Monkey patching a @property

Question

Is it at all possible to monkey patch the value of a @property of an instance of a class that I do not control?

class Foo:
    @property
    def bar         


        

Answer 1

It looks like you need to move on from properties to the realms of data descriptors and non-data descriptors. Properties are just a specialised version of data descriptors. Functions are an example of non-data descriptors -- when you retrieve them from an instance they return a method rather than the function itself.

A non-data descriptor is just an instance of a class that has a __get__ method. The only difference with a data descriptor is that it has a __set__ method as well. Properties initially have a __set__ method that throws an error unless you provide a setter function.

You can achieve what you want really easily just by writing your own trivial non-data descriptor.

class nondatadescriptor:
    """generic nondata descriptor decorator to replace @property with"""
    def __init__(self, func):
        self.func = func

    def __get__(self, obj, objclass):
        if obj is not None:
            # instance based access
            return self.func(obj)
        else:
            # class based access
            return self

class Foo:
    @nondatadescriptor
    def bar(self):
        return "baz"

foo = Foo()
another_foo = Foo()

assert foo.bar == "baz"
foo.bar = 42
assert foo.bar == 42
assert another_foo.bar == "baz"
del foo.bar
assert foo.bar == "baz"
print(Foo.bar)

What makes all this work is that logic under the hood __getattribute__. I can't find the appropriate documentation at the moment, but order of retrieval is:

1. Data descriptors defined on the class are given the highest priority (objects with both __get__ and __set__), and their __get__ method is invoked.
2. Any attribute of the object itself.
3. Non-data descriptors defined on the class (objects with only a __get__ method).
4. All other attributes defined on the class.
5. Finally the __getattr__ method of the object is invoked as a last resort (if defined).