The following code results in refleaks:
import sys
import _elementtree
builder = _elementtree.TreeBuilder()
parser = _elementtree.XMLParser(target=builder)

refcount_before = sys.gettotalrefcount()
parser.__init__(target=builder)
print(sys.gettotalrefcount() - refcount_before)  # should be close to 0

This is because _elementtree_XMLParser___init___impl()
(in Modules/_elementtree.c) doesn't decref before assigning to fields of
`self`.


The following code also results in refleaks:
import sys
import _elementtree
elem = _elementtree.Element(42)
elem.__setstate__({'tag': 42, '_children': list(range(1000))})

refcount_before = sys.gettotalrefcount()
elem.__setstate__({'tag': 42, '_children': []})
print(sys.gettotalrefcount() - refcount_before)  # should be close to -1000

This is because element_setstate_from_attributes() doesn't decref the old
children before storing the new children.


I would open a PR to fix this soon.

Shame on me. I only now found out that Serhiy already mentioned most of the refleaks
in https://bugs.python.org/issue31455#msg302103.

According to Serhiy's advice (https://bugs.python.org/issue31455#msg304338),
this issue now also includes some crashes in _elementtree:


The following code crashes:
import _elementtree
parser = _elementtree.XMLParser.__new__(_elementtree.XMLParser)
parser.close()

This is because _elementtree_XMLParser_close_impl() assumes that the XMLParser
object is initialized, and so it passes `self` to expat_parse(), which assumes
that `self->parser` is valid, and crashes.
Similarly, calling feed(), _parse_whole() or _setevents(), or reading the
`entity` or `target` attribute of an uninitialized XMLParser object would
result in a crash.


ISTM that PR 3956 is more complex, and already not so small, so i would soon
open another PR to fix these crashes.

This has been pending for a while too long, but the fixes look good to me. They should still go at least into Py3.8 and later.

New changeset 402e1cdb132f384e4dcde7a3d7ec7ea1fc7ab527 by Oren Milman in branch 'master':
bpo-31758: Prevent crashes when using an uninitialized _elementtree.XMLParser object (GH-3997)
https://github.com/python/cpython/commit/402e1cdb132f384e4dcde7a3d7ec7ea1fc7ab527

New changeset 61511488cf4e7a1cb57a38efba7e0a84a387fe58 by Miss Islington (bot) in branch '3.8':
bpo-31758: Prevent crashes when using an uninitialized _elementtree.XMLParser object (GH-3997) (GH-19485)
https://github.com/python/cpython/commit/61511488cf4e7a1cb57a38efba7e0a84a387fe58

Let's add it to the last bug fix release of 3.7 as well. It fixes a crash bug, after all.

New changeset 096e41aa4e558b28b7260fe01eb21414b1458b20 by Miss Islington (bot) in branch '3.7':
[3.7] bpo-31758: Prevent crashes when using an uninitialized _elementtree.XMLParser object (GH-3997) (GH-19487)
https://github.com/python/cpython/commit/096e41aa4e558b28b7260fe01eb21414b1458b20

Thanks for the fix, Oren!