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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Multiple Inheritance Makes Mypy think an if branch is unreachable.","articleBody":"In the following example, the branch of an if-statement is not being typechecked.\r\n\r\n```py\r\nclass Parent1: pass\r\n\r\nclass Parent2: pass\r\n\r\nclass Child(Parent1, Parent2): pass\r\n\r\ndef goo(p: Parent1) -\u003e None:\r\n    if isinstance(p, Parent2):\r\n        # this branch is not typechecked\r\n        1 + 'boom'  # no error from mypy here.\r\n\r\n\r\ngoo(Child())\r\n```\r\n\r\nHere is the output from `mypy` and `python`.\r\n```\r\n$ mypy i.py \r\n$ python3 i.py \r\nTraceback (most recent call last):\r\n  File \"i.py\", line 13, in \u003cmodule\u003e\r\n    goo(Child())\r\n  File \"i.py\", line 10, in goo\r\n    1 + 'boom'  # no error from mypy but there's an error at runtime\r\nTypeError: unsupported operand type(s) for +: 'int' and 'str'\r\n```","author":{"url":"https://github.com/ilinum","@type":"Person","name":"ilinum"},"datePublished":"2017-06-23T21:56:18.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":18},"url":"https://github.com/3603/mypy/issues/3603"}