gh-142236: Improve error location for missing comma in string concatenations

When a multi-line string concatenation is missing a comma before the
next argument, the error previously pointed to the first string in the
concatenation, which was confusing because the actual fix needed to be
applied after the last string. This change adds a helper function that
detects multi-line expressions and adjusts the error position to point
to the last line, making it immediately clear where the comma should be
added.

Author: pablogsal

Grammar/python.gram

@@ -1252,7 +1252,7 @@ invalid_expression:
     # Soft keywords need to also be ignored because they can be parsed as NAME NAME
    | !(NAME STRING | SOFT_KEYWORD) a=disjunction b=expression_without_invalid {
         _PyPegen_check_legacy_stmt(p, a) ? NULL : p->tokens[p->mark-1]->level == 0 ? NULL :
-        RAISE_SYNTAX_ERROR_KNOWN_RANGE(a, b, "invalid syntax. Perhaps you forgot a comma?") }
+        _PyPegen_raise_error_for_missing_comma(p, a, b) }
    | a=disjunction 'if' b=disjunction !('else'|':') { RAISE_SYNTAX_ERROR_KNOWN_RANGE(a, b, "expected 'else' after 'if' expression") }
    | a=disjunction 'if' b=disjunction 'else' !expression {
         RAISE_SYNTAX_ERROR_ON_NEXT_TOKEN("expected expression after 'else', but statement is given") }

Lib/test/test_syntax.py

@@ -3336,6 +3336,20 @@ def test_multiline_compiler_error_points_to_the_end(self):
             lineno=3
         )
 
+    def test_multiline_string_concat_missing_comma_points_to_last_string(self):
+        # gh-142236: For multi-line string concatenations with a missing comma,
+        # the error should point to the last string, not the first.
+        self._check_error(
+            "print(\n"
+            '    "line1"\n'
+            '    "line2"\n'
+            '    "line3"\n'
+            "    x=1\n"
+            ")",
+            "Perhaps you forgot a comma",
+            lineno=4,  # Points to "line3", the last string
+        )
+
     @support.cpython_only
     def test_syntax_error_on_deeply_nested_blocks(self):
         # This raises a SyntaxError, it used to raise a SystemError. Context

Misc/NEWS.d/next/Core_and_Builtins/2025-12-06-00-38-37.gh-issue-142236.m3EF9E.rst

@@ -0,0 +1,3 @@
+Improve the "Perhaps you forgot a comma?" syntax error for multi-line string
+concatenations to point to the last string instead of the first, making it
+easier to locate where the comma is missing. Patch by Pablo Galindo.

Parser/action_helpers.c

@@ -947,6 +947,27 @@ _PyPegen_check_legacy_stmt(Parser *p, expr_ty name) {
     return 0;
 }
 
+void *
+_PyPegen_raise_error_for_missing_comma(Parser *p, expr_ty a, expr_ty b)
+{
+    // For multi-line expressions (like string concatenations), point to the
+    // last line instead of the first for a more helpful error message.
+    // Use a->col_offset as the starting column since all strings in the
+    // concatenation typically share the same indentation.
+    if (a->end_lineno > a->lineno) {
+        return RAISE_ERROR_KNOWN_LOCATION(
+            p, PyExc_SyntaxError, a->end_lineno, a->col_offset,
+            a->end_lineno, a->end_col_offset,
+            "invalid syntax. Perhaps you forgot a comma?"
+        );
+    }
+    return RAISE_ERROR_KNOWN_LOCATION(
+        p, PyExc_SyntaxError, a->lineno, a->col_offset,
+        b->end_lineno, b->end_col_offset,
+        "invalid syntax. Perhaps you forgot a comma?"
+    );
+}
+
 static ResultTokenWithMetadata *
 result_token_with_metadata(Parser *p, void *result, PyObject *metadata)
 {

Parser/parser.c

@@ -358,6 +358,7 @@ expr_ty _PyPegen_ensure_real(Parser *p, expr_ty);
 asdl_seq *_PyPegen_join_sequences(Parser *, asdl_seq *, asdl_seq *);
 int _PyPegen_check_barry_as_flufl(Parser *, Token *);
 int _PyPegen_check_legacy_stmt(Parser *p, expr_ty t);
+void *_PyPegen_raise_error_for_missing_comma(Parser *p, expr_ty a, expr_ty b);
 ResultTokenWithMetadata *_PyPegen_check_fstring_conversion(Parser *p, Token *, expr_ty t);
 ResultTokenWithMetadata *_PyPegen_setup_full_format_spec(Parser *, Token *, asdl_expr_seq *, int, int,
                                                          int, int, PyArena *);