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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Prevent an implementation for `int` class from operating on `bool` values","articleBody":"**Context.** Consider the following piece of code.\r\n\r\n```python\r\nfrom classes import typeclass\r\n\r\n@typeclass\r\ndef render(data_value) -\u003e str:\r\n    \"\"\"Pretty-print a value.\"\"\"\r\n\r\n@render.instance(int)\r\ndef _render_int(data_value: int) -\u003e str:\r\n    return f'🔢 {data_value}'\r\n\r\nrender(True) == '🔢 True'\r\n```\r\n*(this should be runnable as-is on `classes 0.4.0`.)*\r\n\r\nI would have expected this code to fail with a `NotImplementedError` because the `bool` case wasn't specified; but instead, that case is handled by the `int` implementation because\r\n\r\n```pycon\r\nIn [3]: issubclass(bool, int)\r\nOut[3]: True\r\n```\r\n\r\n**Decision.** In an `int` implementation, recognize if the provided value is actually a `bool` and refuse to process that value.\r\n\r\n**Consequences.** I have been writing in Python for quite a few years now and I might even have encountered this relationship between `int` and `bool` types before, but that is not quite a type of thing that I keep in my short-term memory to be able to instantly recognize.\r\n\r\nI believe this change can help avert bugs which might be hard to trace otherwise.","author":{"url":"https://github.com/anatoly-scherbakov","@type":"Person","name":"anatoly-scherbakov"},"datePublished":"2021-09-13T13:27:33.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":1},"url":"https://github.com/310/classes/issues/310"}