Max added the comment: I agree about reflected operation, although the wording could be clearer ("will try reflected operation" is better worded as "will return the result of the reflected operation called on the swapped arguments".) But what does it mean "or some other fallback"? And what if the reflected operation or the fallback again return NotImplemented or is actually not implemented. Is it somewhere else in the docs? ---------- _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue15997> _______________________________________

Martin v. Löwis added the comment: The main point is: it depends on the operation. NotImplemented is a way to signal that an operation is not implemented. It can be used for whatever you want to use it for. You can design to call an operation "foo", and, if NotImplemented is returned, call "bar" instead. If you want to know how a specific operation performs its fallback, you have to look in the documentation of the specific operation. As an example for a method where some other fallback is used, see http://docs.python.org/py3k/library/abc.html#abc.ABCMeta.__subclasshook__ ---------- _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue15997> _______________________________________

Terry J. Reedy added the comment: The first three sentences are fine. The problem I have is with the 4th: 'may return' is rather vague. Such methods may raise TypeError instead (the old way), seemingly to the same effect. (See msg170936 in issue #12067, which is about clarifying the comparisons section of the expression chapter.). So the 5th sentence could be misinterpreted wrongly to mean that NotImplemented is needed to get the alternate method try. (Since it is not, I wonder why it was added, since it complicates the internal logic of arithmetic and comparison ops.) Perhaps a better place for any clarification on this point would be in 3.3 in the entry for __le__, etc. ---------- nosy: +terry.reedy _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue15997> _______________________________________

Terry J. Reedy added the comment: I was wrong about TypeError; forget that. The seeming equivalent to 'return NotImplement' is not defining the method. My confusion stemed from the fact that NotImplemented returned from special methods gets intercepted and converted to TypeError exceptions by the operator functions that call them -- unless there is an alternate path *and* the alternate does not also return NotImplemented. Changing "will try reflected operation" to "will return the result of the reflected operation called on the swapped arguments" is wrong as the latter is not completely correct. If the fallback also returns NotImplemented, it is ignored and TypeError is raised the same as if the fallback did not exist. But as Martin said, the details belong in operator documentation. I would like to more closely parallel the None entry, which has "It is used to signify the absence of a value in many situations, e.g. it is returned from functions that don’t explicitly return anything." by replacing "Numeric methods and rich comparison methods may return this value if they do not implement the operation for the operands provided. (The interpreter will then try the reflected operation, or some other fallback, depending on the operator.)" with "It is used to signify that a method does not implement the operation requested for the operands provided. For example, the special methods for arithmetic and and rich comparison may return NotImplemented, and when they do, the interpreter will try the reversed or reflected operation." ---------- _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue15997> _______________________________________