checking if a list is empty

[Steven D'Aprano]

On Sat, 14 May 2011 00:45:29 -0700, rusi wrote:

> On May 14, 12:39 pm, Steven D'Aprano <steve
> +comp.lang.pyt... at pearwood.info> wrote:
>> On Thu, 12 May 2011 23:46:12 -0700, rusi wrote:
>> > Mathematics has existed for millenia. Hindu-arabic numerals (base-10
>> > numbers) have been known for about one millennium The boolean domain
>> > is only a 100 years old. Unsurprisingly it is not quite 'first-class'
>> > yet: See
>> >http://www.cs.utexas.edu/users/EWD/transcriptions/EWD10xx/EWD1070.html
>> > [Lifted fromhttp://c2.com/cgi/wiki?EqualVsTrueFalse]
>>
>> Th money-quote as regards using arbitrary objects in truth tests:
>>
>>     [quote]
>>     All this changed with the introduction of the two-element boolean
>>     domain {true, false} which provides the vocabulary needed to
>>     assign values to boolean expressions: 3<4 is a way for writing
>>     true, 3>4 is a way for writing false, whereas the value of x>0
>>     depends on the value of x ... [end quote]
>>
>> In Python, [1, 2, 3] is another way of writing true, and [] is another
>> way of writing false. Similarly with any other arbitrary objects.
> 
> Well so is [1,2] another way of writing True
> 
> And then we get the interesting result that (True = True) is False

I presume you mean to say:

([1, 2] == True) is False

that is, that one true value is not equal to another true value.

That is correct. However, Python's == operator is not a Boolean Algebra 
operator. If it were, it would probably be called "material 
biconditional", or XNOR, and written ↔ or <-> and would be smart enough 
to recognise that both [1, 2] and True are true.

Or possibly dumb enough... the difficulty is that the equality operator 
knows more about the objects than just their truth value.

And furthermore:

([1, 2] and True) == (True and [1, 2])

is also False, again because == is too smart to recognise that the left 
hand side (True) and the right hand side ([1, 2]) are both true values.

It's not that Python bools aren't first class objects, but that Python 
doesn't have a full set of all 16 possible boolean algebra operators.


-- 
Steven