[Python-Dev] A C implementation of OrderedDict (original) (raw)

Raymond Hettinger raymond.hettinger at gmail.com
Mon Oct 21 05:56:31 CEST 2013

On Oct 20, 2013, at 9:21 AM, Eric Snow <ericsnowcurrently at gmail.com> wrote:

If anyone is interested in having a (faithful) C implementation of OrderedDict for the 3.4 release, I have a patch up [1]. My interest in having the patch applied is relative to proposals that won't apply to 3.4, so I haven't felt the need to advance the patch. However, in case anyone else would find it useful for 3.4, I figured I would point it out.

While the patch isn't all that complicated, it is large and my C/C-API experience isn't proportional to that size. So I don't feel comfortable about moving ahead with the patch without at least one thorough review. Thanks.

I'll look at this in more detail after I've finishing my review of the TransformDict, but my initial impression is that the original show stopper hasn't been overcome: http://bugs.python.org/issue10977

The concrete dict API is very old and is widely used in C extensions outside the standard library. AFAICT, there is no way to prevent that code from bypassing your code and breaking the internal invariants of the ordered dict (that breakage could be silent are muck up the ordering or could fail loudly with a segfault).

If we really want a C implementation, I think the choices boil down to:

  1. living with the problem and writing defensive code so that the ordered dictionary won't segfault when fed to existing external C code and so that the ordered dict notices whether the parent dictionary has different keys than those in the internal linked list.

or 2) biting the bullet and accepting an API change where ordered dicts are no longer a subclass of real dicts.

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