I implemented math.sign in CPython, right now in a fork of Python 3.15a3 in my local repository, and continue by inserting tests into Lib/test/test_math.py. I also plan to add documentation in Doc/library/math.rst. Then I will issue the PR. The function passes all tests. I added to tests theset type that has < and > operations but is not numerical (and produces an exception), sympy.Rational, and big integers.