[Python-Dev] return type of __complex__

[Guido van Rossum]

Math.pow() predates ** by many releases.

On Sunday, October 21, 2012, Steven D'Aprano wrote:

> 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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-- 
--Guido van Rossum (python.org/~guido)
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