Python static property using a non-data descriptor

static_property_descriptor.py

"""
This is an example of how to create a static property using a
non-data descriptor.

https://docs.python.org/2/howto/descriptor.html

This works in python >= 2.3 including 3.x.

In this example the __get_default() method is called the first time
we need to access Widget.default. The __get_default method clobbers
the descriptor and sets the new default Widget directly on the class.
This is so subsequent calls use the same default object.

The documentation will note this, but it's important to remember that
a non-data descriptor can be overridden by the instance dictionary, but
a data descriptor (has a __set__ method) will not be overridden so odd
things may happen if you define a __set__ method.
"""
import time


class Widget(object):

    @classmethod
    def __get_default(cls):
        print('Making default')  # Should only print once
        cls.default = cls('Default {}'.format(time.time()), 'root')
        return cls.default

    # This is simply building a class and instantiating it.
    # We only need the __get__ method for a non-data descriptor.
    default = type('WidgetDefaultGetter', (object,), {
        # See: https://docs.python.org/2/reference/datamodel.html#object.__get__
        '__get__': lambda self, instance, type=None: type.__get_default()
    })()

    def __init__(self, name, parent=None):
        if not parent:
            # Here we use the getter oblivious to the fact it's a static getter
            parent = Widget.default
            # Some sanity that it can be accessed
            assert Widget.default is self.default

        self.name = name
        self.parent = parent

    def __str__(self):
        return '{} -> {}'.format(self.parent, self.name)


# Run our example

print(Widget('First  {}'.format(time.time())))
time.sleep(1)
print(Widget('Second {}'.format(time.time())))

# Output
'''
Making default
root -> Default 1546129640.722508 -> First  1546129640.7224388
root -> Default 1546129640.722508 -> Second 1546129641.722878
'''