Python: Combine two or more dictionaries, creating a list of values for each key

Last update on April 21 2025 12:59:39 (UTC/GMT +8 hours)

68. Combine Dictionaries Creating List of Values per Key

Write a Python program to combine two or more dictionaries, creating a list of values for each key.

Create a new collections.defaultdict with list as the default value for each key and loop over dicts.
Use dict.append() to map the values of the dictionary to keys.
Use dict() to convert the collections.defaultdict to a regular dictionary.

Sample Solution:

Python Code:

# Import the 'defaultdict' class from the 'collections' module.
from collections import defaultdict

# Define a function 'test' that takes any number of dictionaries as arguments (*dicts).
def test(*dicts):
  # Create a 'defaultdict' object 'result' with a default value of an empty list.
  result = defaultdict(list)

  # Iterate through the dictionaries passed as arguments.
  for el in dicts:
    # Iterate through the keys and values in each dictionary.
    for key in el:
      # Append the value associated with the 'key' to the list for that 'key' in 'result'.
      result[key].append(el[key])

  # Convert the 'defaultdict' 'result' back to a regular dictionary and return it.
  return dict(result)

# Define two dictionaries 'd1' and 'd2' with key-value pairs.
d1 = {'w': 50, 'x': 100, 'y': 'Green', 'z': 400}
d2 = {'x': 300, 'y': 'Red', 'z': 600}

# Print the original dictionaries.
print("Original dictionaries:")
print(d1)
print(d2)

# Call the 'test' function with 'd1' and 'd2' and print the combined dictionary.
print("\nCombined dictionaries, creating a list of values for each key:")
print(test(d1, d2))

Sample Output:

Original dictionaries:
{'w': 50, 'x': 100, 'y': 'Green', 'z': 400}
{'x': 300, 'y': 'Red', 'z': 600}

Combined dictionaries, creating a list of values for each key:
{'w': [50], 'x': [100, 300], 'y': ['Green', 'Red'], 'z': [400, 600]}

Flowchart:

For more Practice: Solve these Related Problems:

Write a Python program to merge two dictionaries so that each key maps to a list of values from both dictionaries using defaultdict.
Write a Python program to combine multiple dictionaries into one by appending values for duplicate keys into lists.
Write a Python program to iterate over multiple dictionaries and construct a new dictionary where each key’s value is a list of all corresponding values.
Write a Python program to implement a function that takes several dictionaries and returns a combined dictionary with keys mapping to lists of values.

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