Python: How to Add Items to Dictionary

Introduction

Python dictionaries are a built-in data type for storing key-value pairs. The dictionary elements are mutable and don't allow duplicates. Adding a new element appends it to the end, and in Python 3.7+, the elements are ordered.

Depending on the desired result and given data, there are various ways to add items to a dictionary.

This guide shows how to add items to a Python dictionary through examples.

Prerequisites

• Python version 3.7 or newer.
• A text editor or IDE to write code.
• A method to run the code, such as a terminal or IDE.

How to Add an Item to a Dictionary in Python

Create an example dictionary to test different ways to add items to a dictionary. For example, initialize a dictionary with two items:

my_dictionary = {
  "one": 1,
  "two": 2
}
print(my_dictionary)

The methods below show how to add one or more items to the example dictionary.

Method 1: Using The Assignment Operator

The assignment operator (=) sets a value to a dictionary key:

dictionary_name[key] = value

The assignment operator adds a new key-value pair if the key does not exist. For example:

my_dictionary = {
  "one": 1,
  "two": 2
}
my_dictionary["three"] = 3
print(my_dictionary)

The code shows the updated dictionary contents after adding a new item.

Method 2: Using update()

The update() method adds a new element to an existing dictionary:

dictionary_name.update({key:value})

The method also accepts multiple key-value pairs. To use the update() method, see the example below:

my_dictionary = {
  "one": 1,
  "two": 2
}
my_dictionary.update({"three":3})
print(my_dictionary)

Use this method to add new items or to append a dictionary to an existing one.

Method 3: Using dict() Constructor

The dict() constructor allows creating a new dictionary and adding a value to an existing one. When using the second approach, the method creates a copy of a dictionary and appends an element to it.

The syntax is:

new_dictionary = dict(old_dictionary, key=value)

For example:

my_dictionary = {
  "one": 1,
  "two": 2
}
new_dictionary = dict(my_dictionary, three=3)
print(new_dictionary)

The method preserves the original dictionary and updates the new copy.

Method 4: Using __setitem__

The __setitem__ method is another way to add an item to a dictionary. The syntax is:

dictionary_name.__setitem__(key,value)

For example:

my_dictionary = {
  "one": 1,
  "two": 2
}
my_dictionary.__setitem__("three", 3)
print(my_dictionary)

The method sets the item key as "three" with the value 3.

Method 5: Using The ** Operator

The ** operator merges an existing dictionary into a new dictionary and enables adding additional items. The syntax is:

new_dictionary = {**old_dicitonary, **{key:value}}

For example, to copy an existing dictionary and append a new item, see the following code:

my_dictionary = {
  "one": 1,
  "two": 2
}
new_dictionary = {**my_dictionary, **{"three":3}}
print(new_dictionary)

The new dictionary contains the added item while preserving the old dictionary. This method avoids changing dictionaries and creates a copy for changes instead.

Method 6: Checking If A Key Exists

To avoid overwriting existing data, use an if statement to check whether a key is present before adding a new item to a dictionary. The example syntax is:

if key not in dictionary_name:
    dictionary_name[key] = value

For example:

my_dictionary = {
  "one": 1,
  "two": 2
}
if "three" not in my_dictionary:
  my_dictionary["three"] = 3
print(my_dictionary)

The code checks whether a key with the provided name exists. If the provided key exists, the existing key value does not update, and the dictionary stays unchanged.

Method 7: Using A For Loop

Add key-value pairs to a nested list and loop through the list to add multiple items to a dictionary. For example:

my_dictionary = {
  "one": 1,
  "two": 2
}
my_list = [["three", 3], ["four", 4]]
for key,value in my_list:
    my_dictionary[key] = value
print(my_dictionary)

The for loop goes through the pairs inside the list and adds two new elements.

Method 8: Using zip

Use zip to create dictionary items from two lists. The first list contains keys, and the second contains the values.

For example:

my_dictionary = {
  "one": 1,
  "two": 2
}
my_keys = ["three", "four"]
my_values = [3, 4]
for key,value in zip(my_keys, my_values):
    my_dictionary[key] = value
print(my_dictionary)

The zip function matches elements from two lists by index, creating key-value pairs.

Conclusion

This guide showed how to add items to a Python dictionary. All methods provide unique functionalities, so choose the one that best suits your program and situation.

For more Python tutorials, refer to our article on how to pretty print a JSON file using Python or learn about Python dictionary comprehension.