Python Dictionary

A dictionary is a Python in-built data structure where it allows the storing of data in pairs of key-values. Dictionaries are, therefore very versatile and applied in large scales to jobs that would require the fastest look-up, storage or association of data.

Key Features of a Python Dictionary:

Unordered: Dictionaries in Python 3.7+ maintain the insertion order of items, but are not indexed like lists and cannot be accessed by an index.
Mutable: Items may be added, changed or deleted after the dictionary has been created.
Key-value pairs: Every item within a dictionary is stored in the form of a key: value pair.
Unique Keys: Keys in a dictionary must be unique and immutable (strings, numbers, tuples, etc.).
Dynamic Size: Dictionaries can grow or shrink dynamically as items are added or removed.

Syntax to Create a Dictionary:

You can create a dictionary using curly braces {} or the dict() constructor.

Example:

# Using curly braces
my_dict = {"name": "Alice", "age": 25, "city": "New York"}
# Using the dict() constructor
my_dict2 = dict(name="Bob", age=30, city="San Francisco")

Accessing Values in a Dictionary:

You can retrieve a value using its corresponding key.

Example:

my_dict = {"name": "Alice", "age": 25, "city": "New York"}
print(my_dict["name"]) # Output: Alice

To avoid errors when a key does not exist, you can use the get() method:

print(my_dict.get("country", "Not Found")) # Output: Not Found

Adding or Updating Items:

To add a new key-value pair:

my_dict["country"] = "USA"

To update an existing key:

my_dict["age"] = 26

Removing Items:

Using pop() (removes a key and returns its value):

removed_value = my_dict.pop("city") # Removes "city"

Using del (deletes a key-value pair):

del my_dict["name"]

Using clear() (removes all items):

my_dict.clear()

Iterating Through a Dictionary:

Keys: Use for key in my_dict.

for key in my_dict:
   print(key)

2. Values: Use my_dict.values().

for value in my_dict.values():
   print(value)

3. Key-Value Pairs: Use my_dict.items().

for key, value in my_dict.items():
    print(f"{key}: {value}")

Dictionary Methods:

Here are some useful methods:

Method	Description
`clear()`	Removes all elements from the dictionary.
`copy()`	Returns a shallow copy of the dictionary.
`get(key, [val])`	Returns the value of the key; if key doesn’t exist, returns `val` (default is `None`).
`keys()`	Returns a view object with all the keys.
`values()`	Returns a view object with all the values.
`items()`	Returns a view object with all the key-value pairs.
`pop(key)`	Removes the item with the specified key and returns its value.
`popitem()`	Removes and returns the last inserted key-value pair as a tuple.
`update(dict2)`	Updates the dictionary with key-value pairs from another dictionary or iterable of pairs.

Example Usage:

# Creating a dictionary
student = {"name": "John", "age": 20, "major": "Computer Science"}

# Accessing elements
print(student["name"])         # Output: John
print(student.get("gpa", 4.0)) # Output: 4.0 (default value)

# Adding/updating elements
student["gpa"] = 3.8
student["age"] = 21

# Iterating
for key, value in student.items():
   print(f"{key}: {value}")

# Removing elements
student.pop("major")
print(student) # Output: {'name': 'John', 'age': 21, 'gpa': 3.8}