reversed(tqdm(some_list)) does not yield a reversed list

[harmenwassenaar]

• I have marked all applicable categories:
  • exception-raising bug
  • visual output bug
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  read the [known issues]
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• I have mentioned version numbers, operating system and
  environment, where applicable:

  import tqdm, sys
  print(tqdm.__version__, sys.version, sys.platform)

Version info: 4.65.0 3.10.11 (main, Apr 5 2023, 00:00:00) [GCC 12.2.1 20221121 (Red Hat 12.2.1-4)] linux

When reversed is used on tqdm, it unexpectedly returns everything in the original, unreversed, order

>>> from tqdm import tqdm
>>> list(reversed(tqdm([1, 2, 3, 4])))
100%|██████████████████████████| 4/4 [00:00<00:00, 47798.34it/s]
[1, 2, 3, 4]
>>> 

This is because __reversed__ swaps in a reversed iterator, then stores the resulting generator it gets form calling self.__iter__(), and swaps the original iterator back before returning the generator. Crucially, because __iter__() is a generator, none of its code is executed before the first element is required. So by the time you use the generator, it's using the original iterator again.

There are a number of ways to solve this. The top answer on https://stackoverflow.com/q/5724009/1961666 suggests wrapping most of the code in __iter__ in a generator function, so that __iter__ is not itself a generator and so the first line would be run immediately when called.