I've read some posts on this topic, but this is my first contribution.Thank you Brandt for writing the PEP. I like it's clarity. And thank you Ram for making the suggestion that started the discussion. I fully agree, that we have here a problem worth solving. And many thanks for the many other valuable contributions.

Here's a use-case for having a callback. It's the real-world code example given in the draft PEP 618:

>>> from ast import Constant, Dict, literal_eval
>>> nasty_dict = Dict(keys=[Constant("XXX")], values=[])
>>> literal_eval(nasty_dict) # Like eval('{"XXX": }')
{}

If literal_eval resulted in a call to zip(..., strict=True) the outcome would be something like:

Traceback (most recent call last):

ValueError: zip() argument 2 is too short

It's good, very good, in this situation to get an error message rather than a wrong answer. That would make debugging really hard. And providing a better error message would be even more helpful. How about something like

Traceback (most recent call last):

AstValueError: Dict called with more keys than values

There. Isn't that even better for the programmer who's got to understand where the error came from. Clearly this outcome could be achieved using a custom callback.

Here's the pure-Python implementation in the PEP

if items:
i = len(items) + 1
raise ValueError(f"zip() argument {i} is too short")
sentinel = object()
for i, iterator in enumerate(iterators[1:], 2):
if next(iterator, sentinel) is not sentinel:
raise ValueError(f"zip() argument {i} is too long")

Use this as the template for the custom callback. A simple change to the code allows the result to be an AstValueError, or whatever is considered appropriate.

In the PEP, the only exceptions raised directly by zip are StopIteration and ValueError. I've a question / problem. Wrap zip(..., strict=True) in such a way that the ast example above (taken from the PEP) results in an AstValueError. (Hint: There are traps for the unwary.)

By the way, as the PEP points out, the ValueError can be written as {'AAA':}. So maybe AstSyntaxError would be better.

Jonathan