In [2]:

    

# Base class. Uses a descriptor to set a value
class Descriptor:
    def __init__(self, name=None, **opts):
        self.name = name
        for key, value in opts.items():
            setattr(self, key, value)

    def __set__(self, instance, value):
        instance.__dict__[self.name] = value


# Descriptor for enforcing types
class Typed(Descriptor):
    expected_type = type(None)

    def __set__(self, instance, value):
        if not isinstance(value, self.expected_type):
            raise TypeError('expected ' + str(self.expected_type))
        super().__set__(instance, value)


# Descriptor for enforcing values
class Unsigned(Descriptor):
    def __set__(self, instance, value):
        if value < 0:
            raise ValueError('Expected >= 0')
        super().__set__(instance, value)


class MaxSized(Descriptor):
    def __init__(self, name=None, **opts):
        if 'size' not in opts:
            raise TypeError('missing size option')
        super().__init__(name, **opts)

    def __set__(self, instance, value):
        if len(value) >= self.size:
            raise ValueError('size must be < ' + str(self.size))
        super().__set__(instance, value)


    

In [24]:

    

class Integer(Typed):
    expected_type = int

class UnsignedInteger(Integer, Unsigned):
    pass

class Float(Typed):
    expected_type = float

class UnsignedFloat(Float, Unsigned):
    pass

class String(Typed):
    expected_type = str

class SizedString(String, MaxSized):
    pass


    

In [23]:

    

class Stock:
    # Specify constraints
    name = SizedString('name', size=8)
    shares = UnsignedInteger('shares')
    price = UnsignedFloat('price')

    def __init__(self, name, shares, price):
        self.name = name
        self.shares = shares
        self.price = price
s1 = Stock('s1', 16, 10.2)
s1.name


    

    
Out[23]:





's1'

In [22]:

    

# Class decorator to apply constraints
def check_attributes(**kwargs):
    def _check_decorator(cls):
        for _k, _v in kwargs.items():
            if isinstance(_v, Descriptor):
                setattr(cls, _k, _v)
            else:
                setattr(cls, _k, _v(_k))
        return cls
    return _check_decorator


# decorator setting key value each for cls attributes;
# check_attributes accept kwargs : key is the attr name of Descriptor, value is the name value of Deecriptor
@check_attributes(name=SizedString(size=8), shares=UnsignedInteger, price=UnsignedFloat)
class Stock:
    def __init__(self, name, shares, price):
        self.name = name
        self.shares = shares
        self.price = price
s2 = Stock('s2', 22, 10.8)
s2.name


    

    
Out[22]:





<__main__.SizedString at 0x564fc88>