human
Solve the following python programming problem given the following code:
def as_xml(self, format_datetime = None) : 
    format_datetime = (fmt or json_format_datetime) 
    case = ElementTree.Element ('case') 
    order = ['case_id', 'date_modified', 'create', 'update', 'close', self.CASE_TYPE, 'user_id', 'case_name', 'external_id', 'date_opened', 'owner_id'] 
    def sort_key(item) : 
        (word, _) = item 
        try : 
            i = order.index (word) 
            return (0, i) 
except ValueError : 
            return (1, word) 
def fmt(value) : 
        if (value is None) : 
            return '' 
if isinstance (value, datetime) : 
            return unicode (format_datetime (value)) 
else : 
            if isinstance (value, (basestring, int, date)) : 
                return unicode (value) 
else : 
                raise CaseBlockError ("Can't transform to XML: {}.
unexpected type {}.".format (value, type (value))) 
def dict_to_xml(block, dct) : 
        if dct.has_key ('_attrib') : 
            for (key, value) in dct ['_attrib'].items () : 
                if (value is not CaseBlock.undefined) : 
                    block.set (key, fmt (value)) 
if dct.has_key ('_text') : 
            block.text = unicode (dct ['_text']) 
for (key, value) in sorted (dct.items (), key = sort_key) : 
            if ((value is not CaseBlock.undefined) and (not key.startswith ('_'))) : 
                elem = ElementTree.Element (key) 
                block.append (elem) 
                if isinstance (value, dict) : 
                    dict_to_xml (elem, value) 
else : 
                    elem.text = fmt (value) 
dict_to_xml (case, self) 
    return case 

    
    

    Fix the buggy line: format_datetime = (fmt or json_format_datetime)
assistant
#Here is Python program to fix the buggy line: format_datetime = (fmt or json_format_datetime)
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format_datetime = (format_datetime or json_format_datetime)