5 Best Ways to Combine Python Dictionaries by Adding Values for Common Keys

💡 Problem Formulation: Combining Python dictionaries by summing the values for keys that appear in both is a common task in data processing. For example, you might have two dictionaries like {'a': 1, 'b': 2} and {'b': 3, 'c': 4} and want to combine them into {'a': 1, 'b': 5, 'c': 4}.

Method 1: Using a For Loop

This method consists of iterating over each item in the first dictionary and either adding it to a new dictionary or updating the value if the key already exists. For values of keys appearing in both dictionaries, it adds the values together. This is a simple and straightforward approach suitable for beginners.

Here’s an example:

d1 = {'a': 1, 'b': 2}
d2 = {'b': 3, 'c': 4}
combined_dict = {}

for key in d1:
    combined_dict[key] = d1[key]

for key in d2:
    if key in combined_dict:
        combined_dict[key] += d2[key]
    else:
        combined_dict[key] = d2[key]

The output of this code snippet:

{'a': 1, 'b': 5, 'c': 4}

In this code snippet, the first loop adds all key-value pairs from d1 into the new dictionary combined_dict. The second loop checks if the key from d2 is already in combined_dict. If it is, it adds the value of that key to the existing value. If not, it creates a new key-value pair.

Method 2: Using the collections Module

The collections.Counter is a subclass of dict that’s specially designed for counting hashable objects. It’s a dictionary where elements are stored as dictionary keys and their counts are stored as dictionary values. This can be used conveniently for our task.

Here’s an example:

from collections import Counter

d1 = {'a': 1, 'b': 2}
d2 = {'b': 3, 'c': 4}
combined_dict = Counter(d1) + Counter(d2)

The output of this code snippet:

Counter({'b': 5, 'c': 4, 'a': 1})

In this snippet, Counter objects are created for both dictionaries. The sum of two Counter objects combines them by adding up the values for the keys that exist in both. The resulting Counter object can be used as a regular Python dictionary.

Method 3: Using Dictionary Comprehension

Dictionary comprehension is a concise way to create dictionaries. This method involves creating a new dictionary by combining keys and summing values from both dictionaries in a single line of code using comprehension.

Here’s an example:

d1 = {'a': 1, 'b': 2}
d2 = {'b': 3, 'c': 4}
combined_dict = {key: d1.get(key, 0) + d2.get(key, 0) for key in set(d1) | set(d2)}

The output of this code snippet:

{'a': 1, 'b': 5, 'c': 4}

This code uses set to get all unique keys from both dictionaries and then iterates through them. For each key, it adds the corresponding values from both dictionaries, using the get method to return a default value of 0 if the key is not found.

Method 4: Using a Function

Writing a function for combining dictionaries can make your code reusable. This function iterates through both dictionaries and merges them, adding values for common keys.

Here’s an example:

def combine_dicts(d1, d2):
    result = d1.copy()
    for key, value in d2.items():
        if key in result:
            result[key] += value
        else:
            result[key] = value
    return result

d1 = {'a': 1, 'b': 2}
d2 = {'b': 3, 'c': 4}
combined_dict = combine_dicts(d1, d2)

The output of this code snippet:

{'a': 1, 'b': 5, 'c': 4}

This custom combine_dicts function takes two dictionaries as arguments. It starts by copying the first dictionary, then iterates through the second dictionary, adding values to existing keys and inserting new keys if necessary.

Bonus One-Liner Method 5: Using defaultdict and update

The defaultdict from the collections module provides a default value for the dictionary if the key has not been set yet. It can be combined with the update method for a neat one-liner solution.

Here’s an example:

from collections import defaultdict

d1 = {'a': 1, 'b': 2}
d2 = {'b': 3, 'c': 4}
combined_dict = defaultdict(int, d1)
for key, value in d2.items():
    combined_dict[key] += value

The output of this code snippet:

defaultdict(<class 'int'>, {'a': 1, 'b': 5, 'c': 4})

This code defines combined_dict as a defaultdict with the contents of d1 and zero as the default value for missing keys. It then adds the values from d2 in one go, without the need to check for the existence of the key.

Summary/Discussion

• Method 1: Using a For Loop. It’s simple and easily understandable, but it’s not the most concise or Pythonic way.
• Method 2: Using the collections Module. It’s elegant and succinct thanks to the Counter class, though it may be less clear to those unfamiliar with the collections module.
• Method 3: Using Dictionary Comprehension. It’s idiomatic Python and very concise, but might be slightly harder to read for Python novices.
• Method 4: Using a Function. This method provides a reusable piece of code that’s clean and easy to understand, but it’s a bit lengthier than other methods.
• Bonus Method 5: Using defaultdict and update. This one-liner approach is very Pythonic and efficient, although defaultdict introduces an extra concept that might not be immediately clear to all readers.