Python Dictionaries

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1. What is a Dictionary?

• A dictionary is a collection of key-value pairs that are unordered, mutable (changeable), and indexed by keys.
• Keys must be unique and immutable (e.g., strings, numbers, tuples), while values can be of any type.
• Dictionaries are defined using curly braces {} or the dict() constructor.

person = {"name": "Raj", "age": 25, "city": "Chennai"}

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2. Dictionary Operations

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1. Accessing Values

• Use the key to access the corresponding value.
• If the key does not exist, it raises a KeyError. Use the get() method to avoid this.

person = {"name": "Ram", "age": 30, "city": "Bangalore"}
print(person["name"])  # Output: Ram
print(person.get("age"))  # Output: 30
print(person.get("country", "India"))  # Output: India (default value if key doesn't exist)

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2. Adding/Updating Elements

• Add a new key-value pair or update an existing one using the assignment operator =.

person = {"name": "Anand", "age": 28}
person["city"] = "Mumbai"  # Add new key-value pair
person["age"] = 29  # Update existing key
print(person)  # Output: {'name': 'Anand', 'age': 29, 'city': 'Mumbai'}

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3. Removing Elements

• Use pop() to remove a key-value pair by key (returns the value).
• Use popitem() to remove the last inserted key-value pair (Python 3.7+).
• Use del to delete a key-value pair.
• Use clear() to remove all key-value pairs.

person = {"name": "Bala", "age": 35, "city": "Delhi"}
age = person.pop("age")  # Remove 'age' key
print(age)  # Output: 35
print(person)  # Output: {'name': 'Bala', 'city': 'Delhi'}

person.popitem()  # Remove last inserted key-value pair
print(person)  # Output: {'name': 'Bala'}

del person["name"]  # Delete 'name' key
print(person)  # Output: {}

person.clear()  # Remove all key-value pairs
print(person)  # Output: {}

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4. Checking for Keys

• Use the in keyword to check if a key exists in the dictionary.

person = {"name": "Karthik", "age": 40}
print("name" in person)  # Output: True
print("city" in person)  # Output: False

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5. Iterating Over a Dictionary

• Use loops to iterate over keys, values, or key-value pairs.

person = {"name": "Kumar", "age": 45, "city": "Hyderabad"}

# Iterate over keys
for key in person:
    print(key)  # Output: name, age, city

# Iterate over values
for value in person.values():
    print(value)  # Output: Kumar, 45, Hyderabad

# Iterate over key-value pairs
for key, value in person.items():
    print(f"{key}: {value}")
# Output:
# name: Kumar
# age: 45
# city: Hyderabad

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6. Length

• Use len() to find the number of key-value pairs in a dictionary.

person = {"name": "David", "age": 50}
print(len(person))  # Output: 2

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3. Dictionary Methods

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1. keys()

• Returns a view of all keys in the dictionary.

person = {"name": "Kannan", "age": 55}
print(person.keys())  # Output: dict_keys(['name', 'age'])

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2. values()

• Returns a view of all values in the dictionary.

person = {"name": "Siva", "age": 60}
print(person.values())  # Output: dict_values(['Siva', 60])

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3. items()

• Returns a view of all key-value pairs as tuples.

person = {"name": "Ramesh", "age": 65}
print(person.items())  # Output: dict_items([('name', 'Ramesh'), ('age', 65)])

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4. update()

• Merges another dictionary into the current one (updates existing keys and adds new ones).

person = {"name": "Suresh", "age": 70}
person.update({"city": "Pune", "age": 71})
print(person)  # Output: {'name': 'Suresh', 'age': 71, 'city': 'Pune'}

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5. copy()

• Returns a shallow copy of the dictionary.

person = {"name": "Sathish", "age": 75}
person_copy = person.copy()
print(person_copy)  # Output: {'name': 'Sathish', 'age': 75}

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4. Dictionary Comprehensions

• A concise way to create dictionaries using a single line of code.
• Syntax: {key: value for item in iterable}

squares = {x: x ** 2 for x in range(5)}
print(squares)  # Output: {0: 0, 1: 1, 2: 4, 3: 9, 4: 16}

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5. Nested Dictionaries

• A dictionary can contain other dictionaries, creating a multi-level structure.

people = {
    "Raj": {"age": 25, "city": "Chennai"},
    "Ram": {"age": 30, "city": "Bangalore"}
}
print(people["Raj"]["city"])  # Output: Chennai

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6. Additional Examples

• Creating a Dictionary:
  person = {"name": "Sujatha", "age": 28, "city": "Coimbatore"}
  print(person)  # Output: {'name': 'Sujatha', 'age': 28, 'city': 'Coimbatore'}
  
• Adding/Updating Elements:
  person["occupation"] = "Engineer"
  print(person)  # Output: {'name': 'Sujatha', 'age': 28, 'city': 'Coimbatore', 'occupation': 'Engineer'}
  
• Removing Elements:
  person.pop("age")
  print(person)  # Output: {'name': 'Sujatha', 'city': 'Coimbatore', 'occupation': 'Engineer'}
  
• Iterating Over a Dictionary:
  for key, value in person.items():
      print(f"{key}: {value}")
  # Output:
  # name: Sujatha
  # city: Coimbatore
  # occupation: Engineer
  
• Dictionary Comprehension:
  names = ["Kavitha", "Sathish", "Suresh"]
  name_lengths = {name: len(name) for name in names}
  print(name_lengths)  # Output: {'Kavitha': 7, 'Sathish': 7, 'Suresh': 6}
  

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7. Best Practices

• Use meaningful keys to make dictionaries easy to understand.
• Use get() to avoid KeyError when accessing keys that may not exist.
• Use dictionary comprehensions for concise and readable code.