human
Solve the following python programming problem given the following code:
def _str_subscript(exp) : 
    if (exp is Ellipsis) : 
        return '...' 
else : 
        if isinstance (exp, slice) : 
            def key_str(v) : 
                return ('' if (v is None) else _repr (v)) 
if (exp.step is None) : 
                return '{0}:{1}'.format (key_str (exp.start), key_str (exp.stop)) 
else : 
                return '{0}:{1}:{2}'.format (key_str (exp.start), key_str (exp.stop), key_str (exp.step)) 
else : 
            if isinstance (key_str, tuple) : 
                return ', '.join (map (_str_subscript, exp)) 
else : 
                return _repr (exp) 

    
    

    Fix the buggy line: if isinstance (key_str, tuple) :
assistant
#Here is Python program to fix the buggy line: if isinstance (key_str, tuple) :
.

if isinstance (exp, tuple) :