and [True,True] --> [True, True]?????

[Peter Otten]

Leo wrote:

> Steven D'Aprano wrote:
> 
>> On Mon, 20 Apr 2009 15:13:15 -0500, Grant Edwards wrote:
>> 
>> 
>>> I fail to see the difference between "length greater than 0" and "list
>>> is not empty".  They are, by definition, the same thing, aren't they?
>> 
>> For built-in lists, but not necessarily for arbitrary list-like
>> sequences.
>> 
>> There's also the performance issue. I might have a sequence-like
>> structure where calling len() takes O(N**2) time, (say, a graph) but
>> calling __nozero__ might be O(1). Why defeat the class designer's
>> optimizations?
> Is that true?
> Calling len() actually traverses the whole list?

No. Steven constructed the theoretical case where determining whether a
sequence (not Python's list) is empty is a cheap operation while getting
the actual length is costly. 

The only "popular" data structure I know that implements its length like
this are strings in C.

Peter