Python Program to Merge Two Dictionaries

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Merging dictionaries is a common operation in Python programming, often essential when dealing with data structures and algorithms. While merging dictionaries may seem straightforward, there are nuances to be aware of, particularly regarding key collisions, data integrity, and performance. This comprehensive guide aims to elucidate every aspect of merging dictionaries in Python, complete with code examples, explanations, best practices, and advanced use-cases.

What is a Dictionary in Python?

In Python, a dictionary is an unordered, mutable collection of key-value pairs. Dictionaries are incredibly versatile and are commonly used for tasks such as data retrieval, frequency counting, and many more.

Why Merge Dictionaries?

Before diving into the code, let’s understand why merging dictionaries is important:

  1. Data Aggregation: Combining two or more sets of data into a single data set.
  2. Configuration Management: Merging default settings with user settings.
  3. Dynamic Programming: During runtime, updating an existing dictionary with new key-value pairs.

Basic Dictionary Merging Techniques

Using update( ) Method

The update() method is the most straightforward way to merge two dictionaries. It updates the dictionary with elements from another dictionary object or from an iterable of key-value pairs.

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

# Merge dictionaries

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

Using Dictionary Unpacking (**)

Python 3.5+ introduces a new way to merge dictionaries using the ** unpacking operator.

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

# Merge dictionaries
merged_dict = {**dict1, **dict2}

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

Using the | Merge Operator

Python 3.9 introduces the | operator to merge two dictionaries easily.

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

# Merge dictionaries
merged_dict = dict1 | dict2

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

Advanced Merging Techniques

Conditional Merging

Sometimes you may want to merge dictionaries based on certain conditions. For example, only merging keys that don’t already exist in the first dictionary.

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

# Conditional Merge
for key, value in dict2.items():
    if key not in dict1:
        dict1[key] = value

print(dict1)  # Output: {'a': 1, 'b': 2, 'c': 4}

Merging with a Custom Function

You can create a custom function to determine how to handle key collisions during a merge.

# Custom merge function
def custom_merge(dict1, dict2):
    result = dict1.copy()
    for key, value in dict2.items():
        if key in result:
            result[key] += value  # Add the values together
            result[key] = value
    return result

dict1 = {'a': 1, 'b': 2}
dict2 = {'b': 3, 'c': 4}
merged_dict = custom_merge(dict1, dict2)

print(merged_dict)  # Output: {'a': 1, 'b': 5, 'c': 4}

Common Pitfalls

  1. Key Collision: The most common issue when merging dictionaries is handling key collisions. Always know how your chosen merge method handles them.
  2. Mutability: If you’re merging nested dictionaries, be aware that some methods might only perform a shallow copy, leaving nested objects still pointing to the same memory location.

Practical Applications

  • Data Science: Merging dictionaries is a common operation when aggregating data from multiple sources.
  • Web Development: In frameworks like Django or Flask, merging dictionaries can help manage user sessions and cookies.
  • Configuration Management: Systems like Ansible use dictionaries to merge different configuration files.


Merging dictionaries is a foundational skill in Python programming that is indispensable in a wide range of applications. From data science to web development, understanding how to effectively and efficiently merge dictionaries will serve you well. This guide should equip you with the knowledge and confidence to handle all your dictionary merging needs in Python.

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