How can I test for negative zero in Python?

Question

I want to test if a number is positive or negative, especially also in the case of zero. IEEE-754 allows for -0.0, and it is implemented in Python.

The only workarounds I could find were:

def test_sign(x):
    return math.copysign(1, x) > 0

And maybe (probably takes longer to run):

def test_sign(x):
    math.atan2(x, -1)

I could not find a dedicated function anywhere in the usual libraries, did I overlook something?

Edit: (Why this was relevant)

This is not my current plan anymore, but when asking the question I tried to overload a function depending on whether an argument was positive or negative. Allowing the user to pass negative zeros would resolve the ambiguity what was meant for zero-valued input. And I think this may be of general interest for other use cases as well.

Answer 1

You could use the binary representation:

import struct
def binary(num):
    return ''.join(bin(ord(c)).replace('0b', '').rjust(8, '0') for c in struct.pack('!f', num))

will return you the bit stream

The highest bit is the sign bit (0 is positive, 1 is negative)

However IEEE-754 also states that +0.0 == -0.0 == 0.0. Thus can't be sure that for instance -1.0+1.0 will for instance result in a positive or negative zero.

Answer 2

Within the standard library of Python, your copysign approach is probably as good as it gets in terms of computational cost. It is by design that most things treat positive and negative zero the same except for instances like division by zero, where it makes a huge difference (literally).

If you have numpy, its signbit function does what you want.

import numpy as np


assert(np.signbit(-0.) == True)
assert(np.signbit(0.) == False)
assert(np.signbit(float('nan')) == False)
assert(np.signbit(float('-nan')) == True)

Answer 3

You can use the struct module to test the bit pattern directly.

import struct

def is_neg_zero(n):
    return struct.pack('>d', n) == '\x80\x00\x00\x00\x00\x00\x00\x00'

def is_negative(n):
    return ord(struct.pack('>d', n)[0]) & 0x80 != 0