Hi Stefan,
On Thu, 2013-03-07 at 12:21 +0100, Stefan Behnel wrote:
> Note that Python has an algorithm for shrinking a list on appending, so
> this might not be sufficient for your use case. What do you need it for?
W00t! I didn't know about that.
I'm wrapping a C++ code that should transmit large lists of objects to
Python, while these objects are stored into something vector-like, which
shouldn't get exposed directly. In the past, they did something like
obj = PyList_New(a.size());
for (a.begin(); a.end(); ++a) PyList_SetItem(obj, ...)
I figured I can translate it into a while loop like
obj = []
while (it != a.end()):
obj.append(it)
inc(it)
but then I'm not using the information about the size of a that I
already have, and for huge lists this tends to be quite slow.
I think this must be quite a common use case for bindings...
> And why won't [None]*N help you out? It should be pretty cheap.
It probably will, at least a bit. It just stroke me that if I'm going to
do something along the lines of
idx = 0
obj = [None] * a.size()
while (it != a.end()):
obj[idx] = it
idx += 1
inc(it)
I could also squeeze the last bits of performance by avoiding the
creation of Nones and subsequently populating the list with them.
If you say I have to use PyList_New directly, oh well... It's just that
now since I'm rewriting the bindings in Cython anyways, I'm also trying
to avoid using Python C API directly as much as possible.
> Won't list comprehensions work for you? They could potentially be adapted
> to presize the list.
I guess not.
--
Sincerely yours,
Yury V. Zaytsev
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