How to Add Key to a Dictionary in Python

1. Add a New Key/Value Pair to the Dictionary in Python
2. Update an Existing Key/Value Pair With update() Function in Python

In this tutorial, we will discuss methods to add new keys to a dictionary in Python.

Add a New Key/Value Pair to the Dictionary in Python

The dictionary object holds data in the form of key-value pairs. Adding a new key/value pair to a dictionary is straightforward in Python. The following code example shows us how to add a new key/value pair to a Python dictionary.

dictionary = {"key1": "value1", "key2": "value2", "key3": "value3"}
print(dictionary)

dictionary["key4"] = "value4"
print(dictionary)

dictionary["key2"] = "value4"
print(dictionary)

Output:

{'key1': 'value1', 'key2': 'value2', 'key3': 'value3'}
{'key1': 'value1', 'key2': 'value2', 'key3': 'value3', 'key4': 'value4'}
{'key1': 'value1', 'key2': 'value4', 'key3': 'value3', 'key4': 'value4'}

In the above code, we first initialize a dictionary and then add a new key-value pair to the dictionary using dictionary[key]. If the key does not exist, then this new key-value pair is added to the dictionary. If the key already exists, then the value of the existing key is updated to the new value.

Update an Existing Key/Value Pair With update() Function in Python

In the previous section, we discussed a method that updates an existing key-value pair and adds a new key-value pair to the dictionary if the key is not found. But, it works with only one key/value at a time. If we need to update multiple key-value pairs in the dictionary, we have to use the update() function. The update() function can also add multiple dictionaries into one single dictionary. The following code example shows how we can update multiple key-value pairs in a dictionary with the update() function.

dictionary = {"key1": "value1", "key2": "value2", "key3": "value3"}
print(dictionary)
dictionary.update({"key4": "value4", "key2": "value4"})
print(dictionary)

Output:

{'key1': 'value1', 'key2': 'value2', 'key3': 'value3'}
{'key1': 'value1', 'key2': 'value4', 'key3': 'value3', 'key4': 'value4'}

In the above code, we first initialize a dictionary and then update multiple key/value pairs using the update() function. If the key does not exist, then a new key-value pair is added to the dictionary. If the key already exists, then the existing key has the new value.

As it is clear from the above example, the update() function reduces the code if you want to update multiple key-value pairs simultaneously.