[3.12] gh-106396: Special-case empty format spec to gen empty JoinedStr node (GH-106401) (#106416)

(cherry picked from commit dfe4de203881e8d068e6fc5b8e31075841a86d25)

Co-authored-by: Lysandros Nikolaou <lisandrosnik@gmail.com>

1 files changed, 12 insertions, 0 deletions

diff --git a/Parser/action_helpers.c b/Parser/action_helpers.c
index dbad56b..e68a9ca 100644
--- a/Parser/action_helpers.c
+++ b/Parser/action_helpers.c
@@ -1003,6 +1003,18 @@ _PyPegen_setup_full_format_spec(Parser *p, Token *colon, asdl_expr_seq *spec, in
     if (!spec) {
         return NULL;
     }
+
+    // This is needed to keep compatibility with 3.11, where an empty format spec is parsed
+    // as an *empty* JoinedStr node, instead of having an empty constant in it.
+    if (asdl_seq_LEN(spec) == 1) {
+        expr_ty e = asdl_seq_GET(spec, 0);
+        if (e->kind == Constant_kind
+                && PyUnicode_Check(e->v.Constant.value)
+                && PyUnicode_GetLength(e->v.Constant.value) == 0) {
+            spec = _Py_asdl_expr_seq_new(0, arena);
+        }
+    }
+
     expr_ty res = _PyAST_JoinedStr(spec, lineno, col_offset, end_lineno, end_col_offset, p->arena);
     if (!res) {
         return NULL;