New submission from hubo <>:

The attachment is a script that demonstrates the behavior. The simple unittest 
script should exit very quickly, but in fact, it runs indefinitely.

It uses asyncio to reproduce the result, but other concurrent technologies are 
also affected.

In Python 3, traceback of an exception is mutable. When the exception is 
re-raised, current frames are appended to the traceback. But when the same 
exception object is raised in multiple position (e.g. passed to different 
coroutines with futures), the frames are appended in the same list, so the 
tracebacks are mixed together.

assertRaises in unittest calls traceback.clear_frames internally. When the 
tracebacks are mixed, it may clear a running frame (in a generator), producing 
strange behaviors.

messages: 313735
nosy: hubo1016
priority: normal
severity: normal
status: open
title: raise an exception from multiple positions break the traceback frames
type: behavior
versions: Python 3.6
Added file:

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