[Tutor] comparison bug in python (or do I not get it?)

[Hans Fangohr]

Hi Kent,

> Hans Fangohr wrote:
>
>> In [2]: 2 in [1,2,3] == True
>> Out[2]: False
>>
>> Why does [2] return False? Would people agree that this is a bug?
>
> No, not a bug. Don't be too quick to blame your tools!

That's good news. I'd be worried if this wasn't the desired behaviour
-- I just hadn't understood the logic.

>
> The equivalent expression is
> In [1]: (2 in [1,2,3]) and ([1,2,3]==False)
> Out[1]: False

Ah -- that's the way to read it!

>
> 'in' is considered a comparison operator and can be chained with other
> comparisons. For a clearer example, consider
> In [2]: 2 < 3 < 4
> Out[2]: True
>
> which is not the same as
> In [3]: 2 < (3 < 4)
> Out[3]: False
>
> or
> In [4]: (2 < 3) < 4
> Out[4]: True
>
> It is equivalent to
> In [5]: (2 < 3) and (3 < 4)
> Out[5]: True
>

Well explained -- makes perfect sense now.

Many thanks,

Hans



> See
> http://docs.python.org/ref/comparisons.html
>
> Kent
> _______________________________________________
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>
>
>

--
Hans Fangohr

School of Engineering Sciences 
University of Southampton 
Phone: +44 (0) 238059 8345

Email: fangohr at soton.ac.uk
http://www.soton.ac.uk/~fangohr