Add tests to `test_datastructure`

[TechNiick]

Summary

Add test cases to test_datastructures.py in order to have 100 coverage. Related to this issue and this PR.

Checklist

• [ ] I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• [ ] I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• [ ] I've updated the documentation accordingly.