EHN: fix unsigned int type problem of the result diff()

[zzapzzap]

An unexpected output type(float64) occurs to diff() when using uint
type. So fix it by restoring original dtype after storing it

Fixes #28909

• closes #xxxx
• tests added / passed
• passes black pandas
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• whatsnew entry

What do you think about this problem with the code? @jreback

[MarcoGorelli]

Hi @zzapzzap , and welcome

fix it by restoring original dtype after storing it

We wouldn't necessarily want to do that, the diff operation may have to change the dtype. The original issue seems to be more about documenting the change in the return type and perhaps throwing a warning, than about making any code changes.

As an aside, code changes typically require tests, see contributing to the code base:

pandas is serious about testing and strongly encourages contributors to embrace test-driven development (TDD). This development process “relies on the repetition of a very short development cycle: first the developer writes an (initially failing) automated test case that defines a desired improvement or new function, then produces the minimum amount of code to pass that test.” So, before actually writing any code, you should write your tests. Often the test can be taken from the original GitHub issue. However, it is always worth considering additional use cases and writing corresponding tests.

Adding tests is one of the most common requests after code is pushed to pandas. Therefore, it is worth getting in the habit of writing tests ahead of time so this is never an issue.

[TomAugspurger]

For numeric types, Series.diff() introduces NaNs, which are a float. These can't be cast back to int / uint.

I'd recommend using one of the new nullable integer dtypes.

[zzapzzap]

Thanks for your help.