Merge Python Dictionaries: 10 Approaches You Should Know

Merging dictionaries is a common task in Python, especially when dealing with large datasets, configurations, or managing data. Whether you are combining two dictionaries or multiple, understanding the most efficient and effective ways to do so can enhance your code’s performance and readability. In this blog post, we’ll explore 10 different methods to merge Python dictionaries, each with its own strengths and scenarios for use.

1. Using the | Operator (Python 3.9+)

    dict1 = {'a': 1, 'b': 2}

    dict2 = {'b': 3, 'c': 4}

    merged = dict1 | dict2

    print(merged)  

    Output = {'a': 1, 'b': 3, 'c': 4}

• Pros: Clean and readable.
• Cons: Requires Python 3.9+.

2. Using the update() Method

    dict1 = {'a': 1, 'b': 2}

    dict2 = {'b': 3, 'c': 4}

    dict1.update(dict2)

    print(dict1)  

    Output = {'a': 1, 'b': 3, 'c': 4}

• Pros: In-place merge; no new dictionary is created.
• Cons: Modifies the first dictionary.

3. Using Dictionary Unpacking (Python 3.5+)

    dict1 = {'a': 1, 'b': 2}

    dict2 = {'b': 3, 'c': 4}

    merged = {**dict1, **dict2}

    print(merged)  

    Output = {'a': 1, 'b': 3, 'c': 4}

• Pros: Backward compatible for Python 3.5+.
• Cons: Creates a new dictionary.

4. Using a Dictionary Comprehension

    dict1 = {'a': 1, 'b': 2}

    dict2 = {'b': 3, 'c': 4}

    merged = {k: v for d in [dict1, dict2] for k, v in d.items()}

    print(merged)  

    Output = {'a': 1, 'b': 3, 'c': 4}

• Pros: Customizable merging logic.
• Cons: Less readable for simple merges.

5. Using collections.ChainMap

    from collections import ChainMap

    dict1 = {'a': 1, 'b': 2}

    dict2 = {'b': 3, 'c': 4}

    merged = dict(ChainMap(dict2, dict1))

    print(merged)  

    # {'a': 1, 'b': 3, 'c': 4}

• Pros: Useful for dynamically merging dictionaries.
• Cons: Creates a new dictionary.

6. Using reduce() with operator.or_ (Python 3.9+)

    from functools import reduce

    import operator

    dict1 = {'a': 1, 'b': 2}

    dict2 = {'b': 3, 'c': 4}

    merged = reduce(operator.or_, [dict1, dict2])

    print(merged)  

    Output = {'a': 1, 'b': 3, 'c': 4}

• Pros: Scalable for merging many dictionaries.
• Cons: Requires Python 3.9+.

7. Using reduce() with update()

    from functools import reduce

    dict1 = {'a': 1, 'b': 2}

    dict2 = {'b': 3, 'c': 4}

    merged = reduce(lambda d1, d2: d1.update(d2) or d1, [dict1.copy(), dict2])

    print(merged)  

    # {'a': 1, 'b': 3, 'c': 4}

• Pros: Works in older Python versions.
• Cons: Less readable.

8. Iterative Merging with a Loop

    dict1 = {'a': 1, 'b': 2}

    dict2 = {'b': 3, 'c': 4}

    merged = dict1.copy()

    for k, v in dict2.items():

        merged[k] = v

    print(merged)  

    Output = {'a': 1, 'b': 3, 'c': 4}

• Pros: Clear logic; easy to customize.
• Cons: Verbose for simple merges.

9. Using dict.update() in a Copy

    dict1 = {'a': 1, 'b': 2}

    dict2 = {'b': 3, 'c': 4}

    merged = dict1.copy()

    merged.update(dict2)

    print(merged)  

    Output = {'a': 1, 'b': 3, 'c': 4}

• Pros: Doesn't modify the original dictionary.
• Cons: Slightly less concise than unpacking.

10. Using dict Constructor with .items()

    dict1 = {'a': 1, 'b': 2}

    dict2 = {'b': 3, 'c': 4}

    merged = dict(list(dict1.items()) + list(dict2.items()))

    print(merged)  

    # {'a': 1, 'b': 3, 'c': 4}

• Pros: Works in older Python versions.
• Cons: Less efficient.

Best Approach:

For Python 3.9+, the | operator is the best choice due to:

• Conciseness.
• Readability.
• Built-in functionality.

For Python 3.5–3.8, dictionary unpacking ({**dict1, **dict2}) is a close second.

If compatibility with older versions or dynamic merging is needed, ChainMap is a robust option.

Conclusion:

When merging dictionaries in Python, the choice of method depends on your specific requirements, such as Python version compatibility, performance, and readability.

• For Python 3.9+, the | operator is the most efficient and readable option, making it the recommended approach for merging dictionaries.
• For Python 3.5 to 3.8, dictionary unpacking ({**dict1, **dict2}) is a clean and backward-compatible choice.
• If you need to dynamically manage multiple dictionaries or prioritize specific logic, consider using collections.ChainMap or a dictionary comprehension.

Ultimately, the best approach balances clarity, performance, and compatibility, with modern syntax like | taking the lead for simplicity and elegance in Python 3.9 and beyond.

💫Thank you💫