How to Merge Two Dictionaries Without Overwriting in Python

When working with dictionaries in Python, you might need to merge two dictionaries without overwriting values that already exist. This is especially useful when preserving data integrity while combining multiple sources of information.

In this article, we will explore different ways to merge two dictionaries without overwriting in Python using simple and effective techniques.

Using setdefault()

The setdefault() method allows us to add a key-value pair to a dictionary only if the key does not already exist. Here’s how you can use it to merge two dictionaries:

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

for key, value in dict2.items():
    dict1.setdefault(key, value)

print(dict1)

See the output below:

{'a': 1, 'b': 2, 'c': 4}

Here, the value of 'b' from dict2 is ignored since it already exists in dict1.

Using Dictionary Comprehension

Another way to merge two dictionaries without overwriting in Python is through dictionary comprehension:

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

merged_dict = {key: dict1.get(key, dict2.get(key)) for key in dict1.keys() | dict2.keys()}

print(merged_dict)

See the output below:

{'a': 1, 'b': 2, 'c': 4}

Using the collections.ChainMap

The ChainMap from the collections module provides a clean way to combine dictionaries without modifying the originals, this is my personal pick among the three methods we discussed:

from collections import ChainMap

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

merged_dict = dict(ChainMap(dict1, dict2))

print(merged_dict)

See the output below:

{'b': 2, 'c': 4, 'a': 1}

This method ensures dict1 values are retained even if dict2 has overlapping keys.

Merging dictionaries without overwriting values is essential when dealing with data consistency in Python. We explored different approaches such as setdefault(), dictionary comprehension, and ChainMap to merge two dictionaries without overwriting in Python efficiently.

These methods provide flexibility depending on your use case. Choose the one that best suits your project needs!