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PEP-654: warn that leaf_generator recycles tracebacks #2109

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Oct 16, 2021
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PEP-654: warn that leaf_generator recycles tracebacks #2109

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PEP-654: warn that leaf_generator recycles tracebacks
iritkatriel Oct 16, 2021
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refer to the list and then its contents
iritkatriel Oct 16, 2021
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8 changes: 7 additions & 1 deletion pep-0654.rst
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Expand Up @@ -469,7 +469,13 @@ recursively, as follows:
else:
# exc is a leaf exception and its traceback
# is the concatenation of the traceback
# segments in tbs
# segments in tbs.

# Note: the traceback segments are shallow-copied
# into tbs so they can be shared by multiple leaf
# exceptions. Make a deep copy if your use case
# requires that.
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Huh, this is actually ambiguous (I hadn't realized this before): the singly-linked list of Traceback objects is mutable (you could set any item's tb_next and mutate the chain) and in addition the tbs list is reused by each iteration through the generator.

I'm not particularly worried about people mutating the Traceback objects, but somehow your comment appears to refer to them. To describe the reuse of tbs, maybe reformulate the note like this?

Note: the list returned (tbs) is reused by each iteration through the generator.
Make a copy of your use case holds on to it beyond the current iteration.

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Ah I see, yes. I am a bit concerned also about people concatenating the tb chunks of tbs to make it look like one traceback.


yield exc, tbs
tbs.pop()

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