def docTestLineNumberFix():
    """Modify doctest behaviour to figure out line numbers of strings in __test__
    """
    class MyDocTestFinder(doctest.DocTestFinder):
        def _find_lineno(self, obj, source_lines):
            if isinstance(obj, str):
                #find a unique line in the string, so we know where it is
                for offset,line in enumerate(obj.splitlines(True)):
                    if source_lines.count(line)==1:
                        return source_lines.index(line)-offset
            else: return super()._find_lineno(obj, source_lines)
    doctest.DocTestFinder = MyDocTestFinder