[Python-Dev] PEP 409 and the stdlib

[Ethan Furman]

On 05/20/2013 06:47 AM, Nick Coghlan wrote:
> On 20 May 2013 23:38, Ethan Furman wrote:
>>
>> As a quick reminder, PEP 409 allows this:
>>
>>     try:
>>         ...
>>     except AnError:
>>         raise SomeOtherError from None
>>
>> so that if the exception is not caught, we get the traditional single exception traceback, instead of the new:
>>
>>     During handling of the above exception, another exception occurred
>>
>>
>> My question:
>>
>> How do we go about putting this in the stdlib?  Is this one of the occasions where we don't do it unless we're modifying a module already for some other reason?
>>
>> For that matter, should we?
>>
>> Pros:  Makes tracebacks much less confusing, especially coming from a library
>>
>> Cons:  Could hide bugs unrelated to what is being caught and transformed
>
> Be pretty conservative with this one - we should only use it when we're confident we know the original exception is
> almost certain to be irrelevant noise.
>
> Ensuring the traceback module makes it easy to display both would also be a good preliminary step.

As a case in point, base64.py is currently getting a bug fix, and also contains this code:

def b32decode(s, casefold=False, map01=None):
     .
     .
     .
     for i in range(0, len(s), 8):
         quanta = s[i: i + 8]
         acc = 0
         try:
             for c in quanta:
                 acc = (acc << 5) + b32rev[c]
         except KeyError:
             raise binascii.Error('Non-base32 digit found')
     .
     .
     .
         else:
             raise binascii.Error('Incorrect padding')

Does the KeyError qualify as irrelevant noise?

If we're not going to suppress the originating error I think we should at least change the double trace back message as 
it implies two failures, instead of just one.

--
~Ethan~