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gh-148306: Fix dis.distb() crash when traceback is None #148317

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gh-148306: Fix dis.distb() crash when traceback is None #148317

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7 changes: 6 additions & 1 deletion Lib/dis.py
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Original file line number Diff line number Diff line change
Expand Up @@ -142,11 +142,16 @@ def distb(tb=None, *, file=None, show_caches=False, adaptive=False, show_offsets
if tb is None:
try:
if hasattr(sys, 'last_exc'):
tb = sys.last_exc.__traceback__
exc = sys.last_exc
tb = exc.__traceback__
if isinstance(exc, SyntaxError):
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@00ll00 00ll00 Apr 10, 2026

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Filtering out all SyntaxErrors here doesn’t seem quite right. It should still work if a SyntaxError is raised elsewhere. I think you could check whether tb is None, or just move the while loop below into the try block.

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@Ignyra Ignyra Apr 10, 2026 •
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I may be mistaken but from my understanding, a SyntaxError can not produce a traceback, since it occurs before any bytecode is executed, so my intention was to make the resulting error more informative in that specific case, to distinguish it from the other error that is raised where no exception has occurred at all.

That said, I can switch to the alternative approach if you think that would be more appropriate.

raise RuntimeError("can't disassemble a SyntaxError")
else:
tb = sys.last_traceback
except AttributeError:
raise RuntimeError("no last traceback to disassemble") from None


while tb.tb_next: tb = tb.tb_next
disassemble(tb.tb_frame.f_code, tb.tb_lasti, file=file, show_caches=show_caches, adaptive=adaptive, show_offsets=show_offsets, show_positions=show_positions)

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10 changes: 10 additions & 0 deletions Lib/test/test_dis.py
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Original file line number Diff line number Diff line change
Expand Up @@ -2478,6 +2478,16 @@ def test_distb_empty(self):
with self.assertRaises(RuntimeError):
dis.distb()

def test_distb_syntax_error(self):
try:
compile("???", "", "exec")
except SyntaxError as e:
sys.last_exc = e
sys.last_exc.__traceback__ = None

with self.assertRaises(RuntimeError):
dis.distb()

def test_distb_last_traceback(self):
self.maxDiff = None
# We need to have an existing last traceback in `sys`:
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