human
Solve the following python programming problem given the following code:
def monomial_key(order = None, gens = None) : 
    '
    Return a function defining admissible order on monomials.

    The result of a call to :func:`monomial_key` is a function which should
    be used as a key to :func:`sorted` built-in function, to provide order
    in a set of monomials of the same length.

    Currently supported monomial orderings are:

    1. lex       - lexicographic order (default)
    2. grlex     - graded lexicographic order
    3. grevlex   - reversed graded lexicographic order
    4. ilex, igrlex, igrevlex - the corresponding inverse orders

    If the ``order`` input argument is not a string but has ``__call__``
    attribute, then it will pass through with an assumption that the
    callable object defines an admissible order on monomials.

    If the ``gens`` input argument contains a list of generators, the
    resulting key function can be used to sort SymPy ``Expr`` objects.

    ' 
    if (order is None) : 
        order = lex 
if isinstance (order, Symbol) : 
        order = str (order) 
if isinstance (gens, str) : 
        try : 
            order = _monomial_key [order] 
except KeyError : 
            raise ValueError (("supported monomial orderings are 'lex', 'grlex' and 'grevlex', got %r" % order)) 
if hasattr (order, '__call__') : 
        if (gens is not None) : 
            def _order(expr) : 
                return order (expr.as_poly (* gens).degree_list ()) 
return _order 
return order 
else : 
        raise ValueError (('monomial ordering specification must be a string or a callable, got %s' % order)) 

    
    

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

if isinstance (order, str) :