How do you merge two dictionaries in Python?

Merging two dictionaries in Python can be achieved in multiple ways, depending on your Python version and preference. Here’s a breakdown of the most common methods:

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1. Using the update() Method (Available in All Versions)

The update() method adds the key-value pairs from one dictionary to another. If keys overlap, the values in the second dictionary overwrite those in the first.

dict1 = {'a': 1, 'b': 2}
dict2 = {'b': 3, 'c': 4}

dict1.update(dict2)  # Modifies dict1 in place
print(dict1)  # Output: {'a': 1, 'b': 3, 'c': 4}

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2. Using the {**dict1, **dict2} Syntax (Python 3.5+)

You can use unpacking to merge dictionaries. This creates a new dictionary without modifying the originals.

dict1 = {'a': 1, 'b': 2}
dict2 = {'b': 3, 'c': 4}

merged_dict = {**dict1, **dict2}
print(merged_dict)  # Output: {'a': 1, 'b': 3, 'c': 4}

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3. Using the | Operator (Python 3.9+)

The | operator provides a concise way to merge dictionaries and returns a new dictionary. This is similar to unpacking but more readable.

dict1 = {'a': 1, 'b': 2}
dict2 = {'b': 3, 'c': 4}

merged_dict = dict1 | dict2
print(merged_dict)  # Output: {'a': 1, 'b': 3, 'c': 4}

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4. Using Dictionary Comprehension

For more control, you can use dictionary comprehension to merge dictionaries manually.

dict1 = {'a': 1, 'b': 2}
dict2 = {'b': 3, 'c': 4}

merged_dict = {key: value for d in [dict1, dict2] for key, value in d.items()}
print(merged_dict)  # Output: {'a': 1, 'b': 3, 'c': 4}

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5. Using a Third-Party Library (collections.ChainMap)

The ChainMap class from the collections module groups multiple dictionaries together. It does not create a new dictionary but provides a single view for lookup.

from collections import ChainMap

dict1 = {'a': 1, 'b': 2}
dict2 = {'b': 3, 'c': 4}

merged = ChainMap(dict2, dict1)  # dict2 takes precedence for overlapping keys
print(dict(merged))  # Output: {'a': 1, 'b': 3, 'c': 4}

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Key Points

• Use update() if you want to modify an existing dictionary.
• Use {**dict1, **dict2} or | if you need a new dictionary.
• The ChainMap approach is efficient for lookups but not for creating a standalone dictionary.

Let me know which method suits your needs, or if you’d like to see an example tailored to your use case!