4.5 - Working With Dictionaries

Dictionaries are unordered collections of key-value pairs. In Python, dictionaries are defined with curly braces {} containing keys and values, e.g., {key1: value1, key2: value2}. Keys act as unique identifiers for values, and values can be of any data type.

Dictionaries are mutable, allowing modification of their contents. The keys in a dictionary must be immutable (like strings, numbers, or tuples) and unique, while the values can be of any data type and can be duplicated.

4.5.1 - Creating Dictionaries

Dictionaries are created by enclosing key-value pairs in curly braces {}:

# An empty dictionary
empty_dict = {}

# Dictionary with integer keys
int_key_dict = {1: 'apple', 2: 'banana'}

# Dictionary with mixed key types
mixed_dict = {'name': 'John', 1: [2, 4, 3]}

Alternatively, the dict() constructor can be used:

# Using dict() with keyword arguments
dict_with_keywords = dict(first='apple', second='banana')

# Using dict() with a list of tuples
dict_from_tuples = dict([('first', 'apple'), ('second', 'banana')])

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4.5.2 - Accessing Dictionary Elements

Elements in a dictionary can be accessed using their keys:

# Accessing an element
element = int_key_dict[1]  # 'apple'

# Accessing using get()
element_with_get = int_key_dict.get(1)  # 'apple'

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4.5.3 - Modifying Dictionaries

Dictionaries are mutable. You can add new key-value pairs or modify existing ones:

# Adding a new key-value pair
int_key_dict[3] = 'orange'

# Modifying an existing key
int_key_dict[2] = 'blueberry'

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4.5.4 - Removing Elements from Dictionaries

Elements can be removed from a dictionary using several methods:

• .pop(key): Removes the item with the specified key and returns its value.
• .popitem(): Removes and returns the last key-value pair.
• del: Removes an item or the entire dictionary.
• .clear(): Empties the dictionary.

Example:

removed_value = int_key_dict.pop(3)
int_key_dict.popitem()
del int_key_dict[1]
int_key_dict.clear()

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4.5.5 - Iterating Over Dictionaries

You can iterate over the keys, values, or key-value pairs in a dictionary:

# Iterating over keys
for key in int_key_dict:
    print(key)

# Iterating over values
for value in int_key_dict.values():
    print(value)

# Iterating over key-value pairs
for key, value in int_key_dict.items():
    print(key, value)

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4.5.6 - Checking if a Key Exists

To check if a key exists in a dictionary, use the in keyword:

if 2 in int_key_dict:
    print("Key 2 exists in the dictionary")

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4.5.7 - Dictionary Length

The len() function returns the number of key-value pairs in the dictionary:

length = len(int_key_dict)

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4.5.8 - Nested Dictionaries

Dictionaries can contain other dictionaries, creating nested structures:

nested_dict = {1: {'name': 'John', 'age': 27}, 2: {'name': 'Marie', 'age': 22}}

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4.5.9 - Dictionary Comprehensions

Dictionary comprehensions offer a concise way to create dictionaries:

squared_dict = {x: x**2 for x in range(6)}

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4.5.10 - Merging Dictionaries

Dictionaries can be merged in several ways:

• Using the update() method.
• Using the | operator (Python 3.9+).

Example:

dict1 = {'a': 1, 'b': 2}
dict2 = {'b': 3, 'c': 4}

# Using update()
dict1.update(dict2)  # dict1 is now {'a': 1, 'b': 3, 'c': 4}

# Using the | operator
merged_dict = dict1 | dict2  # {'a': 1, 'b': 3, 'c': 4}

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4.5.11 - Summary Table of Dictionary Operations and Features

Operation/Feature	Description	Example
Creating Dictionaries	Define a dictionary with {} or dict().	d = {1: 'apple', 2: 'banana'}
Accessing Elements	Access elements by their key.	element = d[1]
Modifying Dictionaries	Add or modify elements by key.	d[3] = 'orange'
Removing Elements	Remove elements using various methods.	removed = d.pop(3)
Iterating Over Dictionaries	Iterate over keys, values, or pairs.	for k, v in d.items(): print(k, v)
Checking Key Existence	Check if a key exists.	if 2 in d: ...
Dictionary Length	Get the number of key-value pairs.	len(d)
Nested Dictionaries	Use dictionaries within dictionaries.	nested = {1: {'name': 'John'}}
Dictionary Comprehensions	Concise way to create dictionaries.	squared = {x: x**2 for x in range(6)}
Merging Dictionaries	Merge dictionaries using methods or `	`.