Replace an attribute of an instance with a property

Question

I would like to override an attribute with a property, but only on one instance. Here is how I try to solve it, but it does not do what I would like to:

class Foo:
    def __init__(self):
        self.bar = True
        self._hidden = False

a = Foo()
print(a.bar)
a.bar = property(lambda self: self._hidden)
print(a.bar)


>>> C:\Users\Ilya\AppData\Local\Programs\Python\Python37-32\python.exe C:/Users/Ilya/pydolons/experiment.py
>>> True
>>> <property object at 0x0323B960>

Can it be done? what is the actual mechanics of dunder calls that enable class property, but not the one which is set on the instance?

Following code does what I want to do, but it modifies the class:

class Foo:
    def __init__(self):
        self.bar = True
        self._hidden = False

a = Foo()
print(a.bar)
Foo.bar = property(lambda self: self._hidden)
print(a.bar)

>>> C:\Users\Ilya\AppData\Local\Programs\Python\Python37-32\python.exe C:/Users/Ilya/pydolons/experiment.py
>>> True
>>> False

L3viathan · Accepted Answer · 2018-12-12 13:27:08Z

6

Can it be done?

It can not.

A property is a descriptor, and descriptors can only be declared on the class level.

Allowing instance descriptors was proposed, but rejected.

If you are fine with migrating this instance to a different class, you could do that:

def attach_property(instance, key, prop):
    class Derived(instance.__class__):
        pass
    setattr(Derived, key, prop)
    instance.__class__ = Derived

which you could then use like this:

a = Foo()
print(a.bar)
attach_property(a, "bar", property(lambda self: self._hidden))
print(a.bar)

edited Dec 12, 2018 at 13:27

answered Dec 12, 2018 at 13:19

L3viathan

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5 Comments

ikamen Over a year ago

can I dynamically craft a copy of the class and put it there not to affect other instances?

L3viathan Over a year ago

You can have those instances be instances of a different class, and classes can be dynamically created (e.g. as subclasses of the desired class), so.. I guess?

L3viathan Over a year ago

@ikamen I don't know if I understood what you meant, but I put a suggestion in my edit.

ikamen Over a year ago

awesome! I believe it will even pass the isinstance() check, any drawbacks you know to this solution?

L3viathan Over a year ago

@ikamen Apart from the surprise that the instance is no longer actually the same type as other instances, and the general weirdness of the solution, not really.

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