How to Add/Append Items to a Dictionary in Python [Examples]

This short tutorial will show you how to add single or multiple items (as key:value pairs) to a dictionary in the Python programming language.

What is a Dictionary in Python?

We’ve covered what dictionaries are in our article here.

There are a couple of ways to add items to a dictionary, so we’ve put together this article separately to make sure we’ve got everything covered!

Checking Whether a Key Exists in a Dictionary

As outlined in the article linked above, dictionaries store data as a series of key:value pairs.

Before adding new items to a dictionary, you may want to check whether a value already exists at the given key.

If a key already exists in a dictionary, assigning it a value with any of the below methods will overwrite the existing value.

The in keyword can be used to check whether a key already exists. This will return TRUE if the key exists and FALSE if it does not, so it can be used in an if statement:

myDictionary = {
    "name": "Fred",
    "animal": "cat",
    "colour": "red",
    "age": 3
} 

if 'colour' in myDictionary:
    print('colour exists in dictionary')

Adding a Single Item to a Dictionary

To add a new item to a dictionary, simply assign a value to the new key:

myDictionary = {
        "name": "Fred",
        "animal": "cat",
        "colour": "red",
        "age": 3
    } 

myDictionary["mood"] = "hungry" # Square brackets are used to set the value at a key, or add a new value if it does not exist

print(myDictionary)

Above, a new key:value pair is added to the dictionary for mood, and the change is confirmed by printing the result – which outputs:

{
    "name": "Fred",
    "animal": "cat",
    "colour": "red",
    "age": 3,
    "mood": "hungry"
}

Adding Multiple Items to a Dictionary with the update() Method

Multiple values can be added or updated in a dictionary using the update() method.

Note that if you’re only adding or modifying a single value, the above method results in better performance.

Here’s how the update() method is used:

myDictionary = {
        "name": "Fred",
        "animal": "cat",
        "colour": "red",
        "age": 3
    } 

myDictionary.update({"mood":"sleepy", "fluffiness": "so fluffy"})

print(myDictionary)

After a dictionary is defined, call the update() method from it, passing a new dictionary with the values you wish to have added. The above code results in myDictionary having two values appended to it:

{
    "name": "Fred",
    "animal": "cat",
    "colour": "red",
    "age": 3,
    "mood": "sleepy",
    "fluffiness": "so fluffy"
}

Adding Values to Nested Dictionaries

The above methods for appending values to a dictionary work just as well with dictionaries nested within other dictionaries:

myDictionary = {
        "name": "Fred",
        "animal": "cat",
        "colour": "red",
        "age": 3,
        "collar": {
            "colour": "blue"
            }
    } 

myDictionary["collar"]["style"] = "studded"

myDictionary["collar"].update({"length": 20, "width": 3})

print(myDictionary)

The above code demonstrates appending to a nested dictionary, resulting in the following:

myDictionary = {
    "name": "Fred",
    "animal": "cat",
    "colour": "red",
    "age": 3,
    "collar": {
        "colour": "blue",
        "style": "studded",
        "length": 20, 
        "width": 3
        }
}