[Suggestion] Change around the arguments for the on_x callbacks

[KSneijders]

The first argument for the callbacks is the retriever: Retriever.

From testing and looking at the examples, I can see that it's not used a lot at all.
I've found myself changing the function signatures (from examples) from the first line below to the second.

# From example:
def set_timestamp_repeat(retriever: Retriever, instance: FileHeader):
    ...

# To this (look at retriever parameter)
def set_timestamp_repeat(_, instance: FileHeader):
    ...

Adding them the other way around allows me to leave out the second argument (which would then be Retriever).

Thoughts?

[Divy1211]

This part of the API can definitely do with some improvement because when it was initially made, its usage was not clear - hence the retriever argument has kind of just stuck around. It probably is a good idea to flip the arg order (note: your signature will still need to have the _ as the second arg. The retriever arg does have use cases though, so removing it is not an option)

[KSneijders]

I was under the impression that, like JS, you can leave out trailing arguments... That's unfortunate. I've checked out some SO posts and things like *_ would be the way to go for this it seems.

So moving them around to reflect the frequency of use would be nice but isn't as useful as I had hoped. Either way, would still like to see the change! 🙂