Simple Techniques to Merge Dictionaries in Python

Dictionary is a built-in data structure that stores data in key-value pairs. It not only stores data efficiently but also allows ur to access them easily when needed. In this article we will explore various ways to merge dictionaries in Python.

Using for loop

The most simple and naive approach to merge dictionaries is using a for loop. We can loop over a dictionary items and add the key-value pairs to a other dictionary. In this process, the duplicate keys gets updated and the new keys will be added.

dict1 = {'a': 1, 'b': 2, 'c': 3}
dict2 = {'b': 20, 'c': 30, 'd': 4}

for key, value in dict2.items():
    dict1[key] = value

print(dict1)
# {'a': 1, 'b': 20, 'c': 30, 'd': 4}

Using update method

Python’s dict class has an .update([other]) method that can be used to update a dictionary with the data from other dictionary. The update method takes a dictionary, an iterable or kwargs as argument. When the keys from other is present in the dictionary it updates the value, and when the keys from other is not present in the dictionary it inserts the new data into the dictionary.

dict1 = {'a': 1, 'b': 2, 'c': 3}
dict2 = {'b': 20, 'c': 30, 'd': 4}

dict1.update(dict2)
print(dict1)
# {'a': 1, 'b': 20, 'c': 30, 'd': 4}

We can also pass iterable or kwargs to the dict update method. Following are the example for both of the cases.

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

dict1.update([("b", 20), ("c", 30), ("d", 4)])
print(dict1)
# {'a': 1, 'b': 20, 'c': 30, 'd': 4}

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

dict1.update(b=20, c=30, d=4)
print(dict1)
# {'a': 1, 'b': 20, 'c': 30, 'd': 4}

Using ** operator

The unpacking operator also known as double asterisk operator (**) is used to for packing and unpacking data into and from a dictionary. It is mostly used while calling a function that takes various numbers of kewords arguments.

You can use this unpacking operator to merge two or more dictionaries. Simply unpack both the dictionaries inside a curly braces and it will create a new dictionary containing items from both of the dictionaries.

dict1 = {'a': 1, 'b': 2, 'c': 3}
dict2 = {'b': 20, 'c': 30, 'd': 4}

dict3 = {**dict1, **dict2}
print(dict3)
# {'a': 1, 'b': 20, 'c': 30, 'd': 4}

You can use any number of dictionaries here to merge the data. For the duplicate keys, right most dictiionary will get the precedence.

dict1 = {'a': 1, 'b': 2, 'c': 3}
dict2 = {'b': 20, 'c': 30, 'd': 4}
dict3 = {'c': 300, 'd': 400, 'e': 5}

dict4 = {**dict1, **dict2, **dict3}
print(dict4)
# {'a': 1, 'b': 20, 'c': 300, 'd': 400, 'e': 5}

In the example above, “b” takes value from dict2 as its the right most dictionary for the key “b”, “c” and “d” takes the value from the right most dictionary dict3, and “e” is added into the merged dictionary.

Using | operator

Python 3.9 introduces a new operator which is called union operator ( | ) that merges two dictionaries into one. Using this operator you can create a new dictionary that can contain all the key-value pairs from both of the initial dictionaries. Here also, if the dictionaries share same keys the values from right most dictionary will take precedence.

dict1 = {'a': 1, 'b': 2, 'c': 3}
dict2 = {'b': 20, 'c': 30, 'd': 4}

dict3 = dict1 | dict2
print(dict3)
# {'a': 1, 'b': 20, 'c': 30, 'd': 4}

Similarly, you can use the augmented union operator ( |= ) to merge two dictionary. If you use the augmented union operator, it will perform in-place update to the first dictionary instead of creating a new dictionary. Again for the duplicate keys, right most dictionary will take priority.

dict1 = {'a': 1, 'b': 2, 'c': 3}
dict2 = {'b': 20, 'c': 30, 'd': 4}

dict1 |= dict2
print(dict1)
# {'a': 1, 'b': 20, 'c': 30, 'd': 4}

Using chain() method

The itertools module from Python has a method chain() that can be used to merge two or more dictionaries. It takes multiple iterable objects, merges them and returns one iterable objects.

While merging dictionaries using chain() method you need to pass the key-value pair iterators using the dict.items() method and the chain method will then return one iterator after merging the objects. And finally you can pass that iterator object to the dict() constructor which will converts it into a dictionary.

from itertools import chain

dict1 = {'a': 1, 'b': 2, 'c': 3}
dict2 = {'b': 20, 'c': 30, 'd': 4}

dict3 = dict(chain(dict1.items(), dict2.items()))
print(dict3)
# {'a': 1, 'b': 20, 'c': 30, 'd': 4}

Using ChainMap() method

There is another method ChainMap() which can used to merge dictionaries as well. It works similar to the chain() method, except you don’t need to use the .items() dictionary method. You can directly pass the dictionaries to ChainMap() method and it will work. You need to import the ChainMap() method from collections module.

from collections import ChainMap

dict1 = {'a': 1, 'b': 2, 'c': 3}
dict2 = {'b': 20, 'c': 30, 'd': 4}

dict3 = dict(ChainMap(dict2, dict1))
print(dict3)
# {'a': 1, 'b': 20, 'c': 30, 'd': 4}

Conclusion

In this article, you have learned various methods to merge Python dictionaries. You have also seen them in action in the examples. Some of the merging techniques creates new dictionary after merging, while other techniques performs in-place updates on the dictionary. You have also learned the behavior of those techniques when there are duplicate items in dictionaries.