def _print_Derivative(self, expr):
     if not isinstance(expr.args[1], (_AppliedUndef, sp.Symbol)):
         error("Can only print Derivative code with a single dependent "\
               "variabe. Got: {0}".format(sympycode(expr.args[1])))
     
     if isinstance(expr.args[0], _AppliedUndef):
         return "d%s_d%s" % (expr.args[0].func.__name__, "_".join(\
             self._print(arg) for arg in expr.args[1:]))
     return _StrPrinter._print_Derivative(self, expr)

    def _print_Float(self, expr):

        # If not finite we use parent printer
        if expr.is_zero:
            return "0"
        
        if not expr.is_finite:
            return _StrPrinter._print_Float(self, expr)

        return str(float(expr))

def test_empty_printer():
    str_printer = StrPrinter()
    assert str_printer.emptyPrinter("foo") == "foo"
    assert str_printer.emptyPrinter(x*y) == "x*y"
    assert str_printer.emptyPrinter(32) == "32"

 def __init__(self, namespace=""):
     assert(namespace in ["", "math", "np", "numpy", "ufl"])
     self._namespace = namespace if not namespace else namespace + "."
     _StrPrinter.__init__(self, settings=dict(order=_order))