Issues with Decimal and three-argument pow()

[serhiy-storchaka]

Crash report

1. Three-argument pow() with non-Decimal first argument does not work in the Python implementation.

>>> from decimal import Decimal
>>> pow(10, Decimal(2), 7)
Decimal('2')
>>> from _pydecimal import Decimal
>>> pow(10, Decimal(2), 7)
Traceback (most recent call last):
  File "<python-input-3>", line 1, in <module>
    pow(10, Decimal(2), 7)
    ~~~^^^^^^^^^^^^^^^^^^^
TypeError: unsupported operand type(s) for ** or pow(): 'int', 'Decimal', 'int'

But it works in the C implementation.

2. Three-argument pow() with only Decimal third argument crashes in the C implementation.

>>> from decimal import Decimal
>>> pow(10, 2, Decimal(7))
python: ./Modules/_decimal/_decimal.c:139: find_state_left_or_right: Assertion `mod != NULL' failed.
Aborted (core dumped)

In the Python implementation it only raises TypeError, which is expected.

Linked PRs

• gh-130230: Fix crash in pow() with only Decimal third argument #130237
• [3.13] gh-130230: Fix crash in pow() with only Decimal third argument (GH-130237) #130246
• [3.12] gh-130230: Add new tests for 3-arg pow() with Decimal (GH-130237) #130247

[serhiy-storchaka]

#130237 fixes the second issue. It was a regression in 3.13 (most likely introduced in #106078). In 3.12 and older, pow(10, 2, Decimal(7)) evaluates to Decimal('2').

The first issue is older (it exists from the beginning) and much more complex.

[skirpichev]

The first issue is older (it exists from the beginning) and much more complex.

It's just an instance of #130104. I doubt we can do something on side of the pure-Python module.

BTW, 100% compatibility is not a goal here.

[skirpichev]

Second issue works for me (no crash) for <3.13, removing label.