[yalinli2]

Two changes:

1. For the newly added define_credit function, the doc says it can accept a function, but it couldn't, so I added a helper function for that.
2. For the Unit._flow_fractions attribute, it wasn't included in _init_results function, though other dict were included, It causes problems in qsdsan since it doesn't call Unit.__init__ but _init_results. If Unit._flow_fractions shouldn't be added to _init_results, I can patch up qsdsan, thanks!

[yoelcortes]

Awesome, thanks reviewing recent commits and improving the code!!

[yoelcortes]

@yalinli2, I just realized that the Unit._init_results method is not used in BioSTEAM. A long time ago, we used this method within Unit.__init__. Later, I moved the initialization of results container variables to Unit.__init__ so that Sphinx can automatically add documentation left in #: comments (Sphinx only looks at variables in init).

Would it be better to move the Unit._init_results method to QSDsan?

By the way, I have not been running QSDsan and making sure my changes are backwards compatible (sorry for the trouble!). I will start doing so again with future commits.

Also, for any changes to core classes (e.g., Unit, Stream, TEA, System; not specific unit operations), I will plan on submitting a pull request for your review. Let me know if you have any specific BioSTEAM classes that would be helpful to keep track of with PRs (as opposed to direct commits on my side).

Thank you!

[yalinli2]

Thanks @yoelcortes ! I've updated QSDsan so it doesn't call Unit._init_results, you can just remove it if BioSTEAM doesn't use it. Same for `Unit._init_utils'.

Sounds good on the test and PR review!