How do you merge two dictionaries in Python?

Python Data Structures - Interview Questions and Answers

You can merge two dictionaries in Python using several methods. Here are the most common ways:

1. Using the `update()` Method (Modifies Original Dictionary)

The update() method adds key-value pairs from one dictionary to another.
If a key exists in both dictionaries, the value from the second dictionary overwrites the value in the first.

Example :

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

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

* Best when you want to modify an existing dictionary.

2. Using Dictionary Unpacking (``) (Creates a New Dictionary)**

This method merges dictionaries into a new one without modifying the originals.
If keys overlap, values from later dictionaries overwrite earlier ones.

Example :

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

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

* Best when you want to create a new merged dictionary.

3. Using the `|` Operator (Python 3.9+)

Introduced in Python 3.9, | merges two dictionaries and returns a new one.
Overlapping keys take the value from the second dictionary.

Example (Python 3.9+ Only) :

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

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

* Best when using Python 3.9+ and you need a clean syntax.

4. Using a Dictionary Comprehension (Advanced)

This method gives more control, allowing transformations while merging.

Example :

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

merged_dict = {key: dict2.get(key, dict1.get(key)) for key in dict1.keys() | dict2.keys()}
print(merged_dict)  # Output: {'a': 1, 'b': 3, 'c': 4}

* Best when you need custom merging logic.

Which Method to Use?

Method	Modifies Original?	Creates New Dict?	Python Version
`update()`	Yes	No	Any
`**` (Unpacking)	No	Yes	3.5+
`	` (Union Operator)	No	Yes
Dictionary Comprehension	No	Yes	Any

1. Using the update() Method (Modifies Original Dictionary)

Example :

2. Using Dictionary Unpacking (**) (Creates a New Dictionary)

Example :

3. Using the | Operator (Python 3.9+)

Example (Python 3.9+ Only) :

4. Using a Dictionary Comprehension (Advanced)

Example :

Which Method to Use?

1. Using the `update()` Method (Modifies Original Dictionary)

2. Using Dictionary Unpacking (``) (Creates a New Dictionary)**

3. Using the `|` Operator (Python 3.9+)