[Python-Dev] return type of complex (original) (raw)

Steven D'Aprano steve at pearwood.info
Sun Oct 21 13:02:17 CEST 2012

On 21/10/12 21:11, Antoine Pitrou wrote:

On Sun, 21 Oct 2012 13:38:48 +1100 Chris Angelico<rosuav at gmail.com> wrote:

On Sun, Oct 21, 2012 at 12:53 PM, Steven D'Aprano<steve at pearwood.info> wrote:

Python 2.x legitimately distinguished between floats and complex, e.g.:

py> (-100.0)**0.5 Traceback (most recent call last): File "", line 1, in ValueError: negative number cannot be raised to a fractional power If you wanted a complex result, you can explicitly ask for one: py> (-100.0+0j)**0.5 (6.123031769111886e-16+10j)

I see that behaviour is changed in Python 3. Python 2 (future directives aside) also required you to explicitly ask for floating point. That was also changed in Python 3. That doesn't mean that this is necessarily the right thing to do, but it does have precedent. The square root of a negative number is by nature a complex number. In the set of complex numbers, it is. But in the set of float numbers, a negative number has no square root. I think Steven may be worried about the impact on people who don't know about complex numbers, which is a reasonable concern.

Precisely.

I'm also not sure why we have several variants of the power operator: **, built-in pow(), math.pow().

In Python 3.3, math.pow is the builtin pow. Presumably for backwards compatibility reasons when they were different? If so, it was a LONG time ago:

[steve at ando ~]$ python1.5 Python 1.5.2 (#1, Aug 27 2012, 09:09:18) [GCC 4.1.2 20080704 (Red Hat 4.1.2-52)] on linux2 Copyright 1991-1995 Stichting Mathematisch Centrum, Amsterdam

import math math.pow

And yet strangely there's never been a cmath.pow.

-- Steven

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