Pyhon - merging two list of dictionaries, only last dictionary in the second list returned

Question

I tried doing a bit of search in SO to find a solution, but I'm still stumped. I think I'm fundamentally misunderstanding something about loops, lists and dictionaries.

I'm largely self-taught and by no means an expert, so apologies in advance if this is an incredibly stupid question.

I have various lists of dictionaries like the l1 and l2 samples in the snippet below. My desired output is something like

l3 = [{'A':1, 'B':4},{'A':2,'B':5},{'A':3,'B':6}

However, no matter what I try I always seem to be getting only the last key-value pair from the second dictionary, i.e.

[{'A': 1, 'B': 6}, {'A': 2, 'B': 6}, {'A': 3, 'B': 6}]

This is what I've got (comments to explain what I'm thinking the code is/should be doing)

# First list of dictionaries

l1 = [{'A': 1},{'A': 2},{'A': 3}]
print(l1)

# Second list of dictionaries
l2 = [{'B': 4},{'B': 5},{'B': 6}]
print(l2)

# Empty list - to receive dictionaries in l1 and l2
l3 =[]
print(l3)

# Adding dictionaries from l1 into l3
for dict1 in l1:
    l3.append(dict1)
print(l3)

# Opening l3 to loop through each dictionary, using range(len()) to loop through the index positions
for i in range(len(l3)):

    # Opening l2 to loop through each dictionary
    for dict2 in l2:
        l3[i].update(dict2)
print(l3)

# Tried inverting the lists here, looping through l2 and then looping through l3 to append all dictionaries in l2
for dict2 in l2:
    for i in range(len(l3)):
        l3[i].update(dict2)
print(l3)

I also tried using zip() and ended up with a list of tuples of dictionaries which makes me think I'm either using that incorrectly, or it's too complex a tool for what I need here. From what I could understand doing some research, the problem is that I keep overriding the values I have just written, I assume that's why I consistently end up with only the last value added everywhere.

Any help appreciated!

Answer 1

Your code here is incorrect. For each dict in l3, you update every single dict in l2.

for i in range(len(l3)):

    # Opening l2 to loop through each dictionary
    for dict2 in l2:
        l3[i].update(dict2)

Try this:

for i in range(len(l3)):
    l3[i].update(l2[i])

Here is a better solution: The second list dictionary value will overwrite the first one, if they have the same key.

result = list(map(lambda x,y:{**x, **y}, l1, l2))