Example of a function + lambda to enforce type checking on attr init and settr operations

python_type_checker.py

def typed_property(name, expected_type):
    private_name = '_' + name

    @property
    def prop(self):
        return getattr(self, private_name)

    @prop.setter
    def prop(self, value):
        if not isinstance(value, expected_type):
            raise TypeError('Expected {}'.format(expected_type))
        setattr(self, private_name, value)

    return prop

Integer = lambda name: typed_property(name, int)
Float = lambda name: typed_property(name, float)
String = lambda name: typed_property(name, str)

class Holding(object):
    shares = Integer('shares')
    name = String('name')
    price = Float('price')
    
    #without the lambda's
    # shares = typed_property('shares', int)
    # name = typed_property('name', str)
    # price = typed_property('price', float)

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

    #Long winded approach
    # @property
    # def shares(self):
    #     return self._shares
    #
    # @shares.setter
    # def shares(self, newshares):
    #     if not isinstance(newshares, int):
    #         raise TypeError('Expected a Float')
    #     if newshares < 0:
    #         raise ValueError('Must be > 0')
    #     self._shares = newshares
    #
    # @property
    # def price(self):
    #     return self._price
    #
    # @price.setter
    # def price(self, newprice):
    #     if not isinstance(newprice, float):
    #         raise TypeError('Expected a Float')
    #     if newprice < 0:
    #         raise ValueError('Must be > 0')
    #     self._price = newprice

    @property #avoid having to invoke function
    def value(self):
        return self.price * self.shares

    def __repr__(self):
        return "{!r} {!r} {!r}".format(self.name, self.price, self.shares)

    def __str__(self):
        return "{!r} {!r} {!r}".format(self.name, self.price, self.shares)


x = Holding('AAPL', 300.00, 100) 
print(x)
print(x.value) #not require the invocation now due to property
x.price = '234' #Float required