bpo-34588: Fix an off-by-one error in traceback formatting.benjamin-traceback-obo

The recursive frame pruning code always undercounted the number of elided frames
by one. That is, in the "[Previous line repeated N more times]" message, N would
always be one too few. Near the recursive pruning cutoff, one frame could be
silently dropped. That situation is demonstrated in the OP of the bug report.

The fix is to start the identical frame counter at 1.

1 files changed, 19 insertions, 10 deletions

diff --git a/Lib/traceback.py b/Lib/traceback.py
index afab0a4b91..4e7605d15f 100644
--- a/Lib/traceback.py
+++ b/Lib/traceback.py
@@ -310,6 +310,8 @@ def walk_tb(tb):
         tb = tb.tb_next
 
 
+_RECURSIVE_CUTOFF = 3 # Also hardcoded in traceback.c.
+
 class StackSummary(list):
     """A stack of frames."""
 
@@ -398,18 +400,21 @@ class StackSummary(list):
         last_name = None
         count = 0
         for frame in self:
-            if (last_file is not None and last_file == frame.filename and
-                last_line is not None and last_line == frame.lineno and
-                last_name is not None and last_name == frame.name):
-                count += 1
-            else:
-                if count > 3:
-                    result.append(f'  [Previous line repeated {count-3} more times]\n')
+            if (last_file is None or last_file != frame.filename or
+                last_line is None or last_line != frame.lineno or
+                last_name is None or last_name != frame.name):
+                if count > _RECURSIVE_CUTOFF:
+                    count -= _RECURSIVE_CUTOFF
+                    result.append(
+                        f'  [Previous line repeated {count} more '
+                        f'time{"s" if count > 1 else ""}]\n'
+                    )
                 last_file = frame.filename
                 last_line = frame.lineno
                 last_name = frame.name
                 count = 0
-            if count >= 3:
+            count += 1
+            if count > _RECURSIVE_CUTOFF:
                 continue
             row = []
             row.append('  File "{}", line {}, in {}\n'.format(
@@ -420,8 +425,12 @@ class StackSummary(list):
                 for name, value in sorted(frame.locals.items()):
                     row.append('    {name} = {value}\n'.format(name=name, value=value))
             result.append(''.join(row))
-        if count > 3:
-            result.append(f'  [Previous line repeated {count-3} more times]\n')
+        if count > _RECURSIVE_CUTOFF:
+            count -= _RECURSIVE_CUTOFF
+            result.append(
+                f'  [Previous line repeated {count} more '
+                f'time{"s" if count > 1 else ""}]\n'
+            )
         return result