FrameRange : Prevent creation of FrameRanges with negative steps

[ivanimanishi]

Previously, you could create a FrameRange object with a negative step, but it would go into an infinite loop if you tried to get the frame list

Given that we have the ReversedFrameList that achieves the same as a negative step, it felt like using that was the correct approach.

Note that we still include the support for the negative step in the parse method, so that we can provide a more useful error message during init.

Generally describe what this PR will do, and why it is needed.

• List specific new features and changes to project components

Related Issues

Creating a FrameRange object with a negative frame range would cause an infinite loop when trying to fetch the list of frames.

Dependencies

None.

Breaking Changes

Negative steps were already not supported, since they caused the infinite loop.

Checklist

• I have read the contribution guidelines.
• I have updated the documentation, if applicable.
• I have tested my change(s) in the test suite, and added new test cases where necessary.
• My code follows the Cortex project's prevailing coding style and conventions.

[johnhaddon]

Might be worth adding a test to test/IECore/FrameList.py as well?

[johnhaddon]

Rejecting negative steps (rather than updating asList() to support them) is consistent with the start > end checks we're already doing, so this seems like the right fix. Looks like we also need to apply the step < 0 check in setStep() though, in which case maybe having the constructor call setStep() is simpler than duplicating the checks?

[ivanimanishi]

Added the extra check on setStep() as well.
I decided against changing the constructor to call setStep() as I thought it would also imply we should do the same for setStart() and setEnd(), but then you get into a bunch of details on whether we should pre-initialize the values before calling those methods, and things actually became quite difficult to read.
In the current PR, yes, you end up with duplicated checks, but at least it's fully consistent and readable.

[ivanimanishi]

Might be worth adding a test to test/IECore/FrameList.py as well?

I didn't create the tests before because there were nearly no tests (as indicated by Andrew's comment) in that FrameList.py file, and I didn't want to create all the missing tests :)

I limited myself now to add what I think is a reasonable list of tests specifically for the FrameRange, including the negative step.

Note that the set calls, like setStep(), and setStart() are not available in python, so I couldn't call them directly.