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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"--warn-unreachable false positive with minimal example without types explicitly set","articleBody":"Hello,\r\n\r\nI just downloaded the new mypy v0.720 and am playing with the `--warn-unreachable-flag` in my projects.\r\n\r\nI noticed one occasion where there is something that seems like a false positive, so I am making an issue for it with a minimal reproducible example.\r\n\r\n\r\n#### Version\r\n```\r\nmypy==0.720\r\nmypy-extensions==0.4.1\r\n```\r\n\r\n#### source code example\r\n\r\n```python\r\nimport sys\r\nfrom typing import Optional\r\n\r\nfound_id = None\r\n\r\nsymbols_list = [{'symbol': 'A', 'id': 1}, {'symbol': 'B', 'id': 2},  {'symbol': 'B', 'id': 3}]\r\n\r\nfor entry in symbols_list:\r\n    if entry['symbol'] == 'B':\r\n        if found_id:\r\n            print('error, symbol found multiple times!')\r\n            sys.exit(1)\r\n\r\n        found_id = entry['id']\r\n```\r\n\r\n\r\nThis when tested with `mypy test.py --warn-unreachable` returns:\r\n\u003e test.py:10: error: Statement is unreachable\r\n\r\nSo essentially telling us that `if found_id` is never entered due to the `found_id` type being `None`. But in reality the type should be `Optional[int]` since you can see that it's set to `entry['id']` later.\r\n\r\nIf you explicitly set the types mypy no longer complains:\r\n```python\r\nimport sys\r\nfrom typing import Optional\r\n\r\nfound_id: Optional[int] = None\r\n\r\nsymbols_list = [{'symbol': 'A', 'id': 1}, {'symbol': 'B', 'id': 2},  {'symbol': 'B', 'id': 3}]\r\n\r\nfor entry in symbols_list:\r\n    if entry['symbol'] == 'B':\r\n        if found_id:\r\n            print('error, symbol found multiple times!')\r\n            sys.exit(1)\r\n\r\n        assert isinstance(entry['id'], int)\r\n        found_id = entry['id']\r\n```\r\n\r\n#### Bug or not\r\n\r\nEssentially I am not sure if this is intended behaviour or not. After writing this issue down I realize it all boils down to the matter of mypy not correctly inferring type of `found_id`.","author":{"url":"https://github.com/LefterisJP","@type":"Person","name":"LefterisJP"},"datePublished":"2019-07-13T14:25:43.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":6},"url":"https://github.com/7204/mypy/issues/7204"}