Modifying class __dict__ when shadowed by a property

Question

I am attempting to modify a value in a class __dict__ directly using something like X.__dict__[\'x\'] += 1. It is impossible to do the modification

Answer 1

My main requirement is to have a class attribute that appears to be read only and does not use additional names anywhere. I am not absolutely hung up on the idea of using a metaclass property with an eponymous class dict entry, but that is usually how I hide read only values in regular instances.

What you are asking for is a contradiction: If your example worked, then __class__.__dict__['x'] would be an "additional name" for the attribute. So clearly we need a more specific definition of "additional name." But to come up with that definition, we need to know what you are trying to accomplish (NB: The following goals are not mutually exclusive, so you may want to do all of these things):

• You want to make the value completely untouchable, except within the Class.__init__() method (and the same method of any subclasses): This is unPythonic and quite impossible. If __init__() can modify the value, then so can anyone else. You might be able to accomplish something like this if the modifying code lives in Class.__new__(), which the metaclass dynamically creates in Meta.__new__(), but that's extremely ugly and hard to understand.
• You want the code that manipulates the value to be "nicely encapsulated": Write a method in the metaclass that increments the private value (or does whatever other modification you need), and provide a read-only metaclass property that accesses it under the public name.
• You are concerned about a subclass accidentally clashing names with the private name: Prefix the private name with a double underscore to invoke automatic name mangling. While this is usually seen as a bit unPythonic, it is appropriate for cases where name collisions may be less obvious to subclass authors, such as the internal names of a metaclass colliding with the internal names of a regular class instantiated from it.