gh-160945: Fix _pydecimal losing companion signal flags when a signal is trapped by arpittkhandelwal · Pull Request #160947 · python/cpython

arpittkhandelwal · 2026-10-30T04:13:17Z

_pydecimal lost the flags of companion conditions when an operation
signalled several conditions and one of them was trapped:
Context._raise_error() set one signal's flag and raised at once, so
the calls signalling the remaining conditions never ran. _decimal
records all conditions in the context status before raising. The
exception raised is unchanged; only the flags are now complete.

Context._defer_traps() defers a trapped exception raised inside the
block until the block exits; _raise_error still sets every flag and
returns the default response. The sequential multi-signal sequences in
Decimal._fix, __divmod__, __pow__, quantize,
to_integral_exact and next_toward are wrapped in it.

Tests: new ContextFlags.test_trapped_signal_sets_companion_flags,
covering trapped Overflow, Inexact, Underflow, Subnormal and
DivisionByZero. It runs against both implementations, fails on
_pydecimal without this change, and test_decimal passes (596
tests).

Differential fuzzing against _decimal (5,793 seeded cases) matched
results, exceptions and flags everywhere except 6 cases of a separate
pre-existing difference: with clamp=1 and a zero quotient,
__divmod__ does not finalise the quotient in _pydecimal, so
Clamped is not signalled. Not addressed here.

Disclosure: this change was drafted with AI assistance and reviewed by
the author, who takes responsibility for it.

Issue: _pydecimal loses companion signal flags when a signal is trapped #160945

…signal is trapped

python-cla-bot · 2026-10-30T04:13:22Z

All commit authors signed the Contributor License Agreement.

picnixz

There are unrelated whitespace changes.

picnixz · 2026-10-30T11:31:25Z

+    def _defer_traps(self):
+        """Defer trapped signals until the end of the with block.
+
+        Some operations signal several conditions at once (for


This is clearly AI prose and should have been trimmed.

bedevere-app · 2026-10-30T11:32:28Z

A Python core developer has requested some changes be made to your pull request before we can consider merging it. If you could please address their requests along with any other requests in other reviews from core developers that would be appreciated.

Once you have made the requested changes, please leave a comment on this pull request containing the phrase I have made the requested changes; please review again. I will then notify any core developers who have left a review that you're ready for them to take another look at this pull request.

arpittkhandelwal · 2026-10-30T11:51:12Z

I have made the requested changes; please review again.

Trimmed the _defer_traps docstring and the comments around it, dropped the unrelated comment reflow in _fix, and shortened this description. The remaining whitespace changes are the re-indentation of the code wrapped in with context._defer_traps(): blocks and the line re-wraps that the extra indentation forces. test_decimal passes (596 tests).

bedevere-app · 2026-10-30T11:51:19Z

Thanks for making the requested changes!

@picnixz: please review the changes made to this pull request.

picnixz · 2026-10-30T15:53:23Z

-                        context._raise_error(InvalidOperation, 'x % 0'))
+                with context._defer_traps():
+                    return (context._raise_error(DivisionByZero, 'x // 0',
+                                                 sign),


This is an unrelated whitespace new line change.

picnixz · 2026-10-30T15:53:55Z

        elif name == 'flags' or name == 'traps':
            return self._set_signal_dict(name, value)
-        elif name == '_ignored_flags':
+        elif name in ('_ignored_flags', '_deferred_traps', '_deferred_error'):


Suggested change

elif name in ('_ignored_flags', '_deferred_traps', '_deferred_error'):

elif name in {'_ignored_flags', '_deferred_traps', '_deferred_error'}:

picnixz · 2026-10-30T15:54:16Z

        elif name == 'flags' or name == 'traps':
            return self._set_signal_dict(name, value)
-        elif name == '_ignored_flags':
+        elif name in ('_ignored_flags', '_deferred_traps', '_deferred_error'):


You need test for that as well.

…erred-attrs test

arpittkhandelwal · 2026-10-30T16:00:53Z

I have made the requested changes; please review again.

Restored the original single line in __divmod__ (removed the unrelated newline change).
Used a set literal in Context.__setattr__, as suggested.
Added test_deferred_traps_attributes in PyContextFlags, covering the _deferred_traps and _deferred_error attributes and the state reset after a trapped operation.

bedevere-app · 2026-10-30T16:00:57Z

Thanks for making the requested changes!

@picnixz: please review the changes made to this pull request.

picnixz · 2026-10-30T16:53:27Z

+                    ans = context._raise_error(Overflow, 'above Emax',
+                                               self._sign)


Suggested change

ans = context._raise_error(Overflow, 'above Emax',

self._sign)

ans = context._raise_error(Overflow, 'above Emax', self._sign)

picnixz · 2026-10-30T16:53:57Z

+            with context._defer_traps():
+                # check if rounding pushed the exponent out of range


Suggested change

with context._defer_traps():

# check if rounding pushed the exponent out of range

# check if rounding pushed the exponent out of range

with context._defer_traps():

picnixz · 2026-10-30T16:54:13Z

+                for exception in (Underflow, Subnormal, Inexact, Rounded,
+                                  Clamped):


Suggested change

for exception in (Underflow, Subnormal, Inexact, Rounded,

Clamped):

for exception in Underflow, Subnormal, Inexact, Rounded, Clamped:

…bove with, drop redundant parens

arpittkhandelwal · 2026-10-30T17:48:33Z

I have made the requested changes; please review again.

Joined the context._raise_error(Overflow, 'above Emax', self._sign) call onto a single line in _fix.
Moved the "check if rounding pushed the exponent out of range" comment above the with context._defer_traps(): block.
Dropped the redundant parentheses in the for exception in ... loop in __pow__.

bedevere-app · 2026-10-30T17:48:38Z

Thanks for making the requested changes!

@picnixz: please review the changes made to this pull request.

picnixz · 2026-10-30T19:02:40Z

+            if self._deferred_traps == 1:
+                self._deferred_error = None
+            raise


I wonder: should we really keep self._deferred_error even in the case of an exception? I don't want to keep possible cycles but I don't know whether it's.. reasonable to clear the errors here in general.

Is it possible to see what the C implementation does?

picnixz · 2026-10-30T19:04:04Z

+    def _defer_traps(self):
+        """Defer trapped signals until the end of the with block.
+
+        _raise_error sets the flag of every signal in the block and


Suggested change

_raise_error sets the flag of every signal in the block and

_raise_error() sets the flag of every signal in the block and

arpittkhandelwal · 2026-10-30T19:18:59Z

I have made the requested changes; please review again.

_raise_error() in the _defer_traps docstring, as suggested.
On keeping _deferred_error when an exception propagates: you are right to question it. The C implementation never stores an exception on the context at all — operations only accumulate condition flags in the context status (dec_addstatus ORs them into ctx->status), and the exception is created from those flags once the operation completes; if the operation itself fails, no decimal error is left behind. _defer_traps now clears _deferred_error unconditionally when an exception propagates, at every nesting level: the operation is aborted, so the propagating exception wins, and the context never retains an exception object (the clean-exit path already detached it before raising). The signal flags themselves are still set, as in C. The new test_deferred_traps_exception_discards_error covers the nested case.

bedevere-app · 2026-10-30T19:19:04Z

Thanks for making the requested changes!

@picnixz: please review the changes made to this pull request.

pythongh-160945: Fix _pydecimal losing companion signal flags when a …

324e70e

…signal is trapped

bedevere-app Bot added the awaiting review label Oct 30, 2026

bedevere-app Bot mentioned this pull request Oct 30, 2026

_pydecimal loses companion signal flags when a signal is trapped #160945

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picnixz requested changes Oct 30, 2026

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bedevere-app Bot added awaiting changes and removed awaiting review labels Oct 30, 2026

Trim _defer_traps prose and drop an unrelated comment reflow

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bedevere-app Bot added awaiting change review and removed awaiting changes labels Oct 30, 2026

bedevere-app Bot requested a review from picnixz October 30, 2026 11:51

picnixz reviewed Oct 30, 2026

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Address picnixz review: restore divmod line, use set literal, add def…

22b0491

…erred-attrs test

bedevere-app Bot requested a review from picnixz October 30, 2026 16:01

picnixz reviewed Oct 30, 2026

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Address picnixz review: single-line _raise_error call, move comment a…

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…bove with, drop redundant parens

bedevere-app Bot requested a review from picnixz October 30, 2026 17:48

picnixz reviewed Oct 30, 2026

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Address picnixz review: discard deferred error on any exceptional exit

789b58b

bedevere-app Bot requested a review from picnixz October 30, 2026 19:19

	elif name in ('_ignored_flags', '_deferred_traps', '_deferred_error'):
	elif name in {'_ignored_flags', '_deferred_traps', '_deferred_error'}:

		ans = context._raise_error(Overflow, 'above Emax',
		self._sign)

		with context._defer_traps():
		# check if rounding pushed the exponent out of range

		for exception in (Underflow, Subnormal, Inexact, Rounded,
		Clamped):

	for exception in (Underflow, Subnormal, Inexact, Rounded,
	Clamped):
	for exception in Underflow, Subnormal, Inexact, Rounded, Clamped:

	_raise_error sets the flag of every signal in the block and
	_raise_error() sets the flag of every signal in the block and

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