Cannot use assignment expressions with subscript

Question

if session[\'dp\'] := current_user.avatar :
    ^ SyntaxError: cannot use assignment expressions with subscript

Why Python forbids this use of walr

Answer 1

Because, as the alternative name (named expressions) suggests, the left hand side of the walrus operator is to be a NAME. Therefore, by definition such expressions as noted in your question as well as, for instance, function calls are not allowed to be assigned in this form.

The documentation also specifies:

Single assignment targets other than a single NAME are not supported

To further this argument, one can notice that cPython explicitly checks if the expression is Name_kind:

if (target->kind != Name_kind) {
    const char *expr_name = get_expr_name(target);
    if (expr_name != NULL) {
        ast_error(c, n, "cannot use assignment expressions with %s", expr_name);
    }
    return NULL;
}