Eric Snow wrote:
Guido indicates earlier in the thread that NotImplemented is used so that you know that it came from the function that you directly called, and not from another call inside that function. Why does it matter if it came directly from the function or not? And couldn't the NotImplemented still have come from a call inside the operator, rather than directly?
An exception can bubble up from several levels below the original call. Returning NotImplemented (even if returned from another internal function) still has to be deliberately done by the original method.
Later in the thread it is also stated that while exception handling performance is not too different in C code, it is very much more expensive in Python code.
As to why it would matter if whether the return was from the directly called method vs some nested method... not sure, but I would hazard a guess that by returning NotImplented the interpreter knows everything worked as it should have (assuming no bugs, of course); whereas an exception could easily _not_ have come from the directly called method. Imagine if the methods raised exceptions to signal normal behavior, and some object was buggy -- for __eq__ we'd end up with a False result, instead of getting the exception propagated. (!)
~Ethan~ -- http://mail.python.org/mailman/listinfo/python-list
