Hi
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
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