[Python-Dev] Q: why doesn't list.extend use the sequence interface?

[Fred L. Drake, Jr.]

Barry A. Warsaw writes:
 > Jeremy's right that this can get hairy if your as anal as I am about
 > error checking and refcounting.  In addition to PySequence_Length()
 > ability to lie, there are all sorts of errors that can happen, which
 > should abort the whole process and clean up any temporary objects.
 > Plus you've now got three very similar branches in the code, which
 > adds its own overhead.
 > 
 > So you should decide whether this situation will be used widely enough
 > to warrant the extra coding complexity for the performance win.
 > list.extend() is probably not used too often these days, but it should
 > be 'cause it's cool.  So maybe it's worth it in this case.
 > 
 > On the other hand, this is a situation that comes up often, and if we
 > were to really audit the APIs, we'd probably find even more cases
 > where restrictions to concrete lists or tuples should be relaxed to
 > accept any sequence.  Maybe there's a way we can factor this out in a
 > single function which handles this once and for all?

  How about this: special case the list & tuple flavors (since that'll
be roughly what's in there now), and for other sequences, convert to
either a tuple or a list and let it pass through that special case,
keeping a flag to discard the temporary list when done?
  There are two APIs, PySequence_List() and PySequence_Tuple(), which
can take care of a lot of the machinery in dealing with sequences that
lie about their length or otherwise break (during __getitem__() calls
or whatever), that a large part of the work is done.


  -Fred

-- 
Fred L. Drake, Jr.  <fdrake at beopen.com>
BeOpen PythonLabs Team Member