B035: False positive for comprehensions that use a walrus operator

[Daverball]

        by_payment = {
            payment_id: charge
            for charge in charges
            if (payment_id := charge.metadata.get('payment_id')) is not None
        }

This results in a B035, even though payment_id is definitely not a static value, it's derived from the individual charge object.

I'm not surprised this wasn't caught, because comprehensions and if expressions are some of the few exceptions where the use of a variable can occur before its definition, so it requires special casing when looking up names.

So the error is in in this section of the check

        elif isinstance(node.key, ast.Name):
            if node.key.id not in self._get_dict_comp_loop_var_names(node):
                self.errors.append(
                    B035(node.key.lineno, node.key.col_offset, vars=(node.key.id,))
                )

Instead of only looking for the loop variable names in the target node it also needs to look for any ast.NamedExpr within the ifs nodes of each ast.comprehension.