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Computed Attributes
As was the case when using properties, our first descriptor example of the prior section didn’t do much—it simply printed trace messages for attribute accesses. In practice, descriptors can also be used to compute attribute values each time they are fetched. The following illustrates—it’s a rehash of the same example we coded for properties, which uses a descriptor to automatically square an attribute’s value each time it is fetched:
class DescSquare:
def __init__(self, start): # Each desc has own state
self.value = start
def __get__(self, instance, owner): # On attr fetch
return self.value ** 2
def __set__(self, instance, value): # On attr assign
self.value = value # No delete or docs
class Client1:
X = DescSquare(3) # Assign descriptor instance to class attr
class Client2:
X = DescSquare(32) # Another instance in another client class
# Could also code 2 instances in same class
c1 = Client1()
c2 = Client2()
print(c1.X) # 3 ** 2
c1.X = 4
print(c1.X) # 4 ** 2
print(c2.X) # 32 ** 2
When run, the output of this example is the same as that of the original property-based version, but here a descriptor class object is intercepting the attribute accesses:
9
16
1024
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