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If, after building of Python, libexpat.so (library used by pyexpat module) has been broken/removed or pyexpat module has been broken/removed, then attempt of import of _elementtree module, which requires pyexpat module, will raise strange exceptions in Python 3.
These exceptions can be simulated by setting sys.modules["pyexpat"] = None.
$ python2.6 -c 'import sys; sys.modules["pyexpat"] = None; import _elementtree'
Traceback (most recent call last):
 File "<string>", line 1, in <module>
ImportError: No module named pyexpat
$ python2.7 -c 'import sys; sys.modules["pyexpat"] = None; import _elementtree'
Traceback (most recent call last):
 File "<string>", line 1, in <module>
ImportError: PyCapsule_Import could not import module "pyexpat"
$ python3.1 -c 'import sys; sys.modules["pyexpat"] = None; import _elementtree'
Traceback (most recent call last):
 File "<string>", line 1, in <module>
SystemError: initialization of _elementtree raised unreported exception
$ python3.2 -c 'import sys; sys.modules["pyexpat"] = None; import _elementtree'
Traceback (most recent call last):
 File "<string>", line 1, in <module>
SystemError: initialization of _elementtree raised unreported exception
$ python3.3 -c 'import sys; sys.modules["pyexpat"] = None; import _elementtree'
Traceback (most recent call last):
 File "<string>", line 1, in <module>
 File "<frozen importlib._bootstrap>", line 1286, in _find_and_load
 File "<frozen importlib._bootstrap>", line 1253, in _find_and_load_unlocked
 File "<frozen importlib._bootstrap>", line 432, in _check_name_wrapper
 File "<frozen importlib._bootstrap>", line 347, in set_package_wrapper
 File "<frozen importlib._bootstrap>", line 360, in set_loader_wrapper
 File "<frozen importlib._bootstrap>", line 870, in load_module
RuntimeError: cannot load dispatch table from pyexpat
I suggest to raise ImportError instead of SystemError or RuntimeError.
Third-party modules would rather use only 'except ImportError: pass' instead of 'except (ImportError, SystemError, RuntimeError): pass' when trying to optionally use e.g. xml.etree.cElementTree.
xml.etree.ElementTree in Python 3.3 also ignores only ImportError when trying to import _elementtree.