Sum up values based on two dicts with same keys

I have a Python dictionary like {'user_id': 'user_checkpoint'}

checkpoints_dict= {"id_11": "checkpoint_a", "id_13": "checkpoint_a", "id_15": 
                   "checkpoint_b", "id_22": "checkpoint_c", "id_99": "checkpoint_c", 
                   "id_4": "checkpoint_d", "id_2": None}

>>> checkpoints_dict 
>>> {'id_11': 'checkpoint_a',
     'id_13': 'checkpoint_a',
     'id_15': 'checkpoint_b',
     'id_2': None,
     'id_22': 'checkpoint_c',
     'id_4': 'checkpoint_d',
     'id_99': 'checkpoint_c'}

Note: a user might have no checkpoint like id_2!

I have a second dictionary like {'user_id': time}:

user_time = {"id_11": 40, "id_13": 95, "id_15": 150, "id_22": 12, 
             "id_4": 154, "id_99": 111 , "id_2": 24, "id_3": 43, "id_5": 65, "id_6": 45}

Note: Each user_id from checkpoints_dict can be found within user_time

How do I get the added user_time for each checkpoint within checkpoints_dict? Based on the example I would expect sth. like this:

result_checkpoint_time = {"checkpoint_a": sum([40, 95]),
                          "checkpoint_b": sum([150]),
                          "checkpoint_c": sum([12, 111]),
                          "checkpoint_d": sum([154])}

Since I have no smart idea to approach this I can’t show my solutions to this problem!

>Solution :

You just need to use the user_time dict on the id you get while iterating over the checkpoint dict:

checkpoints_dict = {
    "id_11": "checkpoint_a",
    "id_13": "checkpoint_a",
    "id_15": "checkpoint_b",
    "id_22": "checkpoint_c",
    "id_99": "checkpoint_c",
    "id_4": "checkpoint_d",
    "id_2": None,
}
user_time = {
    "id_11": 40,
    "id_13": 95,
    "id_15": 150,
    "id_22": 12,
    "id_4": 154,
    "id_99": 111,
    "id_2": 24,
    "id_3": 43,
    "id_5": 65,
    "id_6": 45,
}

So:

result = {}
for id_, checkpoint in checkpoints_dict.items():
    if checkpoint is not None:
        result[checkpoint] = result.get(checkpoint, 0) + user_time[id_]