ast wrote at 2020-4-21 14:27 +0200: > From python version 3.6, there is a useful new method >__set_name__ which could be implemented in descriptors. > >This method is called after a descriptor is instantiated >in a class. Parameter "name" is filled with the name >of the attribute refering to the descriptor in the class
> ... >But I discovered that __set_name__ is not called immediately >after the execution of line "value = Descriptor()" but after >all class code execution ... I recently read the Python (3.9a5) documentation - and there I found clearly expressed this behaviour (I no longer can tell you exactly where I read this). The reason comes from the implementation: when the descriptor is instantiated, the name the instance is bound to is not yet known. Class creation is complex in Python (details in the Python documentation): first a so called "namespace" (a mapping from names to values) is constructed; after this construction, the so called metaclass is called with class name, bases, the constructed namespace and potentially additional keyword arguments. It is the typical metaclass (`type`) which calls `__set_name__` for descritors it finds in the namespace. This has several ramifications: * `__set_name__` is not called "inline" (as you observed) * `__set_name__` is not called automatically for descriptors added after class construction * `__set_name_` may not be called at all if the class uses a custom metaclass. -- https://mail.python.org/mailman/listinfo/python-list
