From 966ae8886174a9d56b6769d81e153757679999ab Mon Sep 17 00:00:00 2001
From: lipengyu <lipengyu@kylinos.cn>
Date: Thu, 15 Oct 2026 23:00:14 +0800
Subject: [PATCH] gh-157491: Restore format-specifier state after nested
 replacement fields

---
 Lib/test/test_fstring.py                      | 21 +++++++++++++++++++
 Lib/test/test_tstring.py                      | 18 +++++++++++++++-
 ...-10-15-11-22-44.gh-issue-157491.5BfNWA.rst |  2 ++
 Parser/lexer/lexer.c                          |  2 +-
 4 files changed, 41 insertions(+), 2 deletions(-)
 create mode 100644 Misc/NEWS.d/next/Core_and_Builtins/2026-10-15-11-22-44.gh-issue-157491.5BfNWA.rst

diff --git a/Lib/test/test_fstring.py b/Lib/test/test_fstring.py
index 0f87deebf7a8dd..de2b4ccc788c7e 100644
--- a/Lib/test/test_fstring.py
+++ b/Lib/test/test_fstring.py
@@ -869,6 +869,26 @@ 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):
@@ -1871,6 +1891,7 @@ def test_gh129093(self):
     def test_newlines_in_format_specifiers(self):
         cases = [
             """f'{1:d\n}'""",
+            """f'{x:{y}\n}'""",
             """f'__{
                 1:d
             }__'""",
diff --git a/Lib/test/test_tstring.py b/Lib/test/test_tstring.py
index 1e5f14ca007d1e..6ba2f997cb04a5 100644
--- a/Lib/test/test_tstring.py
+++ b/Lib/test/test_tstring.py
@@ -138,6 +138,19 @@ def test_format_specifiers(self):
         )
         self.assertEqual(fstring(t), "         1")
 
+        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()
@@ -342,7 +355,10 @@ 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)
diff --git a/Misc/NEWS.d/next/Core_and_Builtins/2026-10-15-11-22-44.gh-issue-157491.5BfNWA.rst b/Misc/NEWS.d/next/Core_and_Builtins/2026-10-15-11-22-44.gh-issue-157491.5BfNWA.rst
new file mode 100644
index 00000000000000..6fd2f557d6dc08
--- /dev/null
+++ b/Misc/NEWS.d/next/Core_and_Builtins/2026-10-15-11-22-44.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.
diff --git a/Parser/lexer/lexer.c b/Parser/lexer/lexer.c
index af57deb12cbefb..d3ffcc29f0d284 100644
--- a/Parser/lexer/lexer.c
+++ b/Parser/lexer/lexer.c
@@ -1323,7 +1323,7 @@ tok_get_normal_mode(struct tok_state *tok, tokenizer_mode* current_tok, struct t
             if (c == '}' && current_tok->curly_bracket_depth == current_tok->curly_bracket_expr_start_depth) {
                 current_tok->curly_bracket_expr_start_depth--;
                 current_tok->kind = TOK_FSTRING_MODE;
-                current_tok->in_format_spec = 0;
+                current_tok->in_format_spec = INSIDE_FSTRING_EXPR(current_tok);
                 current_tok->in_debug = 0;
             }
         }