Changes to bools under Numexpr

Ivan Vilata i Balaguer ivilata at carabos.com
Fri Oct 27 02:21:18 CDT 2006


En/na Colin J. Williams ha escrit:

> Ivan Vilata i Balaguer wrote:
>> Hi all.  The attached diff makes some changes to Numexpr support for
>> booleans.  The changes and their rationale are below.
>>
>> 1. New ``True`` and ``False`` boolean constants.  This is so that 1 and
>>    0 are always proper integer constants.  It is also for completeness,
>>    but I don't envision any usage for them that couldn't be expressed
>>    without the constants.
>>   
> I'm puzzled.
> Python already has constants True and False of the bool type.  bool is a 
> subclass of the int type.
> Any instance of the bool type can be converted to the int type.
>  >>> a=1==0
>  >>> type(a)
> <type 'bool'>
>  >>> int(a)
> 0
>  >>> a
> False
>  >>>
> 
> Colin W.
> 
>> 2. The only comparisons supported with booleans are ``==`` and ``!=``,
>>    so that you can compare boolean variables.  Just as NumPy supports
>>    complex order comparisons and Numexpr doesn't, so goes for bools.
>>    Being relatively new, I think there is no need to keep
>>    integer-boolean compatibility in Numexpr.  What was the meaning of
>>    ``True > False`` or ``2 > True`` anyway?

Well, the ``True`` and ``False`` constants where not previously
supported in Numexpr because they had to be defined somewhere.  Now they
are.

Regarding the Python int and bool types and their relationships, it is a
very elegant solution introduced in Python 2.3 since previous versions
didn't have a proper boolean type, so the 0 and 1 ints where used for
that.  What I'm proposing here is, since Numexpr has a recent story and
most probably there isn't much code affected by the change, why not
define the boolean type as a purely logical one and leave its numeric
compatibility issues out?  By the way, it simplifies Numexpr's virtual
machine (less casting opcodes).

I admit again this looks a little baffling, of course, but I don't think
it would mean many noticeable changes to the user.

Regards,

::

	Ivan Vilata i Balaguer   >qo<   http://www.carabos.com/
	       Cárabos Coop. V.  V  V   Enjoy Data
	                          ""

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