Python: Allow comments in subscripts

Once again, the interaction between anchors and extras (specifically
comments) was causing trouble.

The root of the problem was the fact that in `a[b]`, we put `b` in the
`index` field of the subscript node, whereas in `a[b,c]`, we
additionally synthesize a `Tuple` node for `b,c` (which matches the
Python AST).

To fix this, we refactored the grammar slightly so as to make that tuple
explicit, such that a subscript node either contains a single expression
or the newly added tuple node. This greatly simplifies the logic.

python/extractor/tsg-python/python.tsg

@@ -22,7 +22,7 @@
 (assignment !type) @assign
 { let @assign.node = (ast-node @assign "Assign") }
 
-[ (expression_list) (tuple) (tuple_pattern) (pattern_list) ] @tuple
+[ (expression_list) (tuple) (tuple_pattern) (pattern_list) (index_expression_list) ] @tuple
 { let @tuple.node = (ast-node @tuple "Tuple") }
 
 (list_pattern) @list
@@ -2543,66 +2543,16 @@
 
 (subscript
     value: (_) @value
+    subscript: (_) @index
 ) @subscript
 {
     attr (@subscript.node) value = @value.node
     attr (@value.node) ctx = "load"
-}
 
-; Single subscript
-(subscript
-    value: (_)
-    .
-    subscript: (_) @index
-    .
-) @subscript
-{
     attr (@subscript.node) index = @index.node
     attr (@index.node) ctx = "load"
 }
 
-; For expressions of the form `a[b, c]` we must explicitly synthesize an internal tuple node
-; We do this and also hook it up:
-(subscript
-    value: (_)
-    .
-    subscript: (_) @first
-    .
-    subscript: (_)
-) @subscript
-{
-    let @subscript.tuple = (ast-node @first "Tuple")
-    attr (@subscript.tuple) ctx = "load"
-    attr (@subscript.node) index = @subscript.tuple
-    edge @subscript.tuple -> @first.node
-    attr (@subscript.tuple -> @first.node) elts = (named-child-index @first)
-    attr (@first.node) ctx = "load"
-}
-
-(subscript
-    value: (_)
-    .
-    subscript: (_)
-    subscript: (_) @elt
-) @subscript
-{
-    edge @subscript.tuple -> @elt.node
-    attr (@subscript.tuple -> @elt.node) elts = (named-child-index @elt)
-    attr (@elt.node) ctx = "load"
-}
-
-
-; Set the end position correctly
-(subscript
-    value: (_)
-    .
-    subscript: (_)
-    subscript: (_) @last
-    .
-) @subscript
-{
-    attr (@subscript.tuple) _location_end = (location-end @last)
-}
 
 
 
@@ -3448,9 +3398,12 @@
     ; Left hand side of an assignment such as `[foo, bar] = ...`
     (list_pattern element: (_) @elt) @parent
 
-    ; An unadorned tuple (such as in `x = y, z`)
+    ; An unadorned tuple such as in `x = y, z`
     (expression_list element: (_) @elt) @parent
 
+    ; An index containing multiple indices such as in `x[y, z]`
+    (index_expression_list element: (_) @elt) @parent
+
     ; A regular tuple such as `(x, y, z)`
     (tuple element: (_) @elt) @parent
 
@@ -3486,6 +3439,7 @@
     (pattern_list element: (_) @elt)
     (list_pattern element: (_) @elt)
     (expression_list element: (_) @elt)
+    (index_expression_list element: (_) @elt)
     (parenthesized_expression inner: (_) @elt)
     (set element: (_) @elt)
     (match_sequence_pattern (_) @elt)

python/extractor/tsg-python/tsp/grammar.js

@@ -929,11 +929,18 @@ module.exports = grammar({
       field('attribute', $.identifier)
     )),
 
+    _index_expression: $ => choice(
+      $.list_splat,
+      $.expression,
+      $.slice
+    ),
+
+    index_expression_list: $ => open_sequence(field('element', $._index_expression)),
+
     subscript: $ => prec(PREC.call, seq(
       field('value', $.primary_expression),
       '[',
-      commaSep1(field('subscript', choice($.list_splat, $.expression, $.slice))),
-      optional(','),
+      field('subscript', choice($._index_expression, $.index_expression_list)),
       ']'
     )),