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bpo-34588: Fix an off-by-one error in traceback formatting. (GH-9077)

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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.
This commit is contained in:
Benjamin Peterson 2018-09-10 08:43:10 -07:00 • committed by GitHub
parent 54752533b2
commit d545869d08
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4 changed files with 96 additions and 26 deletions
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29
Lib/traceback.py
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@ -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