[Python-Dev] PEP 409 and the stdlib

Steven D'Aprano steve at pearwood.info
Mon May 20 17:39:03 CEST 2013

On 21/05/13 00:12, Ethan Furman wrote:

> 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?

IMO, it is irrelevant noise, and obviously so. The binascii.Error raised is not a bug to be fixed, it is a deliberate exception and part of the API of the binascii module. That it occurs inside an "except KeyError" block is a mere implementation detail. It merely happens to be that digits are converted by looking up in a mapping, another implementation might use a completely different mechanism. In fact, the implementation in Python 3.3 *is* completely different, and there is no KeyError to suppress.

In another reply, R.David Murray answered:

"I don't see that it is of benefit to suppress [the KeyError]."

Can I suggest that it's obviously been a long, long time since you were a beginner to the language, and you've forgotten how intimidating error messages can be? Error messages should be *relevant*. Irrelevant details don't help, they hinder, and I suggest that the KeyError is irrelevant.


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