[SciPy-dev] Arrays as truth values?

Robert Cimrman cimrman3 at ntc.zcu.cz
Tue Nov 1 10:27:11 CST 2005


>>numarray doesn't permit using arrays as truth values since we figured 
>>that it wasn't very clear what people expected to happen (this case is 
>>a good illustration). I'd like to make the argument that scipy_core 
>>also not permit arrays to be used as truth values (i.e., one should get 
>>an exception if one tries to use it that way). I realize that this will 
>>break some old code, but since incompatible changes are being made, 
>>this is the time to make this sort of change.  If left in, it is going 
>>to bite people, often quietly.
>>
>> 
>>
> 
> I agree it can bite people, but I'm concerned that arrays not having a 
> truth value is an odd thing in Python --- you have to implement it by 
> raising an error when __nonzero__ is called right?
> 
> All other objects in Python have truth values (including its built-in 
> array).  My attitude is that its just better to teach people the proper 
> use of truth values, then to break form with the rest of Python.
> 
> I'm would definitely like to hear more opinions though.  It would be 
> very easy to simply raise and error when __nonzero__ is called.

Speaking about 'and' only, my problem with the current implementation of 
it is that it _looks_ like working as '*' in some cases - 'b and c' 
returns an array whose length is that of b and c (if the lengths are 
equal, that is). I would not be against 'b and c' giving a single True 
or False... But this also breaks the Python semantics of 'and'. The same 
holds for other logical ops, of course.

So I don't know :-) - I can live with the current state.

r.




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