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Computed Attributes

The example in the prior section simply traces attribute accesses. Usually, though, properties do much more—computing the value of an attribute dynamically when fetched, for example. The following example illustrates:

class PropSquare:
    def __init__(self, start):
        self.value = start
    def getX(self):                         # On attr fetch
        return self.value ** 2
    def setX(self, value):                  # On attr assign
        self.value = value
    X = property(getX, setX)                # No delete or docs

P = PropSquare(3)       # 2 instances of class with property
Q = PropSquare(32)      # Each has different state information

print(P.X)              # 3 ** 2
P.X = 4
print(P.X)              # 4 ** 2
print(Q.X)              # 32 ** 2

This class defines an attribute X that is accessed as though it were static data, but really runs code to compute its value when fetched. The effect is much like an implicit method call. When the code is run, the value is stored in the instance as state information, but each time we fetch it via the managed attribute, its value is automatically squared:

9
16
1024

Notice that we’ve made two different instances—because property methods automatically receive a self argument, they have access to the state information stored in instances. In our case, this mean the fetch computes the square of the subject instance’s data.