5.6. Protocol Descriptor

5.6.1. Rationale

  • Add managed attributes to objects

  • Outsource functionality into specialized classes

  • Descriptors: classmethod, staticmethod, property, functions in general

  • __del__(self) is reserved when object is being deleted by garbage collector (destructor)

  • __set_name() After class creation, Python default metaclass will call it with parent and classname

5.6.2. Protocol

  • __get__(self, parent, *args) -> self

  • __set__(self, parent, value) -> None

  • __delete__(self, parent) -> None

  • __set_name__(self)

If any of those methods are defined for an object, it is said to be a descriptor.

—Raymond Hettinger

>>> class Descriptor:
...     def __get__(self, parent, *args):
...         return ...
...
...     def __set__(self, parent, value):
...         ...
...
...     def __delete__(self, parent):
...         ...
...
...     def __set_name__(self, parent, classname):
...         ...

5.6.3. Property vs Reflection vs Descriptor

Property:

>>> class Temperature:
...     kelvin = property()
...     _value: float
...
...     @kelvin.setter
...     def myattribute(self, value):
...         if value < 0:
...             raise ValueError
...         else:
...             self._value = value

Reflection:

>>> class Temperature:
...     kelvin: float
...
...     def __setattr__(self, attrname, value):
...         if attrname == 'kelvin' and value < 0:
...             raise ValueError
...         else:
...             super().__setattr__(attrname, value)

Descriptor:

>>> class Kelvin:
...     def __set__(self, parent, value):
...         if value < 0:
...             raise ValueError
...         else:
...             parent._value = value
>>>
>>>
>>> class Temperature:
...     kelvin = Kelvin()
...     _value: float

5.6.4. Example

>>> class MyField:
...     def __get__(self, parent, *args):
...         print('Getter')
...
...     def __set__(self, parent, value):
...         print('Setter')
...
...     def __delete__(self, parent):
...         print('Deleter')
>>>
>>>
>>> class MyClass:
...     value = MyField()
>>>
>>>
>>> my = MyClass()
>>>
>>> my.value = 'something'
Setter
>>>
>>> my.value
Getter
>>>
>>> del my.value
Deleter

5.6.5. Use Case - Kelvin Temperature Validator

>>> class KelvinValidator:
...     def __set__(self, parent, value):
...         if value < 0.0:
...             raise ValueError('Cannot set negative Kelvin')
...         parent._value = value
>>>
>>>
>>> class Temperature:
...     kelvin = KelvinValidator()
...
...     def __init__(self):
...         self._value = None
>>>
>>>
>>> t = Temperature()
>>> t.kelvin = -1
Traceback (most recent call last):
ValueError: Cannot set negative Kelvin

5.6.6. Use Case - Temperature Conversion

>>> class Kelvin:
...     def __get__(self, parent, *args):
...         return round(parent._value, 2)
...
...     def __set__(self, parent, value):
...         parent._value = value
>>>
>>>
>>> class Celsius:
...     def __get__(self, parent, *args):
...         value = parent._value - 273.15
...         return round(value, 2)
...
...     def __set__(self, parent, value):
...         parent._value = value + 273.15
>>>
>>>
>>> class Fahrenheit:
...     def __get__(self, parent, *args):
...         value = (parent._value - 273.15) * 9 / 5 + 32
...         return round(value, 2)
...
...     def __set__(self, parent, fahrenheit):
...         parent._value = (fahrenheit - 32) * 5 / 9 + 273.15
>>>
>>>
>>> class Temperature:
...     kelvin = Kelvin()
...     celsius = Celsius()
...     fahrenheit = Fahrenheit()
...
...     def __init__(self):
...         self._value = 0.0
>>>
>>>
>>> t = Temperature()
>>>
>>> t.kelvin = 273.15
>>> print(t.kelvin)
273.15
>>> print(t.celsius)
0.0
>>> print(t.fahrenheit)
32.0
>>>
>>> t.fahrenheit = 100
>>> print(t.kelvin)
310.93
>>> print(t.celsius)
37.78
>>> print(t.fahrenheit)
100.0
>>>
>>> t.celsius = 100
>>> print(t.kelvin)
373.15
>>> print(t.celsius)
100.0
>>> print(t.fahrenheit)
212.0

5.6.7. Use Case - Value Range Descriptor

>>> class Value:
...     MIN: int
...     MAX: int
...     name: str
...     value: float
...
...     def __init__(self, min, max):
...         self.MIN = min
...         self.MAX = max
...
...     def __set__(self, instance, value):
...         if self.MIN <= value < self.MAX:
...             self.value = value
...         else:
...             raise ValueError(f'{self.name} ({value}) is not in range({self.MIN}, {self.MAX})')
...
...     def __get__(self, instance, owner):
...         return self.value
...
...     def __delete__(self, instance):
...         raise PermissionError
...
...     def __set_name__(self, owner, name):
...         self.name = name
>>>
>>>
>>> class KelvinTemperature:
...     kelvin = Value(min=0, max=99999)
...     celsius = Value(min=-273.15, max=99999)
>>>
>>>
>>> t = KelvinTemperature()
>>>
>>> t.kelvin = 10
>>> t.kelvin = -1
Traceback (most recent call last):
ValueError: kelvin (-1) is not in range(0, 99999)
>>>
>>> t.celsius = -273
>>> t.celsius = -274
Traceback (most recent call last):
ValueError: celsius (-274) is not in range(-273.15, 99999)
>>>
>>> print(t.kelvin)
10
>>> print(t.celsius)
-273

Note __repr__() method and how to access Descriptor value.

>>> from dataclasses import dataclass
>>>
>>>
>>> @dataclass
... class ValueRange:
...     name: str
...     min: float
...     max: float
...     value: float = None
...
...     def __set__(self, parent, value):
...         if value not in range(self.min, self.max):
...             raise ValueError(f'{self.name} is not between {self.min} and {self.max}')
...         self.value = value
>>>
>>>
>>> class Astronaut:
...     name: str
...     age = ValueRange('Age', min=28, max=42)
...     height = ValueRange('Height', min=150, max=200)
...
...     def __init__(self, name, age, height):
...         self.name = name
...         self.height = height
...         self.age = age
...
...     def __repr__(self):
...         name = self.name
...         age = self.age.value
...         height = self.height.value
...         return f'Astronaut({name=}, {age=}, {height=})'
>>>
>>>
>>> Astronaut('Mark Watney', age=38, height=170)
Astronaut(name='Mark Watney', age=38, height=170)
>>>
>>> Astronaut('Melissa Lewis', age=44, height=170)
Traceback (most recent call last):
ValueError: Age is not between 28 and 42
>>>
>>> Astronaut('Rick Martinez', age=38, height=210)
Traceback (most recent call last):
ValueError: Height is not between 150 and 200
>>> from dataclasses import dataclass
>>>
>>>
>>> @dataclass
... class ValueRange:
...     name: str
...     min: int
...     max: int
...
...     def __set__(self, instance, value):
...         print(f'Setter: {self.name} -> {value}')
>>>
>>>
>>> class Point:
...     x = ValueRange('x', 0, 10)
...     y = ValueRange('y', 0, 10)
...     z = ValueRange('z', 0, 10)
...
...     def __init__(self, x, y, z):
...         self.x = x
...         self.y = y
...         self.z = z
...
...     def __setattr__(self, attrname, value):
...         print(f'Setattr: {attrname} -> {value}')
...         super().__setattr__(attrname, value)
>>>
>>>
>>> p = Point(1,2,3)
Setattr: x -> 1
Setter: x -> 1
Setattr: y -> 2
Setter: y -> 2
Setattr: z -> 3
Setter: z -> 3
>>>
>>> p.notexisting = 1337
Setattr: notexisting -> 1337

5.6.8. Inheritance

from typing import Union


class Validator:
    attribute_name: str

    def __set_name__(self, parent, attribute_name):
        self.attribute_name = f'_{attribute_name}'

    def __get__(self, parent, parent_type):
        return getattr(parent, self.attribute_name)

    def __delete__(self, parent):
        setattr(parent, self.attribute_name, None)


class RangeValidator(Validator):
    min: Union[int, float]
    max: Union[int, float]

    def __init__(self, min: float, max: float):
        self.min = min
        self.max = max

    def __set__(self, parent, newvalue):
        if self.min <= newvalue < self.max:
            setattr(parent, self.attribute_name, newvalue)
        else:
            attr = f'{parent.__class__.__name__}.{self.attribute_name.removeprefix("_")}'
            min = self.min
            max = self.max
            raise ValueError(f'{attr} value: {newvalue} is out of range {min=}, {max=}')


class Astronaut:
    firstname: str
    lastname: str
    age: int = RangeValidator(min=27, max=50)
    height: float = RangeValidator(min=150, max=200)
    weight: float = RangeValidator(min=50, max=90)
    _firstname: str
    _lastname: str
    _age: int
    _height: float
    _weight: float


astro = Astronaut()

5.6.9. Use Case - Timezone Converter Descriptor (pytz)

../../_images/protocol-descriptor-timezone.png

Figure 5.6. Comparing datetime works only when all has the same timezone (UTC). More information in Stdlib Datetime Timezone

Descriptor Timezone Converter:

>>> from dataclasses import dataclass
>>> from datetime import datetime
>>> from pytz import timezone
>>>
>>>
>>> class Timezone:
...     def __init__(self, name):
...         self.timezone = timezone(name)
...
...     def __get__(self, parent, *args):
...         return parent.utc.astimezone(self.timezone)
...
...     def __set__(self, parent, new_datetime):
...         local_time = self.timezone.localize(new_datetime)
...         parent.utc = local_time.astimezone(timezone('UTC'))
>>>
>>>
>>> @dataclass
... class Time:
...     utc = datetime.now(tz=timezone('UTC'))
...     warsaw = Timezone('Europe/Warsaw')
...     moscow = Timezone('Europe/Moscow')
...     eastern = Timezone('America/New_York')
...     pacific = Timezone('America/Los_Angeles')
>>>
>>>
>>> t = Time()
>>>
>>> # Launch of a first man to space
>>> t.moscow = datetime(1961, 4, 12, 9, 6, 59)
>>> print(t.utc)
1961-04-12 06:06:59+00:00
>>> print(t.warsaw)
1961-04-12 07:06:59+01:00
>>> print(t.moscow)
1961-04-12 09:06:59+03:00
>>> print(t.eastern)
1961-04-12 01:06:59-05:00
>>> print(t.pacific)
1961-04-11 22:06:59-08:00
>>>
>>> # First man set foot on a Moon
>>> t.warsaw = datetime(1969, 7, 21, 3, 56, 15)
>>> print(t.utc)
1969-07-21 02:56:15+00:00
>>> print(t.warsaw)
1969-07-21 03:56:15+01:00
>>> print(t.moscow)
1969-07-21 05:56:15+03:00
>>> print(t.eastern)
1969-07-20 22:56:15-04:00
>>> print(t.pacific)
1969-07-20 19:56:15-07:00

5.6.10. Use Case - Timezone Converter Descriptor (zoneinfo)

../../_images/protocol-descriptor-timezone.png

Figure 5.7. Comparing datetime works only when all has the same timezone (UTC). More information in Stdlib Datetime Timezone

>>> from dataclasses import dataclass
>>> from datetime import datetime
>>> from zoneinfo import ZoneInfo
>>>
>>>
>>> class Timezone:
...     def __init__(self, name):
...         self.timezone = ZoneInfo(name)
...
...     def __get__(self, parent, *args):
...         return parent.utc.astimezone(self.timezone)
...
...     def __set__(self, parent, new_datetime):
...         local_time = new_datetime.replace(tzinfo=self.timezone)
...         parent.utc = local_time.astimezone(ZoneInfo('UTC'))
>>>
>>>
>>> @dataclass
... class Time:
...     utc = datetime.now(tz=ZoneInfo('UTC'))
...     warsaw = Timezone('Europe/Warsaw')
...     moscow = Timezone('Europe/Moscow')
...     eastern = Timezone('America/New_York')
...     pacific = Timezone('America/Los_Angeles')
>>>
>>>
>>> t = Time()
>>>
>>>pacificeasternt.utc = datetime(1961, 4, 12, 6, 7)  # Gagarin's launch to space
>>> print(t.utc)
1961-04-12 06:07:00
>>> print(t.moscow)
1961-04-12 09:07:00+03:00
>>> print(t.warsaw)
1961-04-12 07:07:00+01:00
>>> print(t.eastern)
1961-04-12 01:07:00-05:00
>>> print(t.pacific)
1961-04-11 22:07:00-08:00
>>>
>>>
>>> t.warsaw = datetime(1969, 7, 21, 3, 56, 15)  # Armstrong's first Lunar step
>>> print(t.utc)
1969-07-21 02:56:15+00:00
>>> print(t.warsaw)
1969-07-21 03:56:15+01:00
>>> print(t.moscow)
1969-07-21 05:56:15+03:00
>>> print(t.eastern)
1969-07-20 22:56:15-04:00
>>> print(t.pacific)
1969-07-20 19:56:15-07:00

5.6.11. Function Descriptor

>>> def hello():
...     pass
>>>
>>>
>>> type(hello)
<class 'function'>
>>> hasattr(hello, '__get__')
True
>>> hasattr(hello, '__set__')
False
>>> hasattr(hello, '__delete__')
False
>>> hasattr(hello, '__set_name__')
False
>>> dir(hello)  
['__annotations__', '__call__', '__class__', '__closure__', '__code__', '__defaults__',
 '__delattr__', '__dict__', '__dir__', '__doc__', '__eq__', '__format__', '__ge__',
 '__get__', '__getattribute__', '__globals__', '__gt__', '__hash__', '__init__',
 '__init_subclass__', '__kwdefaults__', '__le__', '__lt__', '__module__', '__name__',
 '__ne__', '__new__', '__qualname__', '__reduce__', '__reduce_ex__', '__repr__',
 '__setattr__', '__sizeof__', '__str__', '__subclasshook__']
>>> class Astronaut:
...     def hello(self):
...         pass
>>>
>>> type(Astronaut.hello)
<class 'function'>
>>> hasattr(Astronaut.hello, '__get__')
True
>>> hasattr(Astronaut.hello, '__set__')
False
>>> hasattr(Astronaut.hello, '__delete__')
False
>>> hasattr(Astronaut.hello, '__set_name__')
False
>>> dir(Astronaut.hello)  
['__annotations__', '__call__', '__class__', '__closure__', '__code__', '__defaults__',
 '__delattr__', '__dict__', '__dir__', '__doc__', '__eq__', '__format__', '__ge__',
 '__get__', '__getattribute__', '__globals__', '__gt__', '__hash__', '__init__',
 '__init_subclass__', '__kwdefaults__', '__le__', '__lt__', '__module__', '__name__',
 '__ne__', '__new__', '__qualname__', '__reduce__', '__reduce_ex__', '__repr__',
 '__setattr__', '__sizeof__', '__str__', '__subclasshook__']
>>> class Astronaut:
...     def hello(self):
...         pass
>>>
>>> astro = Astronaut()
>>>
>>> type(astro.hello)
<class 'method'>
>>> hasattr(astro.hello, '__get__')
True
>>> hasattr(astro.hello, '__set__')
False
>>> hasattr(astro.hello, '__delete__')
False
>>> hasattr(astro.hello, '__set_name__')
False
>>> dir(astro.hello)  
['__call__', '__class__', '__delattr__', '__dir__', '__doc__', '__eq__', '__format__',
 '__func__', '__ge__', '__get__', '__getattribute__', '__gt__', '__hash__', '__init__',
 '__init_subclass__', '__le__', '__lt__', '__ne__', '__new__', '__reduce__', '__reduce_ex__',
 '__repr__', '__self__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__']

5.6.12. Assignments

Code 5.67. Solution
"""
* Assignment: Protocol Descriptor Simple
* Complexity: easy
* Lines of code: 9 lines
* Time: 13 min

English:
    1. Define descriptor class `Kelvin`
    2. Temperature must always be positive
    3. Use descriptors to check boundaries at each value modification
    4. All tests must pass
    5. Run doctests - all must succeed

Polish:
    1. Zdefiniuj klasę deskryptor `Kelvin`
    2. Temperatura musi być zawsze być dodatnia
    3. Użyj deskryptorów do sprawdzania zakresów przy każdej modyfikacji
    4. Wszystkie testy muszą przejść
    5. Uruchom doctesty - wszystkie muszą się powieść

Tests:
    >>> import sys; sys.tracebacklimit = 0

    >>> class Temperature:
    ...     kelvin = Kelvin()

    >>> t = Temperature()
    >>> t.kelvin = 1
    >>> t.kelvin
    1
    >>> t.kelvin = -1
    Traceback (most recent call last):
    ValueError: Negative temperature
"""


Code 5.68. Solution
"""
* Assignment: Protocol Descriptor ValueRange
* Complexity: easy
* Lines of code: 9 lines
* Time: 13 min

English:
    1. Define descriptor class `ValueRange` with attributes:
        a. `name: str`
        b. `min: float`
        c. `max: float`
        d. `value: float`
    2. Define class `Astronaut` with attributes:
        a. `age = ValueRange('Age', min=28, max=42)`
        b. `height = ValueRange('Height', min=150, max=200)`
    3. Setting `Astronaut` attribute should invoke boundary check of `ValueRange`
    4. Run doctests - all must succeed

Polish:
    1. Zdefiniuj klasę-deskryptor `ValueRange` z atrybutami:
        a. `name: str`
        b. `min: float`
        c. `max: float`
        d. `value: float`
    2. Zdefiniuj klasę `Astronaut` z atrybutami:
        a. `age = ValueRange('Age', min=28, max=42)`
        b. `height = ValueRange('Height', min=150, max=200)`
    3. Ustawianie atrybutu `Astronaut` powinno wywołać sprawdzanie zakresu z `ValueRange`
    6. Uruchom doctesty - wszystkie muszą się powieść

Tests:
    >>> import sys; sys.tracebacklimit = 0

    >>> mark = Astronaut('Mark Watney', 36, 170)

    >>> melissa = Astronaut('Melissa Lewis', 44, 170)
    Traceback (most recent call last):
    ValueError: Age is not between 28 and 42

    >>> alex = Astronaut('Alex Vogel', 40, 201)
    Traceback (most recent call last):
    ValueError: Height is not between 150 and 200
"""

class ValueRange:
    name: str
    min: float
    max: float
    value: float

    def __init__(self, name, min, max):
        pass


class Astronaut:
    age = ValueRange('Age', min=28, max=42)
    height = ValueRange('Height', min=150, max=200)


Code 5.69. Solution
"""
* Assignment: Protocol Descriptor Inheritance
* Complexity: medium
* Lines of code: 25 lines
* Time: 21 min

English:
    1. Define class `GeographicCoordinate`
    2. Use descriptors to check value boundaries
    3. Run doctests - all must succeed

Polish:
    1. Zdefiniuj klasę `GeographicCoordinate`
    2. Użyj deskryptory do sprawdzania wartości brzegowych
    3. Uruchom doctesty - wszystkie muszą się powieść

Tests:
    >>> import sys; sys.tracebacklimit = 0

    >>> place1 = GeographicCoordinate(50, 120, 8000)
    >>> place1
    Latitude: 50, Longitude: 120, Elevation: 8000

    >>> place2 = GeographicCoordinate(22, 33, 44)
    >>> place2
    Latitude: 22, Longitude: 33, Elevation: 44

    >>> place1.latitude = 1
    >>> place1.longitude = 2
    >>> place1
    Latitude: 1, Longitude: 2, Elevation: 8000

    >>> place2
    Latitude: 22, Longitude: 33, Elevation: 44

    >>> GeographicCoordinate(90, 0, 0)
    Latitude: 90, Longitude: 0, Elevation: 0
    >>> GeographicCoordinate(-90, 0, 0)
    Latitude: -90, Longitude: 0, Elevation: 0
    >>> GeographicCoordinate(0, +180, 0)
    Latitude: 0, Longitude: 180, Elevation: 0
    >>> GeographicCoordinate(0, -180, 0)
    Latitude: 0, Longitude: -180, Elevation: 0
    >>> GeographicCoordinate(0, 0, +8848)
    Latitude: 0, Longitude: 0, Elevation: 8848
    >>> GeographicCoordinate(0, 0, -10994)
    Latitude: 0, Longitude: 0, Elevation: -10994

    >>> GeographicCoordinate(-91, 0, 0)
    Traceback (most recent call last):
    ValueError: Out of bounds

    >>> GeographicCoordinate(+91, 0, 0)
    Traceback (most recent call last):
    ValueError: Out of bounds

    >>> GeographicCoordinate(0, -181, 0)
    Traceback (most recent call last):
    ValueError: Out of bounds

    >>> GeographicCoordinate(0, +181, 0)
    Traceback (most recent call last):
    ValueError: Out of bounds

    >>> GeographicCoordinate(0, 0, -10995)
    Traceback (most recent call last):
    ValueError: Out of bounds

    >>> GeographicCoordinate(0, 0, +8849)
    Traceback (most recent call last):
    ValueError: Out of bounds
"""

class GeographicCoordinate:
    def __str__(self):
        return f'Latitude: {self.latitude}, ' +\
               f'Longitude: {self.longitude}, ' +\
               f'Elevation: {self.elevation}'

    def __repr__(self):
        return self.__str__()


"""
latitude - min: -90.0, max: 90.0
longitude - min: -180.0, max: 180.0
elevation - min: -10994.0, max: 8848.0
"""