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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def flip_for(self, twig=None, expression=None, **kwargs):
 """ flip the constraint to solve for for any of the parameters in the expression expression (optional if sympy available, required if not) """


        _orig_expression = self.get_value()

        # try to get the parameter from the bundle
        kwargs['twig'] = twig
        newly_constrained_var = self._get_var(**kwargs)
        newly_constrained_param = self.get_parameter(**kwargs)

        check_kwargs = {k:v for k,v in newly_constrained_param.meta.items() if k not in ['context', 'twig', 'uniquetwig']}
        check_kwargs['context'] = 'constraint'
        if len(self._bundle.filter(**check_kwargs)):
            raise ValueError("'{}' is already constrained".format(newly_constrained_param.twig))

        currently_constrained_var = self._get_var(qualifier=self.qualifier, component=self.component)
        currently_constrained_param = currently_constrained_var.get_parameter() # or self.constrained_parameter

        import constraint
        if self.constraint_func is not None and hasattr(constraint, self.constraint_func):
            # then let's see if the method is capable of resolving for use
            # try:
            if True:
                # TODO: this is not nearly general enough, each method takes different arguments
                # and getting solve_for as newly_constrained_param.qualifier

                lhs, rhs, constraint_kwargs = getattr(constraint, self.constraint_func)(self._bundle, solve_for=newly_constrained_param, **self.constraint_kwargs)
            # except NotImplementedError:
            #     pass
            # else:
                # TODO: this needs to be smarter and match to self._get_var().user_label instead of the current uniquetwig

                expression = rhs._value # safe expression
                #~ print "*** flip by recalling method success!", expression

        # print "***", lhs._value, rhs._value

        if expression is not None:
            expression = expression

        elif _use_sympy:


            eq_safe = "({}) - {}".format(self._value, currently_constrained_var.safe_label)

            #~ print "*** solving {} for {}".format(eq_safe, newly_constrained_var.safe_label)

            expression = sympy.solve(eq_safe, newly_constrained_var.safe_label)[0]

            #~ print "*** solution: {}".format(expression)

        else:
            # TODO: ability for built-in constraints to flip themselves
            # we could access self.kind and re-call that with a new solve_for option?
            raise ValueError("must either have sympy installed or provide a new expression")

        self._qualifier = newly_constrained_param.qualifier
        self._component = newly_constrained_param.component
        self._kind = newly_constrained_param.kind

        self._value = str(expression)
        # reset the default_unit so that set_default_unit doesn't complain
        # about incompatible units
        self._default_unit = None
        self.set_default_unit(newly_constrained_param.default_unit)

        self._update_bookkeeping()

        self._add_history(redo_func='flip_constraint', redo_kwargs={'expression': expression, 'uniqueid': newly_constrained_param.uniqueid}, undo_func='flip_constraint', undo_kwargs={'expression': _orig_expression, 'uniqueid': currently_constrained_param.uniqueid})