math.copysign raises SystemError with non-float x and custom y

[auscompgeek]

BPO	39871
Nosy	@mdickinson, @methane, @auscompgeek, @ZackerySpytz, @miss-islington
PRs	bpo-39871: Fix possible SystemErrors in math.{atan2,copysign,remainder}() #18806 [3.8] bpo-39871: Fix possible SystemError in atan2, copysign and remainder (GH-18806) #18989 [3.7] bpo-39871: Fix possible SystemError in atan2, copysign and remainder (GH-18806) #18990 bpo-39871: Fix an error in a news entry #21749 [3.9] bpo-39871: Fix an error in a news entry (GH-21749) #21760 [3.8] bpo-39871: Fix an error in a news entry (GH-21749) #22333

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = 'https://github.com/mdickinson'
closed_at = <Date 2020-03-14.11:53:04.104>
created_at = <Date 2020-03-06.02:59:01.549>
labels = ['easy', 'type-bug', '3.8', '3.9', 'extension-modules', '3.7']
title = 'math.copysign raises SystemError with non-float x and custom y'
updated_at = <Date 2020-10-26.05:37:20.036>
user = 'https://github.com/auscompgeek'

bugs.python.org fields:

activity = <Date 2020-10-26.05:37:20.036>
actor = 'methane'
assignee = 'mark.dickinson'
closed = True
closed_date = <Date 2020-03-14.11:53:04.104>
closer = 'mark.dickinson'
components = ['Extension Modules']
creation = <Date 2020-03-06.02:59:01.549>
creator = 'auscompgeek'
dependencies = []
files = []
hgrepos = []
issue_num = 39871
keywords = ['patch', 'easy (C)']
message_count = 13.0
messages = ['363477', '363492', '363493', '363495', '363496', '363505', '364150', '364152', '364154', '364155', '374975', '374976', '379640']
nosy_count = 5.0
nosy_names = ['mark.dickinson', 'methane', 'auscompgeek', 'ZackerySpytz', 'miss-islington']
pr_nums = ['18806', '18989', '18990', '21749', '21760', '22333']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue39871'
versions = ['Python 3.7', 'Python 3.8', 'Python 3.9']

[auscompgeek]

If math.copysign(x, y) is passed an x that cannot be converted to a float and a y that implements __float__() in Python, math.copysign() will raise a SystemError from the TypeError resulting from the attempted float conversion of x.

math.copysign() should probably return immediately if converting the first argument to a float raises an error.

Example:

>>> import math
>>> from fractions import Fraction
>>> float(Fraction(-1, 1))  # this is needed to avoid an AttributeError?
-1.0
>>> math.copysign((-1) ** 0.5, Fraction(-1, 1))
TypeError: can't convert complex to float

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3.8/numbers.py", line 291, in __float__
    return self.numerator / self.denominator
SystemError: PyEval_EvalFrameEx returned a result with an error set

[mdickinson]

Nice find. This affects not just copysign, but all of the math module functions that use FUNC2 under the hood. (I think that's just "remainder", "atan2" and "copysign".)

[mdickinson]

@david: do you have any interest in creating a PR?

[auscompgeek]

I'm currently rather busy with other work, but if this happens to still be open in a couple of months I might pick it up.

[mdickinson]

No problem; I'll likely pick it up (if no-one else does) in the next few days.

[ZackerySpytz]

I have created a pull request to fix this issue. Please consider taking a look.

[mdickinson]

New changeset 5208b4b by Zackery Spytz in branch 'master':
bpo-39871: Fix possible SystemError in atan2, copysign and remainder (GH-18806)
5208b4b

[mdickinson]

New changeset e634a8a by Mark Dickinson in branch '3.8':
[3.8] bpo-39871: Fix possible SystemError in atan2, copysign and remainder (GH-18806) (GH-18989)
e634a8a

[mdickinson]

New changeset 4dcfe5f by Mark Dickinson in branch '3.7':
[3.7] bpo-39871: Fix possible SystemError in atan2, copysign and remainder (GH-18806) (GH-18990)
4dcfe5f

[mdickinson]

Zackery: Thanks for the fix! Now merged and backported to 3.8 and 3.7.

[methane]

New changeset 5463635 by Zackery Spytz in branch 'master':
bpo-39871: Fix an error in a news entry (GH-21749)
5463635

[miss-islington]

New changeset fc354ca by Miss Islington (bot) in branch '3.9':
bpo-39871: Fix an error in a news entry (GH-21749)
fc354ca

[methane]

New changeset f6255a2 by Zackery Spytz in branch '3.8':
bpo-39871: Fix an error in a news entry (GH-21749)
f6255a2