What is the most efficient way to create nested dictionaries in Python?

Question

I currently have over 10k elements in my dictionary looks like:

cars = [{'model': 'Ford', 'year': 2010},
        {'model': 'BMW', 'year': 2019},
        ...]

And I have a second dictionary:

car_owners = [{'model': 'BMW', 'name': 'Sam', 'age': 34},
              {'model': 'BMW', 'name': 'Taylor', 'age': 34},
              .....]

However, I want to join together the 2 together to be something like:

combined = [{'model': 'BMW',
             'year': 2019,
             'owners: [{'name': 'Sam', 'age': 34}, ...]
            }]

What is the best way to combine them? For the moment I am using a For loop but I feel like there are more efficient ways of dealing with this.

** This is just a fake example of data, the one I have is a lot more complex but this helps give the idea of what I want to achieve

Answer 1

Iterate over the first list, creating a dict with the key-val as model-val, then in the second dict, look for the same key (model) and update the first dict, if it is found:

cars = [{'model': 'Ford', 'year': 2010}, {'model': 'BMW', 'year': 2019}]
car_owners = [{'model': 'BMW', 'name': 'Sam', 'age': 34}, {'model': 'Ford', 'name': 'Taylor', 'age': 34}]


dd = {x['model']:x for x in cars}

for item in car_owners:
    key = item['model']
    if key in dd:
        del item['model']
        dd[key].update({'car_owners': item})
    else:
        dd[key] = item

print(list(dd.values()))

OUTPUT:

[{'model': 'BMW', 'year': 2019, 'car_owners': {'name': 'Sam', 'age': 34}}, {'model': 'Ford', 'year': 2010, 'car_owners': {'name': 'Taylor', 
'age': 34}}] 

Answer 2

Really, what you want performance wise is to have dictionaries with the model as the key. That way, you have O(1) lookup and can quickly get the requested element (instead of looping each time in order to find the car with model x). If you're starting off with lists, I'd first create dictionaries, and then everything is O(1) from there on out.

models_to_cars = {car['model']: car for car in cars}
models_to_owners = {}
for car_owner in car_owners:
    models_to_owners.setdefault(car_owner['model'], []).append(car_owner)


combined = [{
    **car,
    'owners': models_to_owners.get(model, [])
} for model, car in models_to_cars.items()]

Then you'd have

combined = [{'model': 'BMW',
             'year': 2019,
             'owners': [{'name': 'Sam', 'age': 34}, ...]
            }]

as you wanted