Re: List Performance
- From: Terry Reedy <tjreedy@xxxxxxxx>
- Date: Mon, 30 Jun 2008 16:21:35 -0400
Maric Michaud wrote:
Le Monday 30 June 2008 15:52:56 Gerhard Häring, vous avez écrit :Larry Bates wrote:
If, on the other hand, we knew beforehand how big the list will get
approximately, we could avoid all these reallocations. No problem with
Python's C API:
PyAPI_FUNC(PyObject *) PyList_New(Py_ssize_t size);
But you can't do it directly from Python, unless you (ab)use ctypes.
-- Gerhard
--
http://mail.python.org/mailman/listinfo/python-list
Well, as I posted few days ago, one could envisage, as a pure python optimization for dealing with long list, to replace an algorithm with a lot of append by something like this :
mark = object()
datas = [ mark ] * expected_size
datas = [None] * expected_size
has been a standard idiom since before object() existed ;-)
and works fine *unless* one wants to add None explicitly
and have that be different from 'unused'.
# working with the datas while maintaining the effective currrently used size
Of course one could even subclass list and redefine __len__, append, and some other methods to deal with this "allocated by block" list.
An interesting idea if one does this at least a few times and wants to use .append and .extend instead of explicit indexing.
One could also make such a subclass a 'no-grow' list if appropriate (when an attempt to grow it would indicate a bug).
tjr
.
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