René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Adding lists of subtypes together","articleBody":"Using `mypy==0.620` and Python 3.6.6, the following code:\r\n\r\n```python\r\nfrom typing import List, Optional\r\n\r\nx: List[Optional[int]] = [1, 2, 3]\r\n[None] + x\r\n```\r\n\r\nfails with:\r\n\r\n```\r\na.py:4: error: Unsupported operand types for + (\"List[None]\" and \"List[Optional[int]]\")\r\n```\r\n\r\nThis is presumably because lists are invariant w.r.t their generic parameter, and the declration requires a `List` of the same type\r\n\r\nhttps://github.com/python/typeshed/blob/4e40b035c8337cddcf4f94fcd83a2f0d454488b6/stdlib/3/builtins.pyi#L619.\r\n\r\nI don't know what the right solution here is -- can we just replace the `x: List[_T]` with `x: List[_T_co]` and make it covariant? Or doe we need another typevar `_U = TypeVar(\"U\", bound=_T)` or something?","author":{"url":"https://github.com/alanhdu","@type":"Person","name":"alanhdu"},"datePublished":"2018-08-14T22:25:44.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":3},"url":"https://github.com/2383/typeshed/issues/2383"}