>>> filter_expr = 'element_set=Os,O'
>>> print(element_set_conversion(filter_expr))
( element=" element="O" s"  AND  element="O" )
>>> filter_expr = 'element_set=Nb,N'
>>> print(element_set_conversion(filter_expr))
( element=" element="N" b"  AND  element="N" )

def element_set_conversion(filter_expr):
    """
    Convert element_set filter to multiple element filters 
    Input: 
        :str filter_expr: raw filter expression w/ element_set parameter
            Valid element_set expression: 
                ',': AND operator
                '-': OR operator
                '~': NOT operator
                '(', ')': to change precedence
            Examples:
                element_set=Al;O,H
                element_set=(Mn;Fe),O
    Output:
        :str : converted filter expression
    """
    filter_expr_out = filter_expr
    for els in re.findall("element_set=[\S]*", filter_expr):
        els_in = els
        els_out = ''
        for el in re.findall("[A-Z][a-z]*", els_in):
            bels, els_in = tuple(els_in.split(el, maxsplit=1))
            els_out += bels + ' element="' + el + '" '
        els_out = els_out.replace("element_set=", "")
        els_out = els_out.replace(",", " AND ")
        els_out = els_out.replace("-", " OR ")
        
        filter_expr_out = filter_expr_out.replace(els, "(" + els_out + ")")
    
    return filter_expr_out

>>> filter_expr = 'element_set=Os,O'
>>> print(element_set_conversion(filter_expr))
( element="Os"  AND  element="O" )
>>> filter_expr = 'element_set=Nb,N'
>>> print(element_set_conversion(filter_expr))
( element="Nb"  AND  element="N" )