On 06/10/2021 17:35, Łukasz Langa wrote:
>
>> On 6 Oct 2021, at 18:05, Yury Selivanov <yselivanov...@gmail.com> wrote:
[...]
>> I'll list a few reasons here:
>>
>> 1. `try: .. except group:` is a valid syntax today. And it will continue to
>> be valid syntax. Having both `try: .. except group:` (catch exception
>> `group`) and `try: .. except group E:` (catch exceptions of E into a group)
>> in the same grammar worries me.
>>
>> 1a. It can be especially confusing if someone has a local/global variable
>> called `group`.
>
> This is a valid point, also raised by Pablo over WhatsApp (which happens to
> work today!). The particular hairy example has to do with your next point so
> let's go there first...
>
>
>> 1b. Or, for example, if a user forgets to type `E` and leaves just `except
>> group` it would fallback to the regular try..except behavior. And it would
>> be a runtime error ("group" is undefined).
>
> Right. Worse yet, this wouldn't be a runtime error UNLESS user code raises an
> exception within that try: block. Otherwise Python would happily take the
> unbound name and run with it:
>
> >>> try:
> ... ...
> ... except group:
> ... ...
> ...
> Ellipsis
>
>
> When you raise:
>
> >>> try:
> ... 1/0
> ... except group:
> ... ...
> ...
> Traceback (most recent call last):
> File "<stdin>", line 2, in <module>
> ZeroDivisionError: division by zero
>
> During handling of the above exception, another exception occurred:
>
> Traceback (most recent call last):
> File "<stdin>", line 3, in <module>
> NameError: name 'group' is not defined
>
>
> This is pretty confusing and in my eyes disqualifies the "except group"
> proposal. Pablo also claims it would be very hard to generate good error
> messages due to this and I can see why. My initial idea here was to modify
> this received `NameError` just like we do in other cases with the new "Did
> you mean" helper:
>
> >>> arg = 1
> >>> ar
> Traceback (most recent call last):
> File "<stdin>", line 1, in <module>
> NameError: name 'ar' is not defined. Did you mean: 'arg'?
> >>> def f():
> ... ar
> ...
> >>> f()
> Traceback (most recent call last):
> File "<stdin>", line 1, in <module>
> File "<stdin>", line 2, in f
> NameError: name 'ar' is not defined. Did you mean: 'arg'?
>
> We could potentially do something similar to generate better error messages
> for "except group" confusion, right? Only *we can't* if `group` happens to be
> bound as a name in a reachable scope which Larry points out is a popular
> name. In this scenario any syntax errors would end up with terribly confusing
> TypeErrors or AttributeErrors and so on. This is unacceptable.
Now a moot point, but this could be a SyntaxWarning.
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