#Python Dictionaries (Dictionary)

Dictionaries are a very flexible and powerful built-in data type in Python. They store data in the form of key-value pairs, providing an efficient way to look up, add, and remove entries.

#Dictionary Characteristics

• Key-Value Pairs: Each element consists of a unique "key" and a corresponding "value".
• Unordered (Historical): In versions before Python 3.7, dictionaries were unordered. Starting with Python 3.7, dictionaries maintain the insertion order of elements.
• Mutable: Dictionaries are mutable; you can add, modify, or delete key-value pairs at any time.
• Unique Keys: Keys in a dictionary must be unique and must be of immutable type (such as strings, numbers, or tuples). Values can be of any type and can be repeated.

#Creating Dictionaries

Dictionaries are defined using curly braces {}, containing a series of key: value pairs.

# Create an empty dictionary
empty_dict = {}

# Create a dictionary
person = {
    "name": "John Doe",
    "age": 30,
    "city": "New York",
    "is_student": False
}

# Can also use dict() constructor
car = dict(brand="Ford", model="Mustang", year=1964)

print(person)
print(car)

#Accessing Dictionary Elements

You can access corresponding values through keys.

#Using Square Brackets []

person = {"name": "John Doe", "age": 30}

print(person["name"]) # Output: John Doe

# If the key doesn't exist, raises KeyError
# print(person["country"]) # KeyError: 'country'

#Using .get() Method

The .get() method is safer; if the key doesn't exist, it returns None (or a default value you specify) instead of raising an error.

person = {"name": "John Doe", "age": 30}

print(person.get("age"))        # Output: 30
print(person.get("country"))    # Output: None
print(person.get("country", "USA")) # Output: USA (provided default value)

#Modifying and Adding Elements

You can add new key-value pairs or modify existing values through keys.

person = {"name": "John Doe", "age": 30}

# Modify existing value
person["age"] = 31
print(person) # Output: {'name': 'John Doe', 'age': 31}

# Add new key-value pair
person["city"] = "Boston"
print(person) # Output: {'name': 'John Doe', 'age': 31, 'city': 'Boston'}

#Deleting Elements

• del dict[key]: Deletes the specified key-value pair.
• dict.pop(key): Deletes and returns the value of the specified key.
• dict.popitem(): Deletes and returns the last inserted key-value pair (in Python 3.7+).
• dict.clear(): Empties the dictionary.

#Iterating Over Dictionaries

There are several ways to iterate over all elements in a dictionary.

person = {"name": "John Doe", "age": 30, "city": "New York"}

# Iterate over all keys
for key in person.keys():
    print(key)

# Iterate over all values
for value in person.values():
    print(value)

# Iterate over all key-value pairs (most common)
for key, value in person.items():
    print(f"{key}: {value}")

.keys(), .values(), and .items() return special "view objects" that provide dynamic views of dictionary entries, meaning if the dictionary changes, the views will reflect those changes accordingly.