Merge multiple dictionaries within a list by primary key

Question

Following question:

I have a list of dictionaries like:

l = [{"primary_key": 5164, "attribute1": 2},
     {"primary_key": 5162, "attribute1": 3}, 
     {"primary_key": 5164, "attribute2": 3}]

I want to join them into an output list, but merging the dictionaries that have the same primary-key as:

output_l = [{"primary_key": 5164, "attribute1": 2, "attribute2": 3},
            {"primary_key": 5162, "attribute1": 3}]

After searching a lot I did not find this question somewhere else. Sorry if it is a duplicate. It is very related to this question, but with only one list instead of multiple.

score 2 · Accepted Answer · answered Aug 30 '21 at 14:06

Create a dictionary where you store the ever growing dictionaries by their primary key. The following will work:

dicts = {}
for dct in l:
    dicts.setdefault(dct["primary_key"], {}).update(dct)

l = [*dicts.values()]
# [{'primary_key': 5164, 'attribute1': 2, 'attribute2': 3},
#  {'primary_key': 5162, 'attribute1': 3}]

Note that shared attribute name, the latest entry will "win".

Bhagyesh Dudhediya · Answer 2 · 2021-08-31T10:24:35.990

Here is another approach:

l = [{"primary_key": 5164, "attribute1": 2},
     {"primary_key": 5162, "attribute1": 3}, 
     {"primary_key": 5164, "attribute2": 3},
     {"primary_key": 5162, "attribute2": 3},
     {"primary_key": 5163, "attribute1": 4}] 

# Final list
out_list = []

# Dict to maintain the position where particular primary key was inserted in out_list
visited_keys = {}
index = 0
for item in l:
    # If this key is already in out_list, update it with new key:value
    if item['primary_key'] in visited_keys.keys():
        out_list[visited_keys[item["primary_key"]]].update(item)
    else:
        # Key is not visited, so just append it to the out_list
        out_list.append(item)
        visited_keys[item["primary_key"]] = index
        index += 1
        
print (out_list)

Output:

[{'primary_key': 5164, 'attribute1': 2, 'attribute2': 3}, {'primary_key': 5162, 'attribute1': 3, 'attribute2': 3}, {'primary_key': 5163, 'attribute1': 4}]

Merge multiple dictionaries within a list by primary key

2 Answers2