Combine values of same keys in a list of dicts

Question

I have a list of dictionaries in the following format:

foo = [
    {'a': 'x', 'b': 'y', 'c': 'z'},
    {'a': 'j', 'c': 'z'}
]

I want to group this list of dictionaries into a single dictionary, like:

bar = {
    'a': ['x', 'j'],
    'b': ['y', None],
    'c': ['z', 'z']
}

What I've currently done is, looping through all the dicts in foo and create a list of keys and then looping again over the same to create bar . I wonder whether there is a simpler way to accomplish this. Can anyone help?

Answer 1

bar = {
    k: [d.get(k) for d in foo]
    for k in set().union(*foo)
}

Things to google:

• python list comprehension
• python dict comprehension
• python star
• python dict get
• python set union

Answer 2

I am just going to complement Alex Hall solution here, so that it does not return a lot of "None" values:

def merge_dictionary_list(dict_list):
  return {
    k: [d.get(k) for d in dict_list if k in d] # explanation A
    for k in set().union(*dict_list) # explanation B
  }

Explanation:

• The whole thing inside {} is a dictionary comprehension
• Explanation A : Go through all elements in dictionary list and get values for current key k if the current dictionary ( d ) being evaluated actually has that key.

OBS: Without the if k in d expression there could be a bunch of None values appended to the arrays in case the list of dictionaries contains different types of keys.

• Explanation B : gets all keys from list of dictionary and unite them distinctly by using set().union . After all we can only have distinct elements in set data structure.

If you want to do it the traditional way, just go with:

def merge_list_of_dictionaries(dict_list):
  new_dict = {}
  for d in dict_list:
    for d_key in d:
      if d_key not in new_dict:
        new_dict[d_key] = []
      new_dict[d_key].append(d[d_key])
  return new_dict

I think the first solution looks more elegant, but the second one is more legible/readable.

Kind Regards :)

Answer 3

I would do this in two steps:

1. Collect all keys into a single iterable:

    >>> import operator >>> from functools import reduce >>> all_keys = reduce(operator.or_, (d.keys() for d in foo)) >>> all_keys {'a', 'b', 'c'}

2. Use a dict comprehension to create the desired result:

    >>> bar = {key: [d.get(key) for d in foo] for key in all_keys} >>> bar {'a': ['x', 'j'], 'b': ['y', None], 'c': ['z', 'z']}