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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"dis: FOR_ITER says it no longer pops the stack in 3.12 but it still does when the iterator ended normally","articleBody":"# Documentation\r\n\r\nThe `FOR_ITER` docs in the `dis` module say \"Up until 3.11 the iterator was popped when it was exhausted\". This sounds like in 3.12+ the iterator is not popped anymore:\r\n\r\nhttps://github.com/python/cpython/blob/cecd6012b0ed5dca3916ae341e705ae44172991d/Doc/library/dis.rst?plain=1#L1334-L1342\r\n\r\nInstead there is a new opcode `END_FOR`, which takes care of popping the iterator off the stack. This surprised me because in 3.12 `END_FOR` is supposed to remove 2 elements from the top of the stack. But if the iterator ends normally, there will only be the iterator at the top. I tried to document my thought process in this [godbolt repro](https://godbolt.org/#z:OYLghAFBqd5QCxAYwPYBMCmBRdBLAF1QCcAaPECAMzwBtMA7AQwFtMQByARg9KtQYEAysib0QXACx8BBAKoBnTAAUAHpwAMvAFYTStJg1AAHAJ4EEA0kvrICeAZUboAwqloBXFgxCSAzKTOADJ4DJgAcl4ARpjEIABMGqTGqAqEDgxunt6%2BASlp9gIhYZEsMXGJ1pi2hQxCBEzEBFlePv5VNRn1jQTFEdGxCUkKDU0tOe0jPX2l5UMAlNaoHsTI7BwApACsAEIAXgDU/MQHh6EHxIbAmBBb89sAIhsaAILPb/HxG5/fX68AStghHIALLYA4QyFQ6EaaEHKbIADWIAO2x2j3eyjkQgAEgB9cJyIJBOGkuEI5Go3YMDy0WgY15BADyLweBJeYLJUMuRkwEIpKLRNLppAuV0wDJezNZeJcTPCQgAKlyIVtyQ0kYLqbTaKKeddRVtJS4pSSVXCuOqmJqqTt9Zg8cRMApaQRJQBxbCKvEASUV2H%2B5q5AtthFiTCIxEl0rZADEXkq8S9wmyXEFsC9A6crTa0WHLpHRcL6Vsnq8hAB1F7KIPQ%2BI5ylCnWi/MRkiSnZyH1BNlBH1K82w6Ehpsig6twu2jGl96V6u1qH14caxvasdox4tgjhyOS2NM/6%2B/1Z81cABsy%2Btq52xdFG9LW53JBbgklSoP2Dx8YHKsOl9za66lOD7jtuBbtjOjIsnGCbKuaf5QiOgF3rsm6gU%2BZDjq%2BkFSv23rVso2ApiqS6ISuWo3s2obYQ8j7gVGOEAFKgsoeI7C8LgANJVv8DxklwlpkVeFG3tRbogROEFli8RFxgeC6kkhlF0m%2BVY1gpBykZCFh4AoBwAO50LQRxMHQ7y/BZPyvBwiy0JwWy8D43C8KgnDKOYlgMPCyyrHy3x%2BDwpAEJoNmLMiWwaAAdJIADsMVnmeGhcAAnMlAAcaXxJI8T6JwkiOSFpCuRwvAKCASTBRwWiLHAsBIGgLDGHQsTkJQDVNfQcRmBYAgBTlNC0GBZUQFEhVRKEjSmJwgXjcwxCmEyUTaJgdjTbwDVsIITIMLQU1VbwWBRB4wAuGItBlc5pBYCwVziPtV14E6dh4AAbs6hWYKoK0eNua3kII1SFbQeBRJc81uFghUEMQeAsIVb3EFEqSYA8mA3UYwNGCFixUAYwAKAAangmD6UyxiMH9/CCCIYjsFIMiCIoKjqPdug5QYWPdZ5%2Bgg2VkCLKgxi1BdLkIzDWB8xAiw2CttROAwrjuK0ejBKE/RlIMXDJKk6QCOMPha/kusMDMAxxFrMvPQI3RjErOQW9UstdKMvRq7MmvWC7%2Bt6FMTSmxr5vSz5awSLZ9kFfdxUHO5PVeX4kVcPWEC4IQJCovEAXzLwlXVWFvjJZFWyfH4fjpRoSVcDFWxpblHD5aQTlaEVnCleVQXY2HHDxLwcNcOXDeFcV2cd6QCNpI4khAA%3D%3D).\r\n\r\nReading the generated code for `FOR_ITER`, specifically:\r\n\r\nhttps://github.com/python/cpython/blob/cecd6012b0ed5dca3916ae341e705ae44172991d/Python/generated_cases.c.h#L3069\r\n\r\nhttps://github.com/python/cpython/blob/cecd6012b0ed5dca3916ae341e705ae44172991d/Python/generated_cases.c.h#L3085-L3087\r\n\r\nit looks like there are 2 cases:\r\n\r\n- If the iterator ends normally, `FOR_ITER` pops the iterator off the stack, then it skips the next `END_FOR` (and in 3.13 `POP_TOP`) instructions.\r\n- Otherwise (I'm not sure when that happens?), the iterator ends with both the `iter` and `iter()` on the stack, which are both popped by `END_FOR` (and in 3.13 `POP_TOP`).\r\n\r\nIs it worth documenting the 2 different cases?","author":{"url":"https://github.com/frigus02","@type":"Person","name":"frigus02"},"datePublished":"2024-07-05T11:39:24.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":0},"url":"https://github.com/121399/cpython/issues/121399"}