[Python-Dev] That depends on what the meaning of "is" is (was Re: http://mail.python.org/pipermail/python-dev/2011-December/115172.html)

[PJ Eby]

On Mon, Jan 2, 2012 at 4:07 PM, Jim Jewett <jimjjewett at gmail.com> wrote:

> On Mon, Jan 2, 2012 at 1:16 AM, PJ Eby <pje at telecommunity.com> wrote:
> > On Sun, Jan 1, 2012 at 10:28 PM, Jim Jewett <jimjjewett at gmail.com>
> wrote:
> >>
> >> Given the wording requiring a real dictionary, I would have assumed
> >> that it was OK (if perhaps not sensible) to do pointer arithmetic and
> >> access the keys/values/hashes directly.  (Though if the breakage was
> >> between python versions, I would feel guilty about griping too
> >> loudly.)
>
> > If you're going to be a language lawyer about it, I would simply point
> out
> > that all the spec requires is that "type(env) is dict" -- it says nothing
> > about how Python defines "type" or "is" or "dict".  So, you're on your
> own
> > with that one. ;-)
>
> But the public header file <
> http://hg.python.org/cpython/file/3ed5a6030c9b/Include/dictobject.h >
> defines the typedef structs for PyDictEntry and _dictobject.
>
> What is the purpose of the requiring a "real dict" without also
> promising what the header file promises?
>
>
Er, just because it's in the .h doesn't mean it's in the public API.  But
in any event, if you're actually serious about this, I'd just point out
that:

1. The struct layout doesn't guarantee anything about insertion or lookup
algorithms,
2. If the data structure were changed, the header file would obviously
change as well, and
3. ISTM that Python does not even promise inter-version ABI compatibility
for internals like the dict object layout.

Are you seriously writing code that relies on the C structure layout of
dicts?  Because really, that was SO not the point of the dict type
requirement.  It was so that you could use Python's low-level *API* calls,
not muck about with the data structure directly.  I'm occasionally
considered notorious for abusing Python internals, but even I have to draw
the line somewhere.  ;-)
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