Thanks for this improvement with this clever way to handle the NO_CHANGE case. Before tackling my other review comments here, I think we should discuss again if we really want to settle on the API you have introduced via this PR as it differs from my initial suggestions in #36.

I currently see a problem with this usage example:

# Current usage example
wiz.config(sigfigs=2)  # explicitly set sigfigs value to 2
wiz.config(identifier="result")  # sigfigs value does not change and stays 2
wiz.config(sigfigs=None, identifier=None)  # reset sigfigs to default value
wiz.config(sigfigs=-1)  # explicitly set sigfigs value to -1

I don't think that having values like -1 for sigfigs is a good thing to do. In this case, the -1 is supposed to indicate that we don't want to use explicit sigfigs and instead use the predefined rounding rules. But I think it was a shortcoming in our previous design that we used -1 for this and not None. In my opinion, in a Python world it's way more natural to write wiz.config(sigfigs=None) if you want that sigfigs should not have effects, i.e. reserve None for an actual value-related meaning (and not "reset to default").

But then, this would clash with your changes in this PR where you reserve None for the case reset to default value. I'd like to make more explicit to the caller that they reset a value, e.g. by using the name "reset" or "default" somewhere. Here are a few proposals:

# No separate reset() method (currently implemented in this PR)
wiz.config(sigfigs=None, identifier=None)

# Separate reset() method with True parameters
wiz.reset(sigfigs=True, identifier=True)

# Separate reset() method with String parameters
# We would still get IntelliSense via the IDE by using Literal["sigfigs", "identifier"] as type hint
wiz.reset("sigfigs", "identifier")

# More fluent API
wiz.config.sigfigs.set(2)
wiz.config.sigfigs.reset()
# Advantage: almost like an English sentence
# Big disadvantage: cannot set/reset multiple values at the same time

# Enums
wiz.config(sigfigs=2)
wiz.reset(wiz.config.SIGFIGS, wiz.config.DECIMAL_PLACES)

# Enums other idea without separate reset() method
wiz.config(sigfigs=2)
wiz.config(sigfigs=wiz.config.DEFAULT)

# Allow separate TOML-file for configuration?
# wiz.config.from_file("config.toml")
# Probably don't want that as this would mean one extra file for the user
# to setup and there are not THAT many configuration options in our library.

I currently like wiz.reset(sigfigs=True, identifier=True) and wiz.config(sigfigs=wiz.config.DEFAULT) the most. I would have loved to write wiz.reset(sigfigs, identifier), but that's not possible in Python. Maybe you can also research a bit to come up with more proposals to choose from. Maybe you have some strong opinions on this? My only strong opinion here is that we should rethink if we really want None to indicate Default. As I said, we should reserve it for cases like sigfigs (or other options) where it has an actual meaning related to the value itself. And yes, these questions are kind of philosophical, but that's probably the point of API design ;)

Edit: This StackOverflow answer might be related.

Edit2: When discussing different options, I think we should focus on how easy the API is to use for the user, and not on how difficult it might be to implement.

Edit3: The Python documentation for None just says that None is "frequently used to represent the absence of a value, as when default arguments are not passed to a function".

Edit4: I've implemented a quick preview of how the API could look like with the wiz.config(sigfigs=wiz.CONFIG_DEFAULT) approach. See this commit. You can just check out the branch improve-config-new-api and see the last lines of the playground. Run it and you will get the print statements in your console. You will also get nice IntelliSense with this one where I tried to expose as few internal information as possible to the public: