Issue 1507: complex constructor loses signs of zeros (original) (raw)

In Python2.5 and the current trunk, construction of a complex number from two floats loses the negative sign of a negative zero in the imaginary part.

complex(-0., 0.).real # behaves as expected -0.0 complex(0., -0.).imag # loses sign of zero 0.0

There are situations where it's important to preserve the sign of zero (typically when doing calculations involving functions with branch cuts); there are probably platforms where this is difficult for one reason or another, but on a platform that complies with all the usual standards I can't see any reason for Python not to preserve the signs of zeros, provided that it's straightforward to do so.

The attached patch changes the complex constructor so that if x and y are numeric and neither x nor y is complex (or a subclass of complex) then complex(x, y) simply places x into the real part of the complex number and y into the imaginary part.
For someone who doesn't care about the sign of zeros, this patch will have no noticeable effect. Similarly, on a system that does not have full hardware or system support for signed zeros, this patch should be harmless. Note that the patch does not change the feature that complex(z1, z2) returns z1 + 1j*z2 when z1 and/or z2 is itself complex.

There was some previous discussion of this on python-dev earlier this year. See

http://mail.python.org/pipermail/python-dev/2007-January/070770.html