René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"False negative involving null checks on optional types","articleBody":"Here is a minimal program demonstrating the problem:\r\n```\r\nfoo = None\r\nfor _ in range(2):\r\n  if foo is not None:\r\n    print(1[2])\r\n  foo = 1\r\n\r\n```\r\nExpected behavior:\r\nerror: Value of type \"int\" is not indexable\r\n\r\nActual behavior:\r\nmypy accepts the program as valid, when run with the --strict flag\r\n\r\nThis issue seems to be a problem with how mypy handles reassignment to None type variables, and static evaluation of conditionals.  After line 1, `foo` has the type None according to mypy.  Thus, mypy determines that the condition on line 3 will never hold, and thus doesn't do type checking for the body of the if statement.  On line 5, foo's type changes to `Optional[int]`. This, however, will only be reflected in code that follows line 5.","author":{"url":"https://github.com/person594","@type":"Person","name":"person594"},"datePublished":"2018-08-06T15:01:51.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":6},"url":"https://github.com/5423/mypy/issues/5423"}