Methods (Instance, Class, Static)

class MathUtils:
    pi = 3.14159  # class attribute
    
    def __init__(self, value):
        self.value = value  # instance attribute
    
    # Instance method — operates on instance data
    def square(self):
        return self.value ** 2
    
    # Class method — operates on class data
    @classmethod
    def circle_area(cls, radius):
        return cls.pi * radius ** 2
    
    # Static method — utility, no access to class/instance
    @staticmethod
    def add(a, b):
        return a + b

# Instance method
m = MathUtils(5)
print(f"5² = {m.square()}")

# Class method (can call without instance)
print(f"Circle area (r=3): {MathUtils.circle_area(3):.2f}")

# Static method
print(f"5 + 3 = {MathUtils.add(5, 3)}")

# Practical: alternative constructors
class Date:
    def __init__(self, year, month, day):
        self.year = year
        self.month = month
        self.day = day
    
    @classmethod
    def from_string(cls, date_str):
        year, month, day = map(int, date_str.split("-"))
        return cls(year, month, day)
    
    def __str__(self):
        return f"{self.year}-{self.month:02d}-{self.day:02d}"

d = Date.from_string("2024-01-15")
print(d)  # 2024-01-15