René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Document the order to check chained functions","articleBody":"In [ch3ex6](https://www.datacamp.com/teach/editor/1536/edit/6571140900?branch=master) of \"Machine Learning for Time Series in Python\", the student has to complete this line of code.\r\n\r\n```python\r\nmissing_values = prices.isna().sum()\r\n```\r\n\r\nThe SCT to check this was\r\n\r\n```python\r\nEx().multi(\r\n    check_correct(\r\n        check_object(\"missing_values\").has_equal_value(), \r\n        multi(\r\n            check_function(\"prices.isna.sum\", signature=False), \r\n            check_function(\"prices.isna\")\r\n        )\r\n    )\r\n)\r\n```\r\n\r\nThis gives a bad feedback message when the student types a different function to `is.na()`.\r\n\r\nFor example, the submission\r\n\r\n```python\r\nmissing_values = prices.isnaxxx().sum()\r\n```\r\n\r\nResults in the feedback\r\n\r\n```\r\nDid you call `prices.isna.sum()`?\r\n```\r\n\r\nThis is confusing: it ought to say\r\n\r\n```\r\nDid you call `prices.isna().sum()`?\r\n```\r\n\r\nA partial fix is to perform the checks in reverse order. That is **functions should be checked from left to right, not right to left**.\r\n\r\n```python\r\nEx().multi(\r\n    check_correct(\r\n        check_object(\"missing_values\").has_equal_value(), \r\n        multi(\r\n            check_function(\"prices.isna\"),\r\n            check_function(\"prices.isna.sum\", signature=False)\r\n        )\r\n    )\r\n)\r\n```\r\n\r\nIn this case, the submission \r\n\r\n```python\r\nmissing_values = prices.isnaxxx().sum()\r\n```\r\n\r\ngives the feedback\r\n\r\n```\r\nDid you call `prices.isna()`?\r\n```\r\n\r\nIt isn't perfect though. The submission\r\n\r\n```python\r\nmissing_values = prices.isna().sumxxx()\r\n```\r\n\r\ngives the feedback\r\n\r\n```\r\nDid you call `prices.isna.sum()`?\r\n```\r\n\r\nSo a custom missing message needs to be provided.\r\n```python\r\nEx().multi(\r\n    check_correct(\r\n        check_object(\"missing_values\").has_equal_value(), \r\n        multi(\r\n            check_function(\"prices.isna\"),\r\n            check_function(\"prices.isna.sum\", signature=False, missing_msg = \"Did you call `prices.isna().sum()`?\")\r\n        )\r\n    )\r\n)\r\n```\r\n\r\nSince this is a subtle problem, I think it needs documenting. Ideally, the feedback message also needs fixing so that parentheses are included where the intermediate element is a function.\r\n\r\n","author":{"url":"https://github.com/richierocks","@type":"Person","name":"richierocks"},"datePublished":"2019-01-10T14:22:34.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":0},"url":"https://github.com/370/pythonwhat/issues/370"}