[issue45292] Implement PEP 654: Exception Groups

[Irit Katriel]

Change by Irit Katriel :


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resolution:  -> fixed
stage: patch review -> resolved
status: open -> closed

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[issue45292] Implement PEP 654: Exception Groups

[Irit Katriel]

Irit Katriel  added the comment:


New changeset 9925e70e4811841556747a77acd89c1a70bf344a by Irit Katriel in 
branch 'main':
bpo-45292: [PEP-654] exception groups and except* documentation (GH-30158)
https://github.com/python/cpython/commit/9925e70e4811841556747a77acd89c1a70bf344a


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[issue45292] Implement PEP 654: Exception Groups

[Irit Katriel]

Change by Irit Katriel :


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pull_requests: +28377
pull_request: https://github.com/python/cpython/pull/30158

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[issue45292] Implement PEP 654: Exception Groups

[Irit Katriel]

Irit Katriel  added the comment:

The way these two opcodes are combined by the compiler to implement except* is 
described in the pseudo code here: 

https://github.com/python/cpython/pull/29581#issuecomment-975660029

except* uses JUMP_IF_NOT_EG_MATCH. The excepts (not-*) that collect exceptions 
raised in the except* clauses are virtual.

The do_reraise_star at the end is PREP_RERAISE_STAR followed by 
POP_EXCEPT_AND_RERAISE.

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[issue45292] Implement PEP 654: Exception Groups

[Irit Katriel]

Irit Katriel  added the comment:

The second opcode that the PR adds is PREP_RERAISE_STAR.

This opcode takes a list that contains:
1. all the exceptions that were raised in the executed except* clauses
2. the unmatched part of the exception group

It constructs the exception group that needs to be raised at the end.  This is 
done through a fairly complex operation on the BaseExceptionGroup, which merges 
the re-raised exceptions into the same nesting structure they had in the 
original exception group, so that

try:
   raise eg
except* ValueError:
   raise
except* TypeError:
   raise

is equivalent to just 'raise eg'.


Is there any overlap with existing opcodes?

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[issue45292] Implement PEP 654: Exception Groups

[Irit Katriel]

Irit Katriel  added the comment:

The PR adds two new opcodes. Let's start with the simpler of the two - 
JUMP_IF_NOT_EG_MATCH.   This is the exception-group variation on 
JUMP_IF_NOT_EXC_MATCH.

JUMP_IF_NOT_EXC_MATCH checks for a match by checking if the exception is of the 
given type. The result is boolean.

JUMP_IF_NOT_EG_MATCH checks for a matching by calling .split() on the exception 
group. The result is two exception groups (the matching part and the 
non-matching part).

Can we do this without a new opcode?

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[issue45292] Implement PEP 654: Exception Groups

[Mark Shannon]

Mark Shannon  added the comment:

PR 29581 resulted in a 1% slowdown, which is not terrible, but code not using 
except* should not be slowed down at all.

IMO, the way to avoid the slowdown is to implement except* using the existing 
instruction set (perhaps with a few minor additions)

We already implement try-finally, named except blocks and with statements 
without any complex bytecodes (except perhaps WITH_EXCEPT_START).

These used to involve a lot of state and more complex bytecodes. So it is 
possible to make these simplifications,
but it does take work.


There are a number of techniques we can use:

If any state is needed, push it to the stack as we do with `ctx.__exit__` in 
the with statement, and when pushing f_lasti in exception handlers.
Duplicate code paths when the semantics differ in different cases, as we do for 
finally blocks.
If anything is too complex to handle on the stack, put it in a temporary 
variable.
Be liberal in your use of virtual try-excepts (SETUP_FINALLY, POP_FINALLY 
pairs), as zero-cost exception handling should keep the cost down.


It may be too late for this advice, but if I were writing the `except*` 
implementation from scratch, I would:

1. Sketch out the pseudo Python that a try-except* would map to. This is a good 
opportunity to discover any design bugs that might result in undesirable 
behavior for corner cases.

2. Implement the translation in the compiler, not worrying about any redundancy 
or inefficiency, just correctness.

3. Look to improve the above, either in the compiler front-end, or by replacing 
inefficient code patterns in the back-end. Handling the optimization in the 
backend has the advantage that other code might benefit as well.

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[issue45292] Implement PEP 654: Exception Groups

[Guido van Rossum]

Guido van Rossum  added the comment:

We should have a discussion here about improvements that Mark Shannon would 
like to see, in particular to do more work in the compiler instead of the 
interpreter.

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nosy: +Mark.Shannon, gvanrossum

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[issue45292] Implement PEP 654: Exception Groups

[Irit Katriel]

Irit Katriel  added the comment:


New changeset d60457a6673cf0263213c2f2be02c633ec2e2038 by Irit Katriel in 
branch 'main':
bpo-45292: [PEP-654] add except* (GH-29581)
https://github.com/python/cpython/commit/d60457a6673cf0263213c2f2be02c633ec2e2038


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[issue45292] Implement PEP 654: Exception Groups

[Irit Katriel]

Change by Irit Katriel :


--
pull_requests: +27824
pull_request: https://github.com/python/cpython/pull/29581

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[issue45292] Implement PEP 654: Exception Groups

[Karthikeyan Singaravelan]

Karthikeyan Singaravelan  added the comment:


New changeset 8b06d01507fd708468570eaa43a349828784489a by Irit Katriel in 
branch 'main':
bpo-45292: Use raw strings for regex in tests (GH-29545)
https://github.com/python/cpython/commit/8b06d01507fd708468570eaa43a349828784489a


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[issue45292] Implement PEP 654: Exception Groups

[Irit Katriel]

Change by Irit Katriel :


--
pull_requests: +27794
pull_request: https://github.com/python/cpython/pull/29545

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[issue45292] Implement PEP 654: Exception Groups

[Karthikeyan Singaravelan]

Karthikeyan Singaravelan  added the comment:

The tests emit some deprecation warnings : 

PYTHONWARNINGS=always ./python -Wall -m test test_exception_group
0:00:00 load avg: 0.39 Run tests sequentially
0:00:00 load avg: 0.39 [1/1] test_exception_group
/home/karthikeyan/stuff/python/cpython/Lib/test/test_exception_group.py:41: 
DeprecationWarning: invalid escape sequence '\('
  MSG = 'second argument \(exceptions\) must be a sequence'
/home/karthikeyan/stuff/python/cpython/Lib/test/test_exception_group.py:47: 
DeprecationWarning: invalid escape sequence '\('
  MSG = 'second argument \(exceptions\) must be a non-empty sequence'
/home/karthikeyan/stuff/python/cpython/Lib/test/test_exception_group.py:52: 
DeprecationWarning: invalid escape sequence '\('
  MSG = ('Item [0-9]+ of second argument \(exceptions\)'

== Tests result: SUCCESS ==

1 test OK.

Total duration: 51 ms
Tests result: SUCCESS

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nosy: +xtreak

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[issue45292] Implement PEP 654: Exception Groups

[Erlend E. Aasland]

Change by Erlend E. Aasland :


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nosy: +erlendaasland

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[issue45292] Implement PEP 654: Exception Groups

[Irit Katriel]

Irit Katriel  added the comment:


New changeset 3509b26c916707363c71a1df040855e395cf4817 by Irit Katriel in 
branch 'main':
bpo-45292: [PEP 654] Update traceback display code to work with exception 
groups (GH-29207)
https://github.com/python/cpython/commit/3509b26c916707363c71a1df040855e395cf4817


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[issue45292] Implement PEP 654: Exception Groups

[Irit Katriel]

Change by Irit Katriel :


--
pull_requests: +27475
pull_request: https://github.com/python/cpython/pull/29207

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[issue45292] Implement PEP 654: Exception Groups

[Irit Katriel]

Irit Katriel  added the comment:


New changeset f30ad65dbf3c6b1b5eec14dc954d65ef32327857 by Irit Katriel in 
branch 'main':
bpo-45292: [PEP 654] add the ExceptionGroup and BaseExceptionGroup classes 
(GH-28569)
https://github.com/python/cpython/commit/f30ad65dbf3c6b1b5eec14dc954d65ef32327857


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[issue45292] Implement PEP 654: Exception Groups

[STINNER Victor]

Change by STINNER Victor :


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title: Implement PEP 654 -> Implement PEP 654: Exception Groups

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