There are two different types of descriptors. Data descriptors are defined as objects that define both a
__get__() and a
__set__() method, whereas non-data descriptors only define a
__get__() method. This distinction is important when considering overrides and the namespace of an instance's dictionary. If a data descriptor and an entry in an instance's dictionary share the same name, the data descriptor will take precedence. However, if instead a non-data descriptor and an entry in an instance's dictionary share the same name, the instance dictionary's entry will take precedence.
To make a read-only data descriptor, define both get() and set() with the set() raising an AttributeError when called. Defining the set() method with an exception raising placeholder is enough to make it a data descriptor.
descr.__get__(self, obj, type=None) --> value descr.__set__(self, obj, value) --> None descr.__delete__(self, obj) --> None
An implemented example:
class DescPrinter(object): """A data descriptor that logs activity.""" _val = 7 def __get__(self, obj, objtype=None): print('Getting ...') return self._val def __set__(self, obj, val): print('Setting', val) self._val = val def __delete__(self, obj): print('Deleting ...') del self._val class Foo(): x = DescPrinter() i = Foo() i.x # Getting ... # 7 i.x = 100 # Setting 100 i.x # Getting ... # 100 del i.x # Deleting ... i.x # Getting ... # 7
Descriptor objects can allow related object attributes to react to changes automatically.
Suppose we want to model an oscillator with a given frequency (in Hertz) and period (in seconds). When we update the frequency we want the period to update, and when we update the period we want the frequency to update:
>>> oscillator = Oscillator(freq=100.0) # Set frequency to 100.0 Hz >>> oscillator.period # Period is 1 / frequency, i.e. 0.01 seconds 0.01 >>> oscillator.period = 0.02 # Set period to 0.02 seconds >>> oscillator.freq # The frequency is automatically adjusted 50.0 >>> oscillator.freq = 200.0 # Set the frequency to 200.0 Hz >>> oscillator.period # The period is automatically adjusted 0.005
We pick one of the values (frequency, in Hertz) as the "anchor," i.e. the one that can be set with no conversion, and write a descriptor class for it:
class Hertz(object): def __get__(self, instance, owner): return self.value def __set__(self, instance, value): self.value = float(value)
The "other" value (period, in seconds) is defined in terms of the anchor. We write a descriptor class that does our conversions:
class Second(object): def __get__(self, instance, owner): # When reading period, convert from frequency return 1 / instance.freq def __set__(self, instance, value): # When setting period, update the frequency instance.freq = 1 / float(value)
Now we can write the Oscillator class:
class Oscillator(object): period = Second() # Set the other value as a class attribute def __init__(self, freq): self.freq = Hertz() # Set the anchor value as an instance attribute self.freq = freq # Assign the passed value - self.period will be adjusted