decrement offset when it points to a newline (#10186 followup)

Lib/test/test_traceback.py

@@ -295,6 +295,10 @@ class BaseExceptionReportingTests:
             raise SyntaxError('', ('', 0, 5, 'hello'))
         msg = self.get_report(e).splitlines()
         self.assertEqual(msg[-2], "        ^")
+        def e():
+            exec("x = 5 | 4 |")
+        msg = self.get_report(e).splitlines()
+        self.assertEqual(msg[-2], '              ^')
 
 
 class PyExcReportingTests(BaseExceptionReportingTests, unittest.TestCase):

Python/pythonrun.c

@@ -1344,6 +1344,8 @@ print_error_text(PyObject *f, int offset, const char *text)
 {
     char *nl;
     if (offset >= 0) {
+        if (offset > 0 && offset == strlen(text) && text[offset - 1] == '\n')
+            offset--;
         for (;;) {
             nl = strchr(text, '\n');
             if (nl == NULL || nl-text >= offset)
@@ -1363,7 +1365,7 @@ print_error_text(PyObject *f, int offset, const char *text)
     if (offset == -1)
         return;
     PyFile_WriteString("    ", f);
-    while (--offset)
+    while (--offset > 0)
         PyFile_WriteString(" ", f);
     PyFile_WriteString("^\n", f);
 }