EDIT:
Indeed there was a patch which included sign() in math, but it wasn’t accepted, because they didn’t agree on what it should return in all the edge cases (+/-0, +/-nan, etc)
So they decided to implement only copysign, which (although more verbose) can be used to delegate to the end user the desired behavior for edge cases – which sometimes might require the call to cmp(x,0).
I don’t know why it’s not a built-in, but I have some thoughts.
copysign(x,y): Return x with the sign of y.
Most importantly, copysign is a superset of sign! Calling copysign with x=1 is the same as a sign function. So you could just use copysign and forget about it.
>>> math.copysign(1, -4) -1.0 >>> math.copysign(1, 3) 1.0
If you get sick of passing two whole arguments, you can implement sign this way, and it will still be compatible with the IEEE stuff mentioned by others:
>>> sign = functools.partial(math.copysign, 1) # either of these
>>> sign = lambda x: math.copysign(1, x) # two will work
>>> sign(-4)
-1.0
>>> sign(3)
1.0
>>> sign(0)
1.0
>>> sign(-0.0)
-1.0
>>> sign(float('nan'))
-1.0
Secondly, usually when you want the sign of something, you just end up multiplying it with another value. And of course that’s basically what copysign does.
So, instead of:
s = sign(a) b = b * s
You can just do:
b = copysign(b, a)
And yes, I’m surprised you’ve been using Python for 7 years and think cmp could be so easily removed and replaced by sign! Have you never implemented a class with a __cmp__ method? Have you never called cmp and specified a custom comparator function?
In summary, I’ve found myself wanting a sign function too, but copysign with the first argument being 1 will work just fine. I disagree that sign would be more useful than copysign, as I’ve shown that it’s merely a subset of the same functionality.