Chain exceptions where appropriate

Picking up from #15731, there are many places in numpy where we do something like:

try:
    something_which_throws_typeError
except TypeError:
    raise ValueError("a clearer explanation of what went wrong")

It would produce a marginally clearer error if we could change these to use traceback chaining. Our two options are either:

1. The inner error provides valuable information that is not present in the outer error. We should use from e.

   try:
       something_which_throws_typeError
   except TypeError as e:
       raise ValueError("a clearer explanation of what went wrong") from e
   

2. The inner error provides no valuable information that is not present in the outer error. We should use from None:

   try:
       some_dict[name]
   except KeyError:
       raise ValueError("The name %s is not supported" % name) from None
   

   In that example all the information from the KeyError is already in the message of the new error, so there is no point chaining

An example of such a change would be this line of this otherwise-discarded patch.

For now, I would recommend we only apply this to cases where the exception is thrown unconditionally, as other cases can be more nuanced.

---

Please see this list for potential places to contribute. Special thanks to @nkleinbaer for compiling the list!

[eric-wieser]

It’s very marginally useful, as it removes unhelpful wording from tracebacks like “another exception occurred”, and removes KeyErrors internal to numpy that are just an implementation convenience. I regular see Python novices scared enough by a long traceback that they just read none of it.

This issue seems to be effective at drawing in new contributors, although I haven’t kept track of whether any of them go on to contribute further.

[mattip]

… with that please realize that this is a low priority issue, so PRs may take a while to get reviewed and merged. And as I commented above:

We would like to suggest that contributors show the output from the error. This will require you think about the error: is the code path actually hit? Does it make sense to chain the exception?