[Python-Dev] PEP 344: Exception Chaining and Embedded Tracebacks

[Ka-Ping Yee]

On Mon, 16 May 2005, Aahz wrote:
> I'll comment here in hopes of staving off responses from multiple
> people: I don't think these should be double-underscore attributes.  The
> currently undocumented ``args`` attribute isn't double-underscore, and I
> think that's precedent to be followed.

That isn't the criterion i'm using, though.  Here's my criterion, and
maybe then we can talk about what the right criterion should be:

    System attributes are for protocols defined by the language.

(I'm using the term "system attribute" here to mean "an attribute with
a double-underscore name", which i picked up from something Guido
wrote a while back [1].)

For example, __init__, __add__, __file__, __name__, etc. are all
attributes whose meaning is defined by the language itself as opposed
to the Python library.  A good indicator of this is the fact that
their names are hardcoded into the Python VM.  I reasoned that
__cause__, __context__, and __traceback__ should also be system
attributes since their meaning is defined by Python.

Exceptions are just classes; they're intended to be extended in
arbitrary application-specific ways.  It seemed a good idea to leave
that namespace open.


-- ?!ng

[1] http://mail.python.org/pipermail/python-dev/2003-June/036239.html