descriptors0_delete0.py

"""An example of a descriptor with a ``__delete__()`` method.
The code is for illustration purposes only, and it does not correspond to any
actual implementation.
"""


class ProtectedAttribute:
    """A class attribute that can be protected against deletion"""

    def __set_name__(self, owner, name):
        self.name = name

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

    def __delete__(self, instance):
        raise AttributeError(f"Can't delete {self.name} for {instance!s}")


class ProtectedUser:
    """
    >>> usr = ProtectedUser('jsmith', '127.0.0.1')
    >>> usr.username
    'jsmith'
    >>> del usr.username
    Traceback (most recent call last):
    ...
    AttributeError: Can't delete username for ProtectedUser[jsmith]
    >>> usr.location
    '127.0.0.1'
    >>> del usr.location
    >>> usr.location
    Traceback (most recent call last):
    ...
    AttributeError: 'ProtectedUser' object has no attribute 'location'
    """
    username = ProtectedAttribute()

    def __init__(self, username, location):
        self.username = username
        self.location = location

    def __str__(self):
        return f"{self.__class__.__name__}[{self.username}]"


if __name__ == '__main__':
    import doctest
    doctest.testmod()

descriptors0_get0.py

class Resolution:
    """Represents the resolution for a video display. In case there is no
    resolution set, return a default value, previously indicated.
    """
    def __init__(self, attr_name, default_resolution):
        self.attr_name = attr_name
        self.default_resolution = default_resolution

    def __get__(self, instance, owner):
        if instance is None:
            return self
        return self.default_resolution


class VideoDriver:
    """Contains multiple display devices, each one with a resolution
    configured. If a resolution is not set for a device, return a default one,
    provided by this class, as a fallback.

    >>> media = VideoDriver()
    >>> media.tv
    (1024, 768)
    >>> media.tv = (4096, 2160)
    >>> media.tv
    (4096, 2160)
    >>> del media.tv
    >>> media.tv
    (1024, 768)
    >>> media.screen
    (1920, 1080)
    >>> VideoDriver.tv  # doctest: +ELLIPSIS
    <__main__.Resolution object at 0x...>
    """
    tv = Resolution('tv', (1024, 768))
    screen = Resolution('screen', (1920, 1080))


if __name__ == '__main__':
    import doctest
    doctest.testmod()

descriptors0_set0.py

class TracedProperty:
    """Keep count of how many times an attribute changed its value"""

    def __set_name__(self, owner, name):
        self.name = name
        self.count_name = f'count_{name}'

    def __set__(self, instance, value):
        try:
            current_value = instance.__dict__[self.name]
        except KeyError:
            instance.__dict__[self.count_name] = 0
        else:
            if current_value != value:
                instance.__dict__[self.count_name] += 1

        instance.__dict__[self.name] = value


class Traveller:
    """
    >>> tourist = Traveller('John Smith')
    >>> tourist.city = 'Barcelona'
    >>> tourist.country = 'Spain'

    >>> tourist.count_city
    0
    >>> tourist.count_country
    0

    >>> tourist.city = 'Stockholm'
    >>> tourist.country = 'Sweden'
    >>> tourist.count_city
    1
    >>> tourist.count_country
    1
    >>> tourist.city = 'Gothenburg'
    >>> tourist.count_city
    2
    >>> tourist.count_country
    1
    >>> tourist.country = 'Sweden'
    >>> tourist.count_country
    1
    """
    city = TracedProperty()
    country = TracedProperty()

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


if __name__ == '__main__':
    import doctest
    doctest.testmod()