gh-157491: Resume format-specifier parsing after nested replacement fields

Lib/test/test_fstring.py

@@ -865,6 +865,26 @@ def test_format_specifier_expressions(self):
                              """f'{"s"!{"r"}}'""",
                              ])
 
+    def test_nested_replacement_field_restores_format_specifier(self):
+        class CaptureFormat:
+            def __format__(self, format_spec):
+                self.format_spec = format_spec
+                return ""
+
+        x = CaptureFormat()
+        y = "Y"
+        z = "Z"
+
+        def check_format_spec(result, expected):
+            self.assertEqual(x.format_spec, expected)
+            self.assertEqual(result, "")
+
+        check_format_spec(f'{x:{{y}}}', "{'Y'}")
+        check_format_spec(f'{x:{y}{{z}}}', "Y{'Z'}")
+        check_format_spec(f'{x:{y!s}{{z}}}', "Y{'Z'}")
+        check_format_spec(f'{x:{y=}{{z}}}', "y='Y'{'Z'}")
+        check_format_spec(f'''{x:{y}{{z}}}''', "Y{'Z'}")
+
     def test_custom_format_specifier(self):
         class CustomFormat:
             def __format__(self, format_spec):
@@ -1858,6 +1878,7 @@ def test_gh129093(self):
     def test_newlines_in_format_specifiers(self):
         cases = [
             """f'{1:d\n}'""",
+            """f'{x:{y}\n}'""",
             """f'__{
                 1:d
             }__'""",

Lib/test/test_tokenize.py

@@ -2552,11 +2552,7 @@ def test_degraded_fstring_format_spec(self):
         self.assertEqual(
             [(tok.string, tok.start, tok.end)
              for tok in tokens if tok.type == token.FSTRING_MIDDLE],
-            [
-                ("{", (1, 8), (1, 9)),
-                ("3", (1, 10), (1, 11)),
-                ("}", (1, 12), (1, 13)),
-            ],
+            [("", (1, 13), (1, 13))],
         )
 
         tokens = self._get_tokens('f"{x:{y}}"')

Lib/test/test_tstring.py

@@ -88,6 +88,19 @@ def test_format_specifiers(self):
         )
         self.assertEqual(fstring(t), "Pi: 3.14")
 
+        x = object()
+        y = "Y"
+        z = "Z"
+        for template, expected in (
+            (t"{x:{y}{{z}}}", "Y{'Z'}"),
+            (t"{x:{y!s}{{z}}}", "Y{'Z'}"),
+            (t"{x:{y=}{{z}}}", "y='Y'{'Z'}"),
+            (t'''{x:{y}{{z}}}''', "Y{'Z'}"),
+        ):
+            with self.subTest(template=template):
+                self.assertEqual(template.interpolations[0].format_spec,
+                                 expected)
+
     def test_conversions(self):
         # Test !s conversion (str)
         obj = object()
@@ -313,7 +326,10 @@ def test_syntax_errors(self):
             ("t'{lambda:1}'", "t-string: lambda expressions are not allowed "
                               "without parentheses"),
             ("t'{x:{;}}'", "t-string: expecting a valid expression after '{'"),
-            ("t'{1:d\n}'", "t-string: newlines are not allowed in format specifiers")
+            ("t'{1:d\n}'",
+             "t-string: newlines are not allowed in format specifiers"),
+            ("t'{x:{y}\n}'",
+             "t-string: newlines are not allowed in format specifiers"),
         ):
             with self.subTest(case), self.assertRaisesRegex(SyntaxError, err):
                 eval(case)

Misc/NEWS.d/next/Core_and_Builtins/2026-09-14-11-22-44.gh-issue-157491.5BfNWA.rst

@@ -0,0 +1,2 @@
+Correctly parse braces and report invalid newlines after nested replacement
+fields in f-string and t-string format specifiers.

Parser/lexer/string.c

@@ -78,7 +78,7 @@
     PyObject *res;
     if (comments != NULL && comments->count > 0) {
         Py_ssize_t stripped_size = expr_len;
         _PyTok_Off previous_end = state->expr_span.start;
         Py_ssize_t comment_count = 0;
         for (Py_ssize_t i = 0; i < comments->count; i++) {
             _PyTok_Span comment = comments->spans[i];
@@ -188,7 +188,9 @@
     }
     if (c == '}' && depth == state->replacement_depth - 1) {
         state->replacement_depth--;
-        state->mode = FTSTRING_MODE_MIDDLE;
+        state->mode = state->replacement_depth
+            ? FTSTRING_MODE_FORMAT_SPEC
+            : FTSTRING_MODE_MIDDLE;
         state->debug_expr = 0;
     }
     return 0;