Write a function that takes a list of dicts and combine them

Question

There is an exercise in the book Python Workout: 50 Ten-minute Exercises that asks to write a function to do the following:

Write a function that takes a list of dicts and returns a single dict that combines all of the keys and values. If a key appears in more than one argument, the value should be a list containing all of the values from the arguments.

Here is my code:

def combine_dicts(dict_list):
    combined = {}
    for dict_ in dict_list:
        for key, value in dict_.items():
            if key not in combined.keys():
                combined[key] = [value]
            else:
                combined[key] = combined[key] + [value]
            
    return dict([((key, value[0]) if len(value) == 1 else (key, value)) \
                 for key, value in combined.items()])

what are some ideas that I can use to improve my code, make it more pythonic?

Example input and outputs:

>>> combine_dicts([{'a': [1, 2]}, {'a':[3]}, {'a': [4, 5]}])
{'a': [[1, 2], [3], [4, 5]]}

>>> combine_dicts([{'a': 1, 'b':2}, {'c':2, 'a':3}])
{'a': [1, 3], 'b': 2, 'c': 2}

Answer 1

At first glance your code looks pretty pythonic to me. I would suggest the following changes:

• when you are adding a new value to an existing key's list, you use list concatenation to create a new list: combined[key] = combined[key] + [value]. What you should do instead is append the new value to the existing list: combined[key].append(value)

• in the return statement you create a list of tuples using list comprehension which is then converted to a dictionary. Instead you can directly use a dictionary comprehension to create the dictionary more elegantly: return {key : (value[0] if len(value) == 1 else value) for key, value in combined.items()}

The resulting code looks as follows:

def combine_dicts(dict_list):
     combined = {}
     for dict_ in dict_list:
         for key, value in dict_.items():
              if key not in combined.keys():
                  combined[key] = [value]
              else:
                  combined[key].append(value)
 
     return {
         key : (value[0] if len(value) == 1 else value)
         for key, value in combined.items()
     }