Class Properties

Properties

encapsulation using the @property decorator

import iso6346


class ShippingContainer:

    next_serial = 1337  # lolz

    @staticmethod
    def _make_bic_code(owner_code, serial):
        return iso6346.create(
            owner_code=owner_code,
            serial=str(serial).zfill(6)
        )

    @classmethod
    def _get_next_serial(cls):
        result = cls.next_serial
        cls.next_serial += 1
        return result

    @classmethod
    def create_empty(cls, owner_code, *args, **kwargs):
        return cls(owner_code, contents=None, *args, **kwargs)

    @classmethod
    def create_with_items(cls, owner_code, items, *args, **kwargs):
        return cls(owner_code, contents=list(items), *args, **kwargs)

    def __init__(self, owner_code, contents):
        self.contents = contents
        self.bic = self._make_bic_code(
            owner_code=owner_code,
            serial=ShippingContainer._get_next_serial()
        )


class RefrigeratedShippingContainer(ShippingContainer):

    MAX_CELSIUS = 4.0

    @staticmethod
    def _make_bic_code(owner_code, serial):
        return iso6346.create(
            owner_code=owner_code,
            serial=str(serial).zfill(6),
            category='R'
        )

    def __init__(self, owner_code, contents, celsius):
        super().__init__(owner_code, contents)

        if celsius > RefrigeratedShippingContainer.MAX_CELSIUS:
            raise ValueError("Temperature too hot!")
        self._celsius = celsius

    @property  # celsius getter
    def celsius(self):
        return self._celsius

    @celsius.setter
    def celsius(self, value):
        if value > RefrigeratedShippingContainer.MAX_CELSIUS:
            raise ValueError("Temperature too hot!")
        self._celsius = value

Extended example

import iso6346


class ShippingContainer:

    next_serial = 1337  # lolz

    @staticmethod
    def _make_bic_code(owner_code, serial):
        return iso6346.create(
            owner_code=owner_code,
            serial=str(serial).zfill(6)
        )

    @classmethod
    def _get_next_serial(cls):
        result = cls.next_serial
        cls.next_serial += 1
        return result

    @classmethod
    def create_empty(cls, owner_code, *args, **kwargs):
        return cls(owner_code, contents=None, *args, **kwargs)

    @classmethod
    def create_with_items(cls, owner_code, items, *args, **kwargs):
        return cls(owner_code, contents=list(items), *args, **kwargs)

    def __init__(self, owner_code, contents):
        self.contents = contents
        self.bic = self._make_bic_code(
            owner_code=owner_code,
            serial=ShippingContainer._get_next_serial()
        )


class RefrigeratedShippingContainer(ShippingContainer):

    MAX_CELSIUS = 4.0

    @staticmethod
    def _make_bic_code(owner_code, serial):
        return iso6346.create(
            owner_code=owner_code,
            serial=str(serial).zfill(6),
            category='R'
        )

    @staticmethod
    def _c_to_f(celsius):
        return celsius * 9/5 + 32

    @staticmethod
    def _f_to_c(fahrenheit):
        return (fahrenheit - 32) * 5/9

    def __init__(self, owner_code, contents, celsius):
        super().__init__(owner_code, contents)
        self.celsius = celsius

    @property
    def celsius(self):
        return self._celsius

    @celsius.setter
    def celsius(self, value):
        if value > RefrigeratedShippingContainer.MAX_CELSIUS:
            raise ValueError("Temperature too hot!")
        self._celsius = value

    @property
    def fahrenheit(self):
        return RefrigeratedShippingContainer._c_to_f(self.celsius)

    @fahrenheit.setter
    def fahrenheit(self, value):
        self.celsius = RefrigeratedShippingContainer._f_to_c(value)

Properties and Inheritance

inheritance interaction with the @property decorator

import iso6346


class ShippingContainer:

    HEIGHT_FT = 8.5
    WIDTH_FT = 8.0
    next_serial = 1337

    @staticmethod
    def _make_bic_code(owner_code, serial):
        return iso6346.create(
            owner_code=owner_code,
            serial=str(serial).zfill(6)
        )

    @classmethod
    def _get_next_serial(cls):
        result = cls.next_serial
        cls.next_serial += 1
        return result

    @classmethod
    def create_empty(cls, owner_code, length_ft *args, **kwargs):
        return cls(owner_code, length_ft, contents=None, *args, **kwargs)

    @classmethod
    def create_with_items(cls, owner_code, length_ft items, *args, **kwargs):
        return cls(owner_code, length_ft, contents=list(items), *args, **kwargs)

    def __init__(self, owner_code, length_ft, contents):
        self.contents = contents
        self.length_ft = length_ft
        self.bic = self._make_bic_code(
            owner_code=owner_code,
            serial=ShippingContainer._get_next_serial()
        )

    @property
    def volume_in_cubic_feet(self):
        return ShippingContainer.HEIGHT_FT * ShippingContainer.WIDTH_FT * self.length_ft


class RefrigeratedShippingContainer(ShippingContainer):

    MAX_CELSIUS = 4.0

    FRIDGE_VOLUME_IN_CUBIC_FEET = 100

    @staticmethod
    def _make_bic_code(owner_code, serial):
        return iso6346.create(
            owner_code=owner_code,
            serial=str(serial).zfill(6),
            category='R'
        )

    @staticmethod
    def _c_to_f(celsius):
        return celsius * 9/5 + 32

    @staticmethod
    def _f_to_c(fahrenheit):
        return (fahrenheit - 32) * 5/9

    def __init__(self, owner_code, length_ft, contents, celsius):
        super().__init__(owner_code, length_ft, contents)
        self.celsius = celsius

    @property
    def celsius(self):
        return self._celsius

    @celsius.setter
    def celsius(self, value):
        if value > RefrigeratedShippingContainer.MAX_CELSIUS:
            raise ValueError("Temperature too hot!")
        self._celsius = value

    @property
    def fahrenheit(self):
        return RefrigeratedShippingContainer._c_to_f(self.celsius)

    @fahrenheit.setter
    def fahrenheit(self, value):
        self.celsius = RefrigeratedShippingContainer._f_to_c(value)

    @property
    def volume_in_cubic_feet(self):
        return super().volume_in_cubic_feet - RefrigeratedShippingContainer.FRIDGE_VOLUME_IN_CUBIC_FEET

Extended example (overriding setter)

import iso6346


class ShippingContainer:

    HEIGHT_FT = 8.5
    WIDTH_FT = 8.0
    next_serial = 1337

    @staticmethod
    def _make_bic_code(owner_code, serial):
        return iso6346.create(
            owner_code=owner_code,
            serial=str(serial).zfill(6)
        )

    @classmethod
    def _get_next_serial(cls):
        result = cls.next_serial
        cls.next_serial += 1
        return result

    @classmethod
    def create_empty(cls, owner_code, length_ft *args, **kwargs):
        return cls(owner_code, length_ft, contents=None, *args, **kwargs)

    @classmethod
    def create_with_items(cls, owner_code, length_ft items, *args, **kwargs):
        return cls(owner_code, length_ft, contents=list(items), *args, **kwargs)

    def __init__(self, owner_code, length_ft, contents):
        self.contents = contents
        self.length_ft = length_ft
        self.bic = self._make_bic_code(
            owner_code=owner_code,
            serial=ShippingContainer._get_next_serial()
        )

    @property
    def volume_in_cubic_feet(self):
        return ShippingContainer.HEIGHT_FT * ShippingContainer.WIDTH_FT * self.length_ft


class RefrigeratedShippingContainer(ShippingContainer):

    MAX_CELSIUS = 4.0

    FRIDGE_VOLUME_IN_CUBIC_FEET = 100

    @staticmethod
    def _make_bic_code(owner_code, serial):
        return iso6346.create(
            owner_code=owner_code,
            serial=str(serial).zfill(6),
            category='R'
        )

    @staticmethod
    def _c_to_f(celsius):
        return celsius * 9/5 + 32

    @staticmethod
    def _f_to_c(fahrenheit):
        return (fahrenheit - 32) * 5/9

    def __init__(self, owner_code, length_ft, contents, celsius):
        super().__init__(owner_code, length_ft, contents)
        self.celsius = celsius

    @property
    def celsius(self):
        return self._celsius

    @celsius.setter
    def celsius(self, value):
        if value > RefrigeratedShippingContainer.MAX_CELSIUS:
            raise ValueError(f"Temperature of {value} is too hot!")
        self._celsius = value

    @property
    def fahrenheit(self):
        return RefrigeratedShippingContainer._c_to_f(self.celsius)

    @fahrenheit.setter
    def fahrenheit(self, value):
        self.celsius = RefrigeratedShippingContainer._f_to_c(value)

    @property
    def volume_in_cubic_feet(self):
        return super().volume_in_cubic_feet - RefrigeratedShippingContainer.FRIDGE_VOLUME_IN_CUBIC_FEET


class HeatedRefrigeratedShippingContainer(RefrigeratedShippingContainer):

    MIN_CELSIUS = -20.0

    @RefrigeratedShippingContainer.celsius.setter
    def celsius(self, value):
        if not (HeatedRefrigeratedShippingContainer.MIN_CELSIUS
                <= value
                <= RefrigeratedShippingContainer.MAX_CELSIUS):
            raise ValueError(f"Temperature {value} is out of range!")
        self._celsius = value

Aside - chained relational operators Python allows for chaining relational operators, so that one can write a < b < c in place of (a < b) and (b < c)

Example with pitfalls


class HeatedRefrigeratedShippingContainer(RefrigeratedShippingContainer):

    MIN_CELSIUS = -20.0

    # NOTE: this doesn't work
    @RefrigeratedShippingContainer.celsius.setter
    def celsius(self, value):
        if value < HeatedRefrigeratedShippingContainer.MIN_CELSIUS:
            raise ValueError(f"Temperature {value} is out of range!")
        super().celsius = value  # throws AttributeError: 'super' object has no attribute 'celsius'

    # This works
    @RefrigeratedShippingContainer.celsius.setter
    def celsius(self, value):
        if value < HeatedRefrigeratedShippingContainer.MIN_CELSIUS:
            raise ValueError(f"Temperature {value} is out of range!")
        RefrigeratedShippingContainer.celsius.fset(self, value)

Template Method Pattern

implement skeletal operations in base classes while deferring some details to subclasses

Template Method

Made with