1
  • if the Video ID of that current dictionary exists in the lookup table
  • Assign the matching post_id to a match variable that contains the dictionary of the matching post
  • Check if the title of the matched post is the same as the video's title
  • If they are not the same then update the Title
# Create a lookup table
post_lookup = {d['post_id']: d for d in list_2}

# Iterate over video list and print relevant matches
for video in list_1:
    if video['Video ID'] in post_lookup:
        match = post_lookup[video['Video ID']]
        print(f"Same ID {video['Video ID']} and {match}")
        if video['Title'] != match['title']:
            print(f"Outdated Title {video['Title']} and {match}")
            # do something to update...
        else:
            print('Titles match, do nothing...')
            # this else may not be necessary
    else:
        print(f"{video['Video ID']} Not found...")

Solution 2 (using 2 for loops and range() function)

If you simply want to compare the Video ID of list_1 to every single post_id in list_2 (disregarding efficiency) then you could do the following:

  • Use the range() function to iterate over every dictionary in list_1
  • Use the same function to iterate over every dictionary in list_2
  • Check if the current Video ID of the current dictionary in list_1 matches the 'post_id' of the current dictionary in list_2
for video in range(len(list_1)):
    for post in range(len(list_2)):
        if(list_2[post]['post_id'] == list_1[video]['Video ID']):
            print(f"Video ID:{list_1[video]['Video ID']} matches post_id: {list_2[post]['post_id']}")
            if(list_2[post]['title'] != list_1[video]['Title']):
                print('Titles do not match, please update...')
                # do something to update the video/post title
            else:
                print("Title's match, do nothing...")
                # do something else if the titles match

Solution 3 (Adapted from Answer)

  • Iterate over each dictionary in list_2
  • get the post_id for that current dictionary
  • iterate over each dictionary in list_1
  • if the id's match and the title's don't then print the item and do something to update the title
for dictionary_2 in list_2:
    s = dictionary_2['post_id']
    for dictionary_1 in list_1:
        f = dictionary_1['Video ID']
        if f == s and dictionary_2['title'] != dictionary_1['Title']:
            print(s)
            # do something to update titles
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