Document that multiline display also needs an end line number

Column position information alone is not enough: end_lineno defaults to
lineno, so a frame with colno/end_colno but no end_lineno still shows
only its first physical line.  Say so, and test it.

Author: harjothkhara

Doc/library/traceback.rst

@@ -173,9 +173,10 @@ Module-Level Functions
    for printing.  Each string in the resulting list corresponds to the item with
    the same index in the argument list.  Each string ends in a newline and may
    contain internal newlines.  When an item's source text spans multiple
-   physical lines, only the first physical line is displayed if column position
-   information is unavailable.  Old-style tuples never have column position
-   information.
+   physical lines, every line is displayed only if the item has both column
+   position information and an end line number greater than its line number;
+   otherwise just the first physical line is displayed.  Old-style tuples never
+   have column position information.
 
 
 .. function:: format_exception_only(exc, /[, value], *, show_group=False)
@@ -484,8 +485,10 @@ the module-level functions described above.
       resulting list corresponds to a single :ref:`frame <frame-objects>` from
       the stack.
       Each string ends in a newline and may contain internal newlines.  When a
-      frame's source text spans multiple physical lines, only the first physical
-      line is displayed if column position information is unavailable.
+      frame's source text spans multiple physical lines, every line is displayed
+      only if the frame has both column position information and an end line
+      number greater than its line number; otherwise just the first physical
+      line is displayed.
 
       For long sequences of the same frame and line, the first few
       repetitions are shown, followed by a summary line stating the exact

Lib/test/test_traceback.py

@@ -3438,6 +3438,17 @@ def test_from_list_multiline_with_columns(self):
              '    line 2\n'],
             s.format())
 
+    def test_from_list_multiline_with_columns_without_end_lineno(self):
+        # end_lineno defaults to lineno, so the frame spans a single line
+        # and the trailing physical lines are not displayed.
+        frame = traceback.FrameSummary(
+            'foo.py', 1, 'fred', line='line 1\nline 2',
+            colno=0, end_colno=6)
+        s = traceback.StackSummary.from_list([frame])
+        self.assertEqual(
+            ['  File "foo.py", line 1, in fred\n    line 1\n'],
+            s.format())
+
     def test_format_list_multiline_old_style_tuple(self):
         frames = [
             ('test.py', 10, 'func',

Lib/traceback.py

@@ -85,9 +85,10 @@ def format_list(extracted_list):
     Each string in the resulting list corresponds to the item with the
     same index in the argument list.  Each string ends in a newline and may
     contain internal newlines.  When an item's source text spans multiple
-    physical lines, only the first physical line is displayed if column
-    position information is unavailable.  Old-style tuples never have column
-    position information.
+    physical lines, every line is displayed only if the item has both column
+    position information and an end line number greater than its line number;
+    otherwise just the first physical line is displayed.  Old-style tuples
+    never have column position information.
     """
     return StackSummary.from_list(extracted_list).format()
 
@@ -802,9 +803,10 @@ def format(self, **kwargs):
         Returns a list of strings ready for printing.  Each string in the
         resulting list corresponds to a single frame from the stack.
         Each string ends in a newline and may contain internal newlines.  When
-        a frame's source text spans multiple physical lines, only the first
-        physical line is displayed if column position information is
-        unavailable.
+        a frame's source text spans multiple physical lines, every line is
+        displayed only if the frame has both column position information and an
+        end line number greater than its line number; otherwise just the first
+        physical line is displayed.
 
         For long sequences of the same frame and line, the first few
         repetitions are shown, followed by a summary line stating the exact