[Python-Dev] Caching float(0.0) (original) (raw)

Tim Peters tim.peters at gmail.com
Wed Oct 4 06:42:04 CEST 2006

[skip at pobox.com]

If C90 doesn't distinguish -0.0 and +0.0, how can Python?

With liberal applications of piss & vinegar ;-)

Can you give a simple example where the difference between the two is apparent to the Python programmer?

Perhaps surprsingly, many (well, comparatively many, compared to none ....) people have noticed that the platform atan2 cares a lot:

from math import atan2 as a z = 0.0 # postive zero m = -z # minus zero a(z, z) # the result here is actually +0.0 0.0 a(z, m) 3.1415926535897931 a(m, z) # the result here is actually -0.0 0.0 a(m, m) -3.1415926535897931

It work like that "even on Windows", and these are the results C99's 754-happy appendix mandates for atan2 applied to signed zeroes. I've even seen a /complaint/ on c.l.py that atan2 doesn't do the same when

z = 0.0

is replaced by

z = 0

That is, at least one person thought it was "a bug" that integer zeroes didn't deliver the same behaviors.

Do people actually rely on this? I know I don't, but given that more than just 2 people have remarked on it seeming to like it, I expect that changing this would break /some/ code out there.

BTW, on /some/ platforms all those examples trigger EDOM from the platform libm instead -- which is also fine by C99, for implementations ignoring C99's optional 754-happy appendix.

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