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Re: Bizarre arithmetic results

Albert van der Horst

2/22/2010 6:02:00 PM

In article <mailman.2359.1265890457.28905.python-list@python.org>,
Terrence Cole <terrence@zettabytestorage.com> wrote:
>Can someone explain to me what python is doing here?
>
>Python 3.1.1 (r311:74480, Feb 3 2010, 13:36:47)
>[GCC 4.3.4] on linux2
>Type "help", "copyright", "credits" or "license" for more information.
>>>> -0.1 ** 0.1

Python 4.0
Warning: misleading blank space, expected:
- 0.1**0.1

>-0.7943282347242815
>>>> a = -0.1; b = 0.1
>>>> a ** b
>(0.7554510437117542+0.2454609236416552j)
>>>> -abs(a ** b)
>-0.7943282347242815
>
>Why does the literal version return the signed magnitude and the
>variable version return a complex?
>
>Cheers,
>Terrence
>


--
--
Albert van der Horst, UTRECHT,THE NETHERLANDS
Economic growth -- being exponential -- ultimately falters.
albert@spe&ar&c.xs4all.nl &=n http://home.hccnet.nl/a.w.m.van...
4 Answers

Steven D'Aprano

2/23/2010 8:12:00 AM

0

On Mon, 22 Feb 2010 18:01:44 +0000, Albert van der Horst wrote:

> In article <mailman.2359.1265890457.28905.python-list@python.org>,
> Terrence Cole <terrence@zettabytestorage.com> wrote:
>>Can someone explain to me what python is doing here?
>>
>>Python 3.1.1 (r311:74480, Feb 3 2010, 13:36:47) [GCC 4.3.4] on linux2
>>Type "help", "copyright", "credits" or "license" for more information.
>>>>> -0.1 ** 0.1
>
> Python 4.0
> Warning: misleading blank space, expected:
> - 0.1**0.1
>
>>-0.7943282347242815


Making spaces significant in that fashion is mind-bogglingly awful. Let's
look at a language that does this:

[steve@sylar ~]$ cat ws-example.rb
def a(x=4)
x+2
end

b = 1
print (a + b), (a+b), (a+ b), (a +b), "\n"


[steve@sylar ~]$ ruby ws-example.rb
7773




--
Steven

Mark Dickinson

2/23/2010 1:48:00 PM

0

On Feb 23, 8:11 am, Steven D'Aprano
<ste...@REMOVE.THIS.cybersource.com.au> wrote:
> Making spaces significant in that fashion is mind-bogglingly awful. Let's
> look at a language that does this:
>
> [steve@sylar ~]$ cat ws-example.rb
> def a(x=4)
>     x+2
> end
>
> b = 1
> print (a + b), (a+b), (a+ b), (a +b), "\n"
>
> [steve@sylar ~]$ ruby ws-example.rb
> 7773

Hmm. That's pretty nasty, all right. Not that Python can claim to be
immune to such behaviour:

>>> 3 .real
3
>>> 3. real
File "<stdin>", line 1
3. real
^
SyntaxError: invalid syntax


Though the fact that one of the cases raises an exception (rather than
silently giving some different behaviour) ameliorates things a bit.

--
Mark

Steven D'Aprano

2/24/2010 1:16:00 AM

0

On Tue, 23 Feb 2010 05:48:09 -0800, Mark Dickinson wrote:

> On Feb 23, 8:11 am, Steven D'Aprano
> <ste...@REMOVE.THIS.cybersource.com.au> wrote:
>> Making spaces significant in that fashion is mind-bogglingly awful.
>> Let's look at a language that does this:
>>
>> [steve@sylar ~]$ cat ws-example.rb
>> def a(x=4)
>>     x+2
>> end
>>
>> b = 1
>> print (a + b), (a+b), (a+ b), (a +b), "\n"
>>
>> [steve@sylar ~]$ ruby ws-example.rb
>> 7773
>
> Hmm. That's pretty nasty, all right. Not that Python can claim to be
> immune to such behaviour:
>
>>>> 3 .real
> 3
>>>> 3. real
> File "<stdin>", line 1
> 3. real
> ^
> SyntaxError: invalid syntax
>
>
> Though the fact that one of the cases raises an exception (rather than
> silently giving some different behaviour) ameliorates things a bit.

It ameliorates it *completely* -- you won't get silent errors in Python
because you add or delete whitespace around a dot.


"I find it amusing when novice programmers believe their main job is
preventing programs from crashing. ... More experienced programmers
realize that correct code is great, code that crashes could use
improvement, but incorrect code that doesn't crash is a horrible
nightmare."

http://www.pphsg.org/cdsmith/...


The edge case occurs because dot does double-duty as an operator and as
part of float literals. However, float literals never include whitespace:

>>> 1.5
1.5
>>> 1 . 5
File "<stdin>", line 1
1 . 5
^
SyntaxError: invalid syntax

and likewise for 1. 5 and 1 .5 -- the only way to get a float literal
with a decimal point is by not including whitespace in it. So there is
never any ambiguity about floats. You can even do this:

>>> 1.5.__str__()
'1.5'


And since . is an operator outside of float literals, you can do this:

>>> import sys
>>> sys . platform
'linux2'


although why you'd want to escapes me :)

This actually is a feature, since it is useful when calling methods on
int literals. However this is a very rare thing to do.



--
Steven

aahz

2/27/2010 8:47:00 PM

0

In article <pan.2010.02.23.08.11.51@REMOVE.THIS.cybersource.com.au>,
Steven D'Aprano <steven@REMOVE.THIS.cybersource.com.au> wrote:
>
>[steve@sylar ~]$ cat ws-example.rb

Ahhh, you're a Heroes fan. ;-)
--
Aahz (aahz@pythoncraft.com) <*> http://www.python...

"Many customs in this life persist because they ease friction and promote
productivity as a result of universal agreement, and whether they are
precisely the optimal choices is much less important." --Henry Spencer