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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Conflation of NotImplemented and NotImplementedError","articleBody":"The contract for binary methods such as `__add__`, `__eq__`, etc. is such that they are expected to return `NotImplemented` error to signal lack of support for a particular type, in which case the interpreter will try flipping the operands, and finally fall back to object comparison (in the case of `==`).\r\n\r\nI believe that `VirtualMachine::call_or_unsupported` is `RustPython`'s implementation of this behavior, but instead of using the `NotImplemented` built-in constant, it expects implementors to raise a `NotImplementedError`. This can be seen by calling the magic methods directly:\r\n\r\nRustPython:\r\n```\r\n\u003e\u003e\u003e\u003e\u003e (3).__sub__('')\r\nTraceback (most recent call last):\r\n  File \u003cunknown\u003e, line 0, in \u003cmodule\u003e\r\nNotImplementedError: Cannot substract 'int' object and 'str' object\r\n```\r\n\r\nCPython:\r\n```\r\n\u003e\u003e\u003e (3).__sub__('')\r\nNotImplemented\r\n```\r\n\r\nThis was discovered in #410.","author":{"url":"https://github.com/OddCoincidence","@type":"Person","name":"OddCoincidence"},"datePublished":"2019-02-09T18:16:05.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":2},"url":"https://github.com/419/RustPython/issues/419"}