Resolve a variable name in a possibly local context

        Parameters
        ----------
        key : str
            A variable name
        is_local : bool
            Flag indicating whether the variable is local or not (prefixed with
            the '@' symbol)

        Returns
        -------
        value : object
            The value of a particular variable

def resolve(self, key, is_local):
        """Resolve a variable name in a possibly local context

        Parameters
        ----------
        key : str
            A variable name
        is_local : bool
            Flag indicating whether the variable is local or not (prefixed with
            the '@' symbol)

        Returns
        -------
        value : object
            The value of a particular variable
        """
        try:
            # only look for locals in outer scope
            if is_local:
                return self.scope[key]

            # not a local variable so check in resolvers if we have them
            if self.has_resolvers:
                return self.resolvers[key]

            # if we're here that means that we have no locals and we also have
            # no resolvers
            assert not is_local and not self.has_resolvers
            return self.scope[key]
        except KeyError:
            try:
                # last ditch effort we look in temporaries
                # these are created when parsing indexing expressions
                # e.g., df[df > 0]
                return self.temps[key]
            except KeyError:
                raise compu.ops.UndefinedVariableError(key, is_local)