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GH-98363: Presize the list for batched() by rhettinger · Pull Request #98419 · python/cpython · GitHub
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GH-98363: Presize the list for batched() #98419

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Merged
rhettinger merged 6 commits into from
Oct 20, 2022
Merged

GH-98363: Presize the list for batched() #98419

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ea9511e
Presize the list for batched()
rhettinger Oct 18, 2022
88fc1b1
Fast path for the common case (zero new items).
rhettinger Oct 19, 2022
0e05aa4
Cleaner handling of empty result
rhettinger Oct 19, 2022
646630e
Let NULL error flow through the return
rhettinger Oct 19, 2022
5e38c09
Use n variable consistently throughout
rhettinger Oct 19, 2022
fd5b508
Remove spurious include directive
rhettinger Oct 19, 2022
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Cleaner handling of empty result
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@rhettinger
rhettinger committed Oct 19, 2022
commit 0e05aa486104ca6639e43f7e93fca7708696dacc
19 changes: 10 additions & 9 deletions Modules/itertoolsmodule.c
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Original file line number Diff line number Diff line change
Expand Up @@ -142,37 +142,38 @@ static PyObject *
batched_next(batchedobject *bo)
{
Py_ssize_t i;
Py_ssize_t n = bo->batch_size;
PyObject *it = bo->it;
PyObject *item;
PyObject *result;

if (it == NULL) {
return NULL;
}
result = PyList_New(bo->batch_size);
result = PyList_New(n);
if (result == NULL) {
return NULL;
}
for (i=0 ; i < bo->batch_size ; i++) {
for (i=0 ; i < n ; i++) {
item = PyIter_Next(it);
if (item == NULL) {
break;

@sweeneyde sweeneyde Oct 20, 2022

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Do we care how this handles strange iterators that keep going after having raised StopIteration?

>>> from itertools import batched
>>> class A:
...     def __init__(self):
...         self.i = 0
...     def __iter__(self):
...         return self
...     def __next__(self):
...         i = self.i
...         self.i += 1
...         if i == 7:
...             raise StopIteration()
...         return i
...
>>> x = batched(A(), 5)
>>> next(x)
[0, 1, 2, 3, 4]
>>> next(x)
[5, 6]
>>> next(x)
[8, 9, 10, 11, 12]

Maybe the Py_CLEAR(bo->it) could be moved to before this break;.

@rhettinger rhettinger Oct 20, 2022 •
edited
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I'm thinking of leaving this as is. The current PR matches the behavior of the pure python version and the strange iterator works the same way with other tools:

>>> it = zip(range(10), A())
>>> list(it)
[(0, 0), (1, 1), (2, 2), (3, 3), (4, 4), (5, 5), (6, 6)]
>>> list(it)
[(8, 8), (9, 9)]

>>> it = enumerate(A())
>>> list(it)
[(0, 0), (1, 1), (2, 2), (3, 3), (4, 4), (5, 5), (6, 6)]
>>> next(it)
(7, 8)
>>> next(it)
(8, 9)

That said, I'm not really sure about this one and could easily be persuaded to add a Py_CLEAR(bo->it) to the code path for the short list.

@sweeneyde sweeneyde Oct 20, 2022 •
edited
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This does seem like an edge case that could go either way.

I'm noticing that some iterators do Py_CLEAR(obj->it) as soon as the next() call fails, while other objects have internal iterators that are never NULL. My concern is that batched() clears the iterator sometimes (when the first next() of a batch fails), but not other times (when a subsequent next() fails).

So I would think to do one or the other, but not both:

  1. Clear the iterator immediately after any internal next() call fails, similar to pairwise, islice, chain, and cycle (sort of).

  2. Never clear the iterator and remove the if (it == NULL) check altogether, similar to dropwhile, takewhile, accumulate, compress, zip and enumerate. Unfortunately, it would mean allocating and throwing away PyList_New(n), but that's in the rare case that someone is abusing the iterator.

Maybe removing the Py_CLEAR and the NULL check is most natural?

}
PyList_SET_ITEM(result, i, item);
}
if (i > 0 && i < bo->batch_size) {
if (i == 0) {
Py_CLEAR(bo->it);
Py_DECREF(result);
return NULL;
}
if (i < bo->batch_size) {
PyObject *short_list = PyList_GetSlice(result, 0, i);
Py_SETREF(result, short_list);
if (result == NULL) {
return NULL;
}
}
if (i > 0) {
return result;
}
Py_CLEAR(bo->it);
Py_DECREF(result);
return NULL;
return result;
}

static PyTypeObject batched_type = {
Expand Down