[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, 18 insertions, 0 deletions

diff --git a/Lib/test/test_fstring.py b/Lib/test/test_fstring.py
index ba223ae..cb14bba 100644
--- a/Lib/test/test_fstring.py
+++ b/Lib/test/test_fstring.py
@@ -496,6 +496,24 @@ x = (
         self.assertEqual(wat2.end_col_offset, 17)
         self.assertEqual(fstring.end_col_offset, 18)
 
+    def test_ast_fstring_empty_format_spec(self):
+        expr = "f'{expr:}'"
+
+        mod = ast.parse(expr)
+        self.assertEqual(type(mod), ast.Module)
+        self.assertEqual(len(mod.body), 1)
+
+        fstring = mod.body[0].value
+        self.assertEqual(type(fstring), ast.JoinedStr)
+        self.assertEqual(len(fstring.values), 1)
+
+        fv = fstring.values[0]
+        self.assertEqual(type(fv), ast.FormattedValue)
+
+        format_spec = fv.format_spec
+        self.assertEqual(type(format_spec), ast.JoinedStr)
+        self.assertEqual(len(format_spec.values), 0)
+
     def test_docstring(self):
         def f():
             f'''Not a docstring'''