Left join two lists of dictionaries on a single key [duplicate]

Question

Given n lists with m dictionaries as their elements, I would like to produce a new list, with a joined (left join) set of dictionaries. Each dictionary is guaranteed to have a key called index, but could have an arbitrary set of keys beyond that. For example, imagine the following two lists:

l1 = [{"index":1, "b":2}, 
      {"index":2, "b":3},
      {"index":3, "b":"10"},
      {"index":4, "c":"7"}]

l2 = [{"index":1, "c":4},
      {"index":2, "c":5},
      {"index":6, "c":8},
      {"index":7, "c":9}]

I would like to produce a joined list:

l3 = [{"index":1, "b":2, "c":4}, 
      {"index":2, "b":3, "c":5}, 
      {"index":3, "b":10},
      {"index":4, "c":7}]

What is the most efficient way to do this in Python?

Currently I have this piece of code, but it only does an inner join, how can I modify this to give me a left join?

def left_join(left, right, key):
    merged = {}
    for item in left+right:
        if item[key] in merged:
            merged[item[key]].update(item)
        else:
            merged[item[key]] = item
    return [val for (_, val) in merged.items()]

Answer 1

The following snippet simply converts them to dictionaries for faster merge, and reconverts to the merged dictionary into a list to match your expected output.

l1_dict = {item['index']: item for item in l1}
l2_dict = {item['index']: item for item in l2}

for item in l1_dict:
    l1_dict[item].update(l2_dict.get(item, {}))

l3 = list(l1_dict.values())
print(l3)