Dictionary in Python - PlanetLearning

Python Collections Data Types

Dictionaries in Python

Characteristics:

Mutable: Dictionaries can be modified after their creation. You can add, remove, or change items.
Unordered (Python < 3.7): Prior to Python 3.7, dictionaries did not maintain the order of elements.
Ordered (Python >= 3.7): Starting with Python 3.7, dictionaries maintain the order of items based on the order of insertion.
Key-Value Pairs: Dictionaries consist of key-value pairs, where keys are unique and used to access corresponding values.
Heterogeneous: Both keys and values can be of any data type, and different key-value pairs can have different types.

Syntax:

Dictionaries are defined by placing a comma-separated sequence of key-value pairs inside curly braces {}.

my_dict = {"key1": "value1", "key2": "value2"}

Advantages of Dictionaries

Fast Lookup: Dictionaries provide fast lookups for keys, making them ideal for scenarios where quick access to data is required.
Flexible Data Storage: Dictionaries can store a variety of data types as keys and values, offering great flexibility.
Readable Code: Using meaningful keys makes code more readable and self-documenting.

Use Cases for Dictionaries

Storing Configuration Settings: Dictionaries are commonly used to store configuration settings.
Implementing Caches: Due to their fast lookup capabilities, dictionaries are often used to implement caches.
Counting Occurrences: Dictionaries can be used to count occurrences of items in a collection.
Associating Data: Dictionaries are perfect for associating pairs of data, like names and ages, words and definitions, etc.

Creating Dictionaries:

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# Empty dictionaryempty_dict = {}​# Dictionary with integer keysint_keys_dict = {1: "one", 2: "two", 3: "three"}​# Dictionary with string keysstr_keys_dict = {"name": "Alice", "age": 30, "city": "New York"}​# Mixed keys dictionarymixed_keys_dict = {1: "one", "two": 2, 3.0: [1, 2, 3]}​# Using the dict() functiondict_func = dict(name="Alice", age=30, city="New York")    #From a List of Tuples:list_of_tuples = [("name", "Charlie"), ("age", 35), ("city", "Chicago")]dict_from_tuples = dict(list_of_tuples)

Accessing Elements:

Elements in a dictionary can be accessed using keys.
You can also use the get() method to avoid KeyError if the key is not found.

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person = {"name": "Alice", "age": 30, "city": "New York"}print(person["name"])  # Output: Aliceprint(person["age"])  # Output: 30    print(my_dict.get("name"))  # Output: Aliceprint(my_dict.get("country", "Unknown"))  # Output: Unknown

Adding and Updating Elements:

You can add new key-value pairs or modify existing ones.

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person = {"name": "Alice", "age": 30}​# Adding a new key-value pairperson["city"] = "New York"print(person)  # Output: {'name': 'Alice', 'age': 30, 'city': 'New York'}​# Modifying an existing valueperson["age"] = 31print(person)  # Output: {'name': 'Alice', 'age': 31, 'city': 'New York'}

Removing Elements

You can remove elements using del, pop(), popitem(), and clear().

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person = {"name": "Alice", "age": 30, "city": "New York"}​# Using deldel person["city"]print(person)  # Output: {'name': 'Alice', 'age': 30}​# Using pop()age = person.pop("age")print(age)  # Output: 30print(person)  # Output: {'name': 'Alice'}​# Using popitem()name = person.popitem()print(name)  # Output: ('name', 'Alice')print(person)  # Output: {}​# Using clear()person = {"name": "Alice", "age": 30, "city": "New York"}person.clear()print(person)  # Output: {}

Dictionary Methods

Dictionaries have many built-in methods for various operations. Here are some of the most commonly used ones:

keys()	Returns a view object containing the dictionary's keys. `person = {"name": "Alice", "age": 30} keys = person.keys() print(keys) # Output: dict_keys(['name', 'age'])`
values()	Returns a view object of the dictionary's values. `values = person.values() print(values) # Output: dict_values(['Alice', 30])`
items()	Returns a view object containing the dictionary's key-value pairs. `items = person.items() print(items) # Output: dict_items([('name', 'Alice'), ('age', 30)])`
get()	Returns the value for a specified key if the key is in the dictionary; otherwise, it returns a default value. `age = person.get("age", "Not Available") print(age) # Output: 30 city = person.get("city", "Not Available") print(city) # Output: Not Available`
update()	Updates the dictionary with elements from another dictionary or an iterable of key-value pairs. `person.update({"city": "New York", "email": "alice@example.com"}) print(person) # Output: {'name': 'Alice', 'age': 30, 'city': 'New York', 'email': 'alice@example.com'}`
pop()	Removes the specified key and returns its value. If the key is not found, it returns a default value if provided. `city = person.pop("city", "Not Found") print(city) # Output: New York print(person) # Output: {'name': 'Alice', 'age': 30, 'email': 'alice@example.com'}`
popitem()	Removes and returns the last key-value pair inserted into the dictionary. `last_item = person.popitem() print(last_item) # Output: ('email', 'alice@example.com') print(person) # Output: {'name': 'Alice', 'age': 30}`
setdefault()	Returns the value of a specified key. If the key does not exist, it inserts the key with a specified value. `email = person.setdefault("email", "default@example.com") print(email) # Output: default@example.com print(person) # Output: {'name': 'Alice', 'age': 30, 'email': 'default@example.com'}`

Code Example

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# Creating dictionariesperson = {"name": "Alice", "age": 30, "city": "New York"}print(person)  # Output: {'name': 'Alice', 'age': 30, 'city': 'New York'}​# Accessing elementsprint(person["name"])  # Output: Aliceprint(person.get("age"))  # Output: 30​# Adding and modifying elementsperson["email"] = "alice@example.com"person["age"] = 31print(person)  # Output: {'name': 'Alice', 'age': 31, 'city': 'New York', 'email': 'alice@example.com'}​# Removing elementsperson.pop("city")print(person)  # Output: {'name': 'Alice', 'age': 31, 'email': 'alice@example.com'}​# Dictionary methodsprint(person.keys())  # Output: dict_keys(['name', 'age', 'email'])print(person.values())  # Output: dict_values(['Alice', 31, 'alice@example.com'])print(person.items())  # Output: dict_items([('name', 'Alice'), ('age', 31), ('email', 'alice@example.com')])​# Updating the dictionaryperson.update({"city": "New York", "phone": "123-456-7890"})print(person)  # Output: {'name': 'Alice', 'age': 31, 'email': 'alice@example.com', 'city': 'New York', 'phone': '123-456-7890'}​# Dictionary comprehensionsquares = {x: x*x for x in range(1, 6)}print(squares)  # Output: {1: 1, 2: 4, 3: 9, 4: 16, 5: 25}

Dictionary Operations

Dictionary Comprehension
You can create dictionaries using dictionary comprehensions, which are similar to list comprehensions.
```
squares = {x: x*x for x in range(1, 6)}
print(squares)  # Output: {1: 1, 2: 4, 3: 9, 4: 16, 5: 25}
```
Checking Key Existence
You can check if a key exists in a dictionary using the in keyword.
```
print("name" in person)  # Output: True
print("address" in person)  # Output: False
```