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[Python-Dev] return type of __complex__

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 "<stdin>", line 1, in<module>
>>> 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
<built-in function pow>


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



-- 
Steven
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