gh-157778: Include opening brace line in multiline f-string errors (#157779)

Author: pablogsal

Lib/test/test_fstring.py

@@ -617,6 +617,34 @@ def test_unterminated_string(self):
                              r"""f'{("x}'""",
                              ])
 
+    def test_unclosed_multiline_replacement_field(self):
+        for prefix in ('f', 't', 'rf', 'rt'):
+            for quote in ('"', "'"):
+                triple = quote * 3
+                cases = (
+                    # The apparent closing quotes open a string in the field.
+                    ('var = "abc"\na = PREFIXQUOTE {var} extern "C" { QUOTE\n'
+                     'b = QUOTE string QUOTE', 2),
+                    # Parentheses and dictionaries inside the field must not
+                    # change which opening brace the diagnostic identifies.
+                    ('a = PREFIXQUOTE{\n(QUOTE', 1),
+                    ('a = PREFIXQUOTE{\n{0: QUOTE', 1),
+                    # Use the innermost format field or formatted string.
+                    ('a = PREFIXQUOTE{0:\n{1\nQUOTE', 2),
+                    ('a = f"{\nPREFIXQUOTE{1\nQUOTE', 2),
+                    # Account for parentheses outside the formatted string.
+                    ('a = (PREFIXQUOTE{1\nQUOTE', 1),
+                )
+                for source, lineno in cases:
+                    source = source.replace('PREFIX', prefix).replace('QUOTE', triple)
+                    with self.subTest(source=source):
+                        with self.assertRaises(SyntaxError) as cm:
+                            compile(source, '<test>', 'exec')
+                        self.assertEqual(
+                            cm.exception.msg,
+                            f"{prefix[-1]}-string: expecting '}}' to close '{{' "
+                            f"on line {lineno}")
+
     @unittest.skipIf(support.is_wasi, "exhausts limited stack on WASI")
     def test_mismatched_parens(self):
         self.assertAllRaise(SyntaxError, r"closing parenthesis '\}' "

Misc/NEWS.d/next/Core_and_Builtins/2026-09-19-12-00-00.gh-issue-157778.fstring-brace.rst

@@ -0,0 +1,3 @@
+Include the line number of the opening brace in the :exc:`SyntaxError` for
+an unclosed f-string or t-string replacement field when a matching quote
+on a later line is interpreted as the start of an unterminated string.

Parser/lexer/string.c

@@ -355,9 +355,19 @@ _PyLexer_scan_string(struct tok_state *tok, struct token *token, int c)
 
             const ftstring_state *state = _PyLexer_CurrentFTString(tok);
             if (state != NULL) {
-                /* A matching quote belongs to the surrounding formatted
-                 * string, so the expression is missing its closing brace. */
+                /* A matching quote may have been intended to close the
+                 * surrounding formatted string instead of opening a string
+                 * inside a replacement field. */
                 if (state->quote == quote && state->quote_size == quote_size) {
+                    int level = state->paren_level + state->replacement_depth - 1;
+                    assert(level >= 0 && level < tok->level);
+                    assert(tok->parenstack[level] == '{');
+                    int lineno = tok->parenlinenostack[level];
+                    if (lineno != tok->lineno) {
+                        return MAKE_TOKEN(_PyTokenizer_syntaxerror(tok,
+                            "%c-string: expecting '}' to close '{' on line %d",
+                            _PyLexer_StringPrefix(state->kind), lineno));
+                    }
                     return MAKE_TOKEN(_PyTokenizer_syntaxerror(tok,
                         "%c-string: expecting '}'",
                         _PyLexer_StringPrefix(state->kind)));