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

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".)

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

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.

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

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

New changeset 5208b4b37953a406db0ed6a9db545c2948dde989 by Zackery Spytz in branch 'master':
bpo-39871: Fix possible SystemError in atan2, copysign and remainder (GH-18806)
https://github.com/python/cpython/commit/5208b4b37953a406db0ed6a9db545c2948dde989

New changeset e634a8ac1f8a2904a2241b83fc5e4c9f40d5c7ca by Mark Dickinson in branch '3.8':
[3.8] bpo-39871: Fix possible SystemError in atan2, copysign and remainder (GH-18806) (GH-18989)
https://github.com/python/cpython/commit/e634a8ac1f8a2904a2241b83fc5e4c9f40d5c7ca

New changeset 4dcfe5f2e4bad8e03990de75d8d4ec20e8aa23b3 by Mark Dickinson in branch '3.7':
[3.7] bpo-39871: Fix possible SystemError in atan2, copysign and remainder (GH-18806) (GH-18990)
https://github.com/python/cpython/commit/4dcfe5f2e4bad8e03990de75d8d4ec20e8aa23b3

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

New changeset 54636355805dd2877bb54fbad8d967e1ddd8b553 by Zackery Spytz in branch 'master':
bpo-39871: Fix an error in a news entry (GH-21749)
https://github.com/python/cpython/commit/54636355805dd2877bb54fbad8d967e1ddd8b553

New changeset fc354ca51d0b6f24f2871788dfa7e35c56129f8b by Miss Islington (bot) in branch '3.9':
bpo-39871: Fix an error in a news entry (GH-21749)
https://github.com/python/cpython/commit/fc354ca51d0b6f24f2871788dfa7e35c56129f8b

New changeset f6255a2ccb55a63194caf698f471c803c08be359 by Zackery Spytz in branch '3.8':
bpo-39871: Fix an error in a news entry (GH-21749)
https://github.com/python/cpython/commit/f6255a2ccb55a63194caf698f471c803c08be359