human
Solve the following python programming problem given the following code:
def _format_errors(self) : 
    'Returns a string listing all errors within a document' 
    def generate_key(value, prefix = '') : 
        if isinstance (value, list) : 
            value = ' '.join ([generate_key (k) for k in value]) 
else : 
            if isinstance (value, dict) : 
                value = ' '.join ([generate_key (v, k) for (k, v) in value.iteritems ()]) 
results = (('%s.%s' % (prefix, value)) if prefix else value) 
        return results 
error_dict = defaultdict (list) 
    for (k, v) in self.to_dict ().iteritems () : 
        error_dict [generate_key (v)].append (k) 
return ' '.join ([('%s: %s' % (k, generate_key)) for (k, v) in error_dict.iteritems ()]) 

    
    

    Fix the buggy line: return ' '.join ([('%s: %s' % (k, generate_key)) for (k, v) in error_dict.iteritems ()])
assistant
#Here is Python program to fix the buggy line: return ' '.

return ' '.join ([('%s: %s' % (k, v)) for (k, v) in error_dict.iteritems ()])