improve comment and simplify phrasing of genexp grammar in the documentation

Author: adqm

Doc/reference/expressions.rst

@@ -418,7 +418,7 @@ Generator expressions
 A generator expression is a compact generator notation in parentheses:
 
 .. productionlist:: python-grammar
-   generator_expression: "(" (`expression` | "*"  `or_expr`) `comp_for` ")"
+   generator_expression: "(" `starred_expression` `comp_for` ")"
 
 A generator expression yields a new generator object.  Its syntax is the same as
 for comprehensions, except that it is enclosed in parentheses instead of

Python/codegen.c

@@ -4522,7 +4522,6 @@ codegen_sync_comprehension_generator(compiler *c, location loc,
             break;
         case COMP_DICTCOMP:
             if (val == NULL){
-                // dict unpacking case
                 VISIT(c, expr, elt);
                 ADDOP_I(c, elt_loc, DICT_UPDATE, depth+1);
             }
@@ -4621,8 +4620,9 @@ codegen_async_comprehension_generator(compiler *c, location loc,
         switch (type) {
         case COMP_GENEXP:
             if (elt->kind == Starred_kind){
-                // ADD_YIELD_FROM won't work here so we need an explicit loop
-                // this block is loosely based on codegen_for
+                /* ADD_YIELD_FROM won't work here like it does in the
+                synchronous case, so use an explicit loop to mimic the
+                bytecode for (z async for y in x for z in y). */
                 NEW_JUMP_TARGET_LABEL(c, async_unpack_start);
                 NEW_JUMP_TARGET_LABEL(c, async_unpack_end);
                 VISIT(c, expr, elt->v.Starred.value);
@@ -4664,7 +4664,6 @@ codegen_async_comprehension_generator(compiler *c, location loc,
             break;
         case COMP_DICTCOMP:
             if (val == NULL){
-                // dict unpacking case
                 VISIT(c, expr, elt);
                 ADDOP_I(c, elt_loc, DICT_UPDATE, depth+1);
             }