Dictionaries

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StevenSwiniarski
Published Jun 4, 2021Updated May 11, 2025
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A dictionary is a data set of key-value pairs. It maps pieces of data to each other and allows quick access to values associated with keys.

In Python, dictionaries are dynamic and mutable, meaning they can change in size or content.

Note: As of Python version 3.7, dictionaries maintain insertion order. This behavior was not guaranteed in previous versions.

Creating a Dictionary

Syntax for creating an empty dictionary:

dictionary_name = {}

The dict() function can also be used:

dictionary_name = dict()

Syntax for creating a dictionary with entries:

dictionary_name = { key1: value1,  key2: value2,  key3: value3, ... }

Each item is a key-value pair, separated by a comma.

Keys must be immutable types, such as numbers and strings. Using mutable types like lists as keys will raise a TypeError.

Values can be of any type, including strings, numbers, lists, and even dictionaries.

Example

# Create a dictionary
coffee_shop = { "cold brew": 3.50, "latte": 4.25, "cappucino": 3.99 }


# Print the dictionary
print(coffee_shop)

Output:

{'cold brew': 3.5, 'latte': 4.25, 'cappucino': 3.99}

Accessing Items in a Dictionary

Access a value by referencing its key:

coffee_shop = { "cold brew": 3.50, "latte": 4.25, "cappucino": 3.99 }
print(coffee_shop["cold brew"])

Output:

3.5

Attempting to retrieve a value with a non-existent key raises a KeyError. Assigning a value to a new key adds a new pair, while assigning to an existing key updates its value.

Adding Items to a Dictionary

To add an item:

dictionary_name[new_key] = new_value

Example

coffee_shop = { "cold brew": 3.50, "latte": 4.25, "cappucino": 3.99 }
coffee_shop["espresso"] = 4.72
print(coffee_shop)

Output:

{'cold brew': 3.5, 'latte': 4.25, 'cappucino': 3.99, 'espresso': 4.72}

Changing Items in a Dictionary

To change a value:

coffee_shop = { "cold brew": 3.50, "latte": 4.25, "cappucino": 3.99 }
coffee_shop["cold brew"] = 3.18
print(coffee_shop)

Output:

{'cold brew': 3.18, 'latte': 4.25, 'cappucino': 3.99}

Removing Items from a Dictionary

Use the del statement:

del dictionary_name[key]

Example

coffee_shop = { "cold brew": 3.50, "latte": 4.25, "cappucino": 3.99 }
del coffee_shop["latte"]
print(coffee_shop)

Output:

{'cold brew': 3.5, 'cappucino': 3.99}

Iterating a Dictionary

Common methods for iteration:

  • .keys(): Accesses dictionary keys.
  • .values(): Accesses dictionary values.
  • .items(): Accesses key-value pairs.

Syntax

dictionary_name.keys()
dictionary_name.values()
dictionary_name.items()

Codebyte Example

Code
Output
Loading...


Output:


cold brew
latte
cappucino
3.5
4.25
3.99
('cold brew', 3.5)
('latte', 4.25)
('cappucino', 3.99)
cold brew 3.5
latte 4.25
cappucino 3.99




Frequently Asked Questions


1. Are dict() and {} the same?


Yes. Both create a dictionary, but differ in syntax:


    

  • {} creates an empty dictionary directly.
    • 

  • dict() creates an empty dictionary and can also take keyword arguments or an iterable of key-value pairs.
    • 



2. How do I merge two dictionaries in Python?


Use the .update() method:


dict1.update(dict2)




3. How to clear a dictionary in Python?


Use the .clear() method:


dictionary_name.clear()




Dictionaries
.clear()
Removes all entries in a dictionary.
.copy()
Returns a copy of a dictionary.
.fromkeys()
Returns a dictionary with the specified keys.
.get()
Retrieves the value for a specified key from a dictionary with an optional default value.
.items()
Returns a list of tuples for each key-value pair in a dictionary.
.keys()
Returns a list of keys for a dictionary.
.pop()
Returns the value of a specified key, then removes the key-value pair from a dictionary.
.popitem()
Returns the last inserted key-value pair from a dictionary as a tuple, and removes the entry.
.setdefault()
Returns the value of a specified key. If the key does not exist, it is inserted with the specified value.
.update()
Updates the dictionary with key-value pairs from another dictionary or iterable, overwriting existing keys if they exist.
.values()
Returns a view of values for a dictionary.
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