How to merge two Python dictionaries in a single expression?

In Python, a Dictionary is an unordered and mutable data structure which stores data as key-value pairs. In some cases, we may need to merge two dictionaries into a single dictionary using a single expression. Python provides several methods to accomplish this task efficiently.

Using | Merge Operator (Python 3.9+)

The | merge operator is the most modern and readable way to merge dictionaries in a single expression. It returns a new dictionary containing all key-value pairs from both dictionaries. If both dictionaries have the same key, the value from the right-side dictionary takes precedence ?

# Two sample dictionaries
employee_info = {
    "name": "Niharikaa",
    "department": "Developer"
}

employee_update = {
    "department": "Programming",
    "location": "New York"
}

# Merging using the | operator in a single expression
merged_dict = employee_info | employee_update

print("Merged Dictionary =", merged_dict)

Merged Dictionary = {'name': 'Niharikaa', 'department': 'Programming', 'location': 'New York'}

Using Double Asterisk (**) Unpacking Operator

The ** unpacking operator provides a concise way to merge dictionaries in a single expression. This method works in Python 3.5+ and creates a new dictionary by unpacking all key-value pairs ?

# Two sample dictionaries
employee_info = {
    "name": "Niharikaa",
    "department": "Developer"
}

employee_update = {
    "department": "Programming",
    "location": "New York"
}

# Merging using the ** unpacking operator in a single expression
merged_dict = {**employee_info, **employee_update}
print("Merged Dictionary =", merged_dict)

Merged Dictionary = {'name': 'Niharikaa', 'department': 'Programming', 'location': 'New York'}

Using collections.ChainMap() Method

The collections.ChainMap() method groups multiple dictionaries into a single view without creating a new dictionary. ChainMap searches each dictionary in order and returns the value from the first dictionary containing the key ?

from collections import ChainMap

# Two sample dictionaries
employee_info = {
    "name": "Niharikaa",
    "department": "Developer"
}

employee_update = {
    "department": "Programming",
    "location": "New York"
}

# Creating a ChainMap and converting to dictionary
combined = ChainMap(employee_update, employee_info)
merged_dict = dict(combined)

print("Merged Dictionary =", merged_dict)

Merged Dictionary = {'department': 'Programming', 'location': 'New York', 'name': 'Niharikaa'}

Using update() Method

The update() method modifies the original dictionary by adding key-value pairs from another dictionary. This method doesn't create a new dictionary but updates the existing one in place ?

# Two sample dictionaries
employee_info = {
    "name": "Niharikaa",
    "department": "Developer"
}

employee_update = {
    "department": "Programming",
    "location": "New York"
}

# Merging using the update() method
employee_info.update(employee_update)

print("Merged Dictionary =", employee_info)

Merged Dictionary = {'name': 'Niharikaa', 'department': 'Programming', 'location': 'New York'}

Comparison

Conclusion

Use the | operator for the most readable syntax in Python 3.9+, or ** unpacking for older versions. Use update() when you want to modify the original dictionary, and ChainMap for memory-efficient dictionary views.