How To Add To A Dictionary In Python

Dictionaries are an essential part of Python programming and are commonly used to store and manipulate key-value pairs.

A dictionary is an unordered collection of items, where each item has a unique key with an associated value.

Adding to a dictionary is a common operation in Python and knowing how to do so efficiently and effectively can greatly improve the performance and readability of your code. In this tutorial, we will explore different ways to add to a dictionary in Python.

Method 1: Direct Assignment

The simplest way to add a new key-value pair to a dictionary is by directly assigning a value to a new key. Here is an example:

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animals = {
    "dog": "Brown",
    "cat": "White",
    "bear": "Grey"
}
 
animals["lion"] = "Yellow"
print(animals)

In the above example, we added a new key “lion” with the value “Yellow” to the animals dictionary. The resulting dictionary now contains this new key-value pair:

{"dog": "Brown", "cat": "White", "bear": "Grey", "lion": "Yellow"}

Method 2: Using the update() Method

Another way to add new key-value pairs to a dictionary is by using the update() method. This method is often used when you need to add multiple items at once. Here is an example:

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animals = {
    "dog": "Brown",
    "cat": "White",
    "bear": "Grey"
}
 
new_animals = {
    "lion": "Yellow",
    "tiger": "Black"
}
 
animals.update(new_animals)
print(animals)
 

In the above example, we merged the new_animals dictionary into the animals dictionary using the update() method. The resulting dictionary now contains the key-value pairs from both dictionaries:

{"dog": "Brown", "cat": "White", "bear": "Grey", "lion": "Yellow", "tiger": "Black"}

Method 3: Using Dictionary Comprehension

If you need to add new key-value pairs to a dictionary based on some logic or by applying a function, you can use dictionary comprehension. This allows you to generate a new dictionary by iterating through other objects and evaluating conditions. Here is an example:

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def make_plural(word):
    return word + "s"
 
animals_single = ["dog", "cat", "bear"]
animals_plural = {animal: make_plural(animal) for animal in animals_single}
 
print(animals_single)
print(animals_plural)

In the above example, we used dictionary comprehension to create a new dictionary based on the items in the list animals_single, applying the make_plural function to each item:

['dog', 'cat', 'bear']
{'dog': 'dogs', 'cat': 'cats', 'bear': 'bears'}

Method 4: Using defaultdict

If you want to add to a dictionary with a default value when the key is not present, you can use the defaultdict class from the collections module. This allows you to initialize the dictionary with a default factory function, so whenever you try to access a non-existent key, it will be added to the default function’s output. Here is an example:

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from collections import defaultdict
 
animals = defaultdict(int)
animals["dog"] += 1
animals["cat"] += 1
animals["bear"] += 1

In the example above, even though we didn’t explicitly set initial values for the keys “dog”, “cat”, and “bear”, the defaultdict class initialized them with the default value 0 and incremented the value by 1.

Conclusion

In this tutorial, we have explored four different ways to add to a dictionary in Python:

  1. Direct Assignment
  2. The update() Method
  3. Dictionary Comprehension
  4. defaultdict

Choose the appropriate method based on your specific use case and requirements to create efficient and readable code.