How to determine if a Python class attribute is read only/writeable?

Question

I'm trying to use inspect.getmembers() to list all the writeable attributes of a class. Is there a way to query if a class attribute is readonly or writeable?

Edit: I'm using the first solution from https://stackoverflow.com/a/3818861/2746401 and get an error when trying to copy read-only attributes (e.g. __doc__). I want to filter our these read-only properties and only copy the writeable ones

Answer 1

Most names will be settable by default. The exceptions are the data descriptor objects and classes that define the __setattr__ hook.

A data descriptor is any attribute of the class that has a __get__ method and at least either a __set__ or a __delete__ method. These methods are used when you try to set or delete the attribute on an instance of the class.

Data descriptors with a __set__ method may not be writeable, and it'll depend on the exact descriptor implementation what they'll allow. A property object is such a data descriptor; if the property has a fset attribute set to None it won't be writeable. If property().fset is not set to None but to a callable, then that callable determines if the attribute is writeable.

In addition, any data descriptor with only a __delete__ method can't be written either, the lack of a __set__ method results in a AttributeError when you try to set the attribute name on the instance.

A __setattr__ hook intercepts all attribute setting, and can prevent any such action to fail. There is a corresponding __delattr__ hook too, which matters if you want to count deleting attributes as writing.

In other words, this is really hard to determine, not in a generic way, once descriptors or a __setattr__ hook are involved.