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doubly_linked_list.py: Add doctests #2052

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doubly_linked_list.py: Add doctests #2052

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@cclauss
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cclauss commented May 31, 2020

Describe your change:

  • Add an algorithm?
  • Add tests to an existing algorithm?
  • Documentation change?

Checklist:

  • I have read CONTRIBUTING.md.
  • This pull request is all my own work -- I have not plagiarized.
  • I know that pull requests will not be merged if they fail the automated tests.
  • This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
  • All new Python files are placed inside an existing directory.
  • All filenames are in all lowercase characters with no spaces or dashes.
  • All functions and variable names follow Python naming conventions.
  • All function parameters and return values are annotated with Python type hints.
  • All functions have doctests that pass the automated testing.
  • All new algorithms have a URL in its comments that points to Wikipedia or other similar explanation.
  • If this pull request resolves one or more open issues then the commit message contains Fixes: #{$ISSUE_NO}.
cclauss and others added 2 commits May 31, 2020
@cclauss
doubly_linked_list.py: Add doctests
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fixup! Format Python code with psf/black push
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cclauss added 2 commits May 31, 2020
@cclauss
Update doubly_linked_list.py
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Update doubly_linked_list.py
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@cclauss
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cclauss commented May 31, 2020

@WilliamHYZhang @AlexDvorak
Could you please look at why these tests are failing? Am I doing something wrong?
@TheSuperNoob
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TheSuperNoob commented Jun 2, 2020

Could you please look at why these tests are failing? Am I doing something wrong?

@cclauss
I believe the problem here is with the code, not the tests. line 44: self.head.previous = None
The problem here is that on line 41: self.head = self.head.next will be None, since there is only one element left in the list, head no longer has a next element.

Similarly the problem is also there in deleteTail on line 58 and 59, but this time tail will have no previous element.
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cclauss commented Jun 2, 2020

https://travis-ci.com/github/TheAlgorithms/Python/builds/168928811#L754 is even worse than that. It is saying that self.head is None.
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TheSuperNoob commented Jun 2, 2020

https://travis-ci.com/github/TheAlgorithms/Python/builds/168928811#L754 is even worse than that. It is saying that self.head is None.

Yes, because on line 41 self.head is set to self.head.next which would be None in this case, since there is no next node.
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cclauss commented Jun 2, 2020

So, Can you suggest a fix that allows the tests to pass?
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TheSuperNoob commented Jun 2, 2020

You can make the tests pass by not trying to use deleteHead or deleteTail on the final element in the list, however I'd recommend fixing the algorithm so this is possible. I can open a PR later today that fixes this.
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