When the _elementtree module is in use, the TreeBuilder class will raise an IndexError in treebuilder_handle_end if __init__ was passed "None".

I discovered this while writing a subclass of TreeBuilder with a modified __init__ method that delegated to TreeBuilder:

    class MyTreeBuilder(ET.TreeBuilder):
        def __init__(self, element_factory=None):
            super().__init__(element_factory)

Used as a target, this class (and also simply "TreeBuilder(None)") will cause the IndexError when "parser.feed(data)" is called.

>>> import xml.etree.ElementTree as ET
>>> parser = ET.XMLParser(target=ET.TreeBuilder(None))
>>> parser.feed('<file><line>22</line></file>')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IndexError: pop from empty stack

The error is raised from treebuilder_handle_end, but the cause appears to be in treebuilder_handle_start.

    if (self->element_factory) {
        node = PyObject_CallFunction(self->element_factory, "OO", tag, attrib);
    } else {
        node = create_new_element(tag, attrib);
    }

I included a patch adding a check against Py_None to the "if" test above which seems to fix the issue. I also included a simple test case for it.

New changeset c430bea30457 by Eli Bendersky in branch '3.3':
Issue #17901: fix TreeBuilder construction for an explicit element_factory=None
http://hg.python.org/cpython/rev/c430bea30457

New changeset e79df5d1f680 by Eli Bendersky in branch 'default':
Issue #17901: fix TreeBuilder construction for an explicit element_factory=None
http://hg.python.org/cpython/rev/e79df5d1f680

I committed a fix with a simplified test. Thanks for the contribution, Aaron.

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