Python Basics

Python Fundamentals

• Data types, Variables, Scope

• Immutable vs Mutable Objects

Immutable Objects

• Cannot be changed after creation. Any modification creates a new object.

• Examples:

  • Numbers: int, float, complex

  • Strings: str

  • Tuples: tuple

  • Frozen Sets: frozenset

• Immutability Example:

  x = "hello"
  x = x + " world"  # Creates a new string
  print(x)  # Output: 'hello world'

• Explanation: Strings are immutable; concatenation creates a new string rather than modifying the original.

Mutable Objects

• Can be changed in place without creating a new object.

• Examples:

  • Lists: list

  • Dictionaries: dict

  • Sets: set

  • User-defined objects (if attributes are modifiable)

• Mutability Example:

  lst = [1, 2, 3]
  lst.append(4)  # Modifies the list in place
  print(lst)  # Output: [1, 2, 3, 4]

Key Differences

Implications of Mutability

1. Performance Considerations:

   • Frequent changes to immutable objects can be inefficient.

2. Safety in Multi-threading:

   • Immutable objects prevent accidental modifications, making them safer.

3. Dictionary Keys & Set Elements:

   • Only immutable objects can be used as keys in dictionaries or elements in sets.

Examples of Valid and Invalid Keys:

# Valid dictionary keys
 d = { (1, 2): "valid" }
 frozenset([4, 5]): "frozenset key"

# Invalid keys
 d = { [1, 2]: "list key" }  # TypeError
 d = { {3: 4}: "dict key" }  # TypeError

The Role of hash and eq

1. __hash__:

   • Returns a unique integer representing the hash value of an object.

   • Required for objects to be used as dictionary keys or set elements.

   • Example: hash(42).

2. __eq__:

   • Defines equality between objects and is used for checking if different objects have the same hash value.

class CustomKey:
    def __init__(self, value):
        self.value = value
    def __hash__(self):
        return hash(self.value)
    def __eq__(self, other):
        return isinstance(other, CustomKey) and self.value == other.value

# Dictionary using custom objects as keys
 d = {CustomKey(10): "value1"}