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validate.py

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class Validator:

@classmethod

def check(cls, value):

return value

class Typed(Validator):

expected_type = object

@classmethod

def check(cls, value):

if not isinstance(value, cls.expected_type):

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

return super().check(value)

class Integer(Typed):

expected_type = int

class Float(Typed):

expected_type = float

class String(Typed):

expected_type = str

class Positive(Validator):

@classmethod

def check(cls, value):

if value < 0:

raise ValueError('Must be >= 0')

return super().check(value)

class NonEmpty(Validator):

@classmethod

def check(cls, value):

if len(value) == 0:

raise ValueError('Must be non-empty')

return super().check(value)

class PositiveInteger(Integer, Positive):

pass

class PositiveFloat(Float, Positive):

pass

class NonEmptyString(String, NonEmpty):

pass

# Examples

if __name__ == '__main__':

def add(x, y):

Integer.check(x)

Integer.check(y)

return x + y

class Stock:

__slots__ = ('name','_shares','_price')

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

self.name = name

self.shares = shares

self.price = price

def __repr__(self):

return f'Stock({self.name!r}, {self.shares!r}, {self.price!r})'

@property

def shares(self):

return self._shares

@shares.setter

def shares(self, value):

self._shares = PositiveInteger.check(value)

@property

def price(self):

return self._price

@price.setter

def price(self, value):

self._price = PositiveFloat.check(value)

@property

def cost(self):

return self.shares * self.price

def sell(self, nshares):

self.shares -= nshares