encapsulation.py

class TankError(Exception):
    pass


class Tank:
    def __init__(self, capacity):
        self.capacity = capacity
        self.__level = 0

    @property
    def level(self):
        return self.__level

    @level.setter
    def level(self, amount):
        if amount > 0:
            # fueling
            if amount <= self.capacity:
                self.__level = amount
            else:
                raise TankError('Too much liquid in the tank')
        elif amount < 0:
            raise TankError('Not possible to set negative liquid level')

    @level.deleter
    def level(self):
        if self.__level > 0:
            print('It is good to remember to sanitize the remains from the tank!')
        self.__level = None

# our_tank object has a capacity of 20 units
our_tank = Tank(20)
# our_tank's current liquid level is set to 10 units
our_tank.level = 10

our_tank.__level=1

print('Current liquid level:', our_tank.level)

# adding additional 3 units (setting liquid level to 13)
our_tank.level += 3
print('Current liquid level:', our_tank.level)

# let's try to set the current level to 21 units
# this should be rejected as the tank's capacity is 20 units
try:
    our_tank.level = 21
except TankError as e:
    print('Trying to set liquid level to 21 units, result:', e)

# similar example - let's try to add an additional 15 units
# this should be rejected as the total capacity is 20 units
try:
    our_tank.level += 15
except TankError as e:
    print('Trying to add an additional 15 units, result:', e)

# let's try to set the liquid level to a negative amount
# this should be rejected as it is senseless
try:
    our_tank.level = -3
except TankError as e:
    print('Trying to set liquid level to -3 units, result:', e)

print('Current liquid level:', our_tank.level)

del our_tank.level