class Typed:

expected_type = object

def __init__(self, name):

self.name = name

def __get__(self, instance, cls):

return instance.__dict__[self.name]

def __set__(self, instance, value):

if not isinstance(value, self.expected_type):

raise TypeError(f'Expected {self.expected_type}')

instance.__dict__[self.name] = value

class Integer(Typed):

expected_type = int

class Float(Typed):

expected_type = float

class Holding:

shares = Integer('shares')

price = Float('price')

def __init__(self, name, shares, price):

self.name = name

self.shares = shares

self.price = price

@property

def cost(self):

return self.price * self.shares

if __name__ == '__main__':

h = Holding('IBM', 25, 34.5)

print(f'Share {h.shares} : cost : {h.cost}')

# h.price = '24' # TypeError: Expected <class 'float'>

# h.shares = 'a lot' # TypeError: Expected <class 'int'>

h.shares = 26

print(f'After change: share {h.shares} : cost : {h.cost}')

# t = Integer('test')

# t = 4 # This would override the existing reference

# this would work only inside class

h.append