Encapsulation & Properties

class Temperature:
    def __init__(self, celsius=0):
        self._celsius = celsius  # "private" convention
    
    @property
    def celsius(self):
        return self._celsius
    
    @celsius.setter
    def celsius(self, value):
        if value < -273.15:
            raise ValueError("Below absolute zero!")
        self._celsius = value
    
    @property
    def fahrenheit(self):
        return self._celsius * 9/5 + 32
    
    @fahrenheit.setter
    def fahrenheit(self, value):
        self._celsius = (value - 32) * 5/9

# Usage
temp = Temperature(25)
print(f"{temp.celsius}°C = {temp.fahrenheit}°F")

temp.fahrenheit = 212
print(f"{temp.celsius}°C = {temp.fahrenheit}°F")

# temp.celsius = -300  # ValueError!

# Name mangling with double underscore
class Secret:
    def __init__(self):
        self.__hidden = "Can't access directly"
    
    def reveal(self):
        return self.__hidden

s = Secret()
print(s.reveal())  # Works
# print(s.__hidden)  # AttributeError!
print(s._Secret__hidden)  # Works (name mangling)