[Python-checkins] cpython: Touch up exception messaging

[Ezio Melotti]

Hi,
I'm not sure these changes are an improvement.

On Fri, Jan 25, 2013 at 8:49 PM, brett.cannon
<python-checkins at python.org> wrote:
> http://hg.python.org/cpython/rev/792810303239
> changeset:   81735:792810303239
> user:        Brett Cannon <brett at python.org>
> date:        Fri Jan 25 13:49:19 2013 -0500
> summary:
>   Touch up exception messaging
>      [...]
>      magic = data[:4]
>      raw_timestamp = data[4:8]
>      raw_size = data[8:12]
>      if magic != _MAGIC_BYTES:
> -        msg = 'bad magic number in {!r}: {!r}'.format(name, magic)
> +        msg = 'incomplete magic number in {!r}: {!r}'.format(name, magic)

Here 2 things could go wrong:
 1) magic is less than 4 bytes (so it could be "incomplete");
 2) magic is 4 bytes, but it's different (so it's "invalid" or "bad");
For this to be "incomplete" the size of the whole file should be less
than 4 bytes, and it's unlikely that in a 3bytes-long file the content
is an incomplete magic number.  It's much more likely than it's not a
magic number at all, or that it is a bad/wrong/invalid 4-bytes magic
number, so the previous error looked better to me.

>          raise ImportError(msg, **exc_details)
>      elif len(raw_timestamp) != 4:
> -        message = 'bad timestamp in {!r}'.format(name)
> +        message = 'incomplete timestamp in {!r}'.format(name)
>          _verbose_message(message)
>          raise EOFError(message)
>      elif len(raw_size) != 4:
> -        message = 'bad size in {!r}'.format(name)
> +        message = 'incomplete size in {!r}'.format(name)

If we arrived here the magic number was right, so this are probably
the timestamp and size, but for this to fail the whole file should be
less than 8 or 12 bytes (unless I misread the code).
Something like "reached EOF while reading timestamp/size" would
probably be more informative.

Best Regards,
Ezio Melotti