Add a new function to the math module sign/signum https://en.wikipedia.org/wiki/Sign_function

See math.copysign().

https://docs.python.org/3/library/math.html#math.copysign

sign/signum isn't the same as copysign, and doesn't support complex numbers

sgn(x) is the same as copysign(1.0, x).

The math module doesn't support complex numbers and shouldn't.

If this is added, should this signum function be the two value version that returns 1 for zero, or the three value version that returns 0? Should it distinguish between signed zeroes +0.0 and -0.0? What should it do for NANs (raise, return a NAN, copy the sign bit from the NAN)?

The argument for adding this will be slightly stronger if other languages have this, and especially if it is part of the standard C maths library.

In any case, there's only a few more days until the 3.6 feature freeze, so unless you have an implementation and tests ready to go, the earliest it could be added will be 3.7.

Serhiy: if this gets added, then it would make sense to add cmath.signum to handle complex numbers.

copysign() is part of the standard C maths library (that is why it was added to the math module).

sgn/sign/signum was used in old languages that didn't have copysign() (Basic, Pascal, etc).

This has been previously discussed and rejected, in issue 829370.

Thanks David.