René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Preferred idiom for exhaustiveness checking of unions","articleBody":"* Are you reporting a bug, or opening a feature request?\r\n\r\nA question...\r\n\r\n* Please insert below the code you are checking with mypy.\r\n\r\nI have some union, e.g.\r\n\r\n```python\r\nfrom typing import Union\r\n\r\nMyUnion = Union[int, str]\r\n```\r\n\r\nand I have a function which takes the union and matches on it:\r\n\r\n```python\r\ndef handle_my_union(x: MyUnion) -\u003e None:\r\n    if isinstance(x, int): ...\r\n    elif isinstance(x, str): ...\r\n```\r\n\r\nNow suppose that the union gets added a new member:\r\n\r\n```diff\r\n- MyUnion = Union[int, str]\r\n+ MyUnion = Union[int, str, bytes]\r\n```\r\n\r\nI have to remember to add it to all of the places which match on the union, for example `handle_my_union`. As written above, mypy doesn't warn (and has no reason to). So what I use is this function (stolen mostly verbatim [from TypeScript](https://www.typescriptlang.org/docs/handbook/advanced-types.html#exhaustiveness-checking)):\r\n\r\n```python\r\nfrom typing import NoReturn\r\n\r\ndef assert_never(x: NoReturn) -\u003e NoReturn:\r\n    assert False, \"Unhandled type: {}\".format(type(x).__name__)\r\n```\r\n\r\nand use it as follows:\r\n\r\n```diff\r\n def handle_my_union(x: MyUnion) -\u003e None:\r\n     if isinstance(x, int): ...\r\n     elif isinstance(x, str): ...\r\n+    else:\r\n+        assert_never(x)\r\n```\r\n\r\n* What is the actual behavior/output?\r\n\r\nNow mypy warns:\r\n\r\n```\r\nerror: Argument 1 to \"assert_never\" has incompatible type \"bytes\"; expected \"NoReturn\"\r\n```\r\n\r\nWhen I add a branch to handle `bytes`:\r\n\r\n```diff\r\n def handle_my_union(x: MyUnion) -\u003e None:\r\n     if isinstance(x, int): ...\r\n     elif isinstance(x, str): ...\r\n+    elif isinstance(x, bytes): ...\r\n     else:\r\n         assert_never(x)\r\n```\r\n\r\nthe error is gone.\r\n\r\n* What is the behavior/output you expect?\r\n\r\nI am happy with this solution, I just wonder if this is something that is considered appropriate to rely on, or if there's another preferable way to achieve exhaustiveness checking?\r\n\r\n* What are the versions of mypy and Python you are using?\r\n\r\nPython 3.5.5, mypy 0.630.\r\n\r\n* What are the mypy flags you are using? (For example --strict-optional)\r\n\r\n`no_implicit_optional, check_untyped_defs, warn_unused_ignores`.\r\n","author":{"url":"https://github.com/bluetech","@type":"Person","name":"bluetech"},"datePublished":"2018-10-22T09:30:12.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":21},"url":"https://github.com/5818/mypy/issues/5818"}