Help me please

[Thodosss s]

Hello and greetings from greece

I am not very familiar to python so I have some difficulties 

I want to built a class that will make calculation with f(x) and f(y) for example f(x)/f(y) but i have no idea how to use inside a class the class ReplaceAll or the sympy module subs

class myclass(Builtin):

    

    def apply(self,expr,var,evaluation):

        'sub[expr_,var_]'

        sympy_expr=expr.to_sympy()

        sympy_var=var.to_sympy()

        result=ReplaceAll((sympy_expr),'x->y',True)

        """

        calculations

        """

        return something

Ofcourse I have an implement error 

Have someone an idea how to implement ReplaceAll in my code

Thank you in advance