Clarify error message when unpacking objects with `*` and `**`

[cool-RR]

When attempting to unpack a list (or any other non-mapping) as a mapping, one gets this error message:

builtins.TypeError: 'list' object is not a mapping

It's confusing, especially since sometimes the unpacking is done on a long multi-line statement, and it's not clear that the unpacking is the source of the error.

I suggest modifying to:

builtins.TypeError: Can't unpack 'list' object using ** because it's not a mapping

I'll be happy to submit a pull request if you agree with this change.

[StanFromIreland]

Good change in my opinion always good to improve error messages.

[picnixz]

I'm not in favor of this change because users do rely on exception messages but I can definitely understand the confusion. What we could say is "cannot unpack 'list' object using **" (you can only unpack mapping-objects so it would be redundant). Wouldn't it be better if the bad code was highlighted instead of just having ^^^ carrets?

[cool-RR]

I'm not in favor of this change because users do rely on exception messages but I can definitely understand the confusion. What we could say is "cannot unpack 'list' object using **"

I think it's not as informative as the text I suggested, but maybe other people will chime in.

(you can only unpack mapping-objects so it would be redundant).

Isn't * also unpacking?

Wouldn't it be better if the bad code was highlighted instead of just having ^^^ carrets?

It would be better, but it seems like a separate discussion.

[picnixz]

Isn't * also unpacking?

Yes, but if you mention **, then you expect a mapping to be unpacked (that's the redundancy I was talking about).

I think it's not as informative as the text I suggested, but maybe other people will chime in.

We generally weigh between how much information you need and how much it's needed. in general, we don't specify a reason if that reason can be inferred from the language specifications (in particular, ** only works on mapping objects)

[cool-RR]

Isn't * also unpacking?

Yes, but if you mention **, then you expect a mapping to be unpacked (that's the redundancy I was talking about).

I see. However, the text cannot unpack 'list' object using ** does not convey the core of the problem, which is that list does not support the mapping protocol. This is conveyed by the current error message. Even though you and I could easily guess from the text that this is the case, I believe that error messages should also be easily understandble to beginners or people will less experience with Python.

The error message Cannot unpack 'list' object using ** because it is not a mapping conveys both pieces of information. If you don't like that it's long, we could shorten it to something like Cannot unpack non-mapping 'list' object using **.

[picnixz]

Ah, I see your point now. I still think that my message would be sufficient if in addition we have a TypeError, but I agree that I could have been biased. I like your alternative formulation, namely cannot unpack non-mapping 'list' object using **.

Asking some docs, typing and core devs on that matter: @JelleZijlstra @AlexWaygood @ncoghlan @willingc @serhiy-storchaka @encukou (or maybe we should open a discourse thread for bikeshedding the error message and/or it would be better if we change that error message).

As a matter of fact, we write T object is not iterable when trying to unpack it using *, so it could also be helpful to change that one to cannot *-unpack non-iterable 'T' object (and cannot **-unpack non-mapping 'T' object). Whatever we choose for **, we should also reflect it for *.

[cool-RR]

Great. Agreed regarding the * error message.