list vs. tuple [Re: len() should always return something]

[Terry Reedy]

Steven D'Aprano wrote:
> On Fri, 24 Jul 2009 15:03:29 -0400, Roy Smith wrote:
> 
>> In article <mailman.3674.1248461573.8015.python-list at python.org>,
>>  Terry Reedy <tjreedy at udel.edu> wrote:
>>
>>> Better:    if isinstance(x, (int, float, complex)):
>> I never noticed this before, but it seems odd that the second argument
>> to isinstance() should be a tuple.  Using the normal arguments made
>> about tuples vs. lists, it seems like a list would be the right data
>> structure here.

I ignore the 'normal arguments' and it seems that Guido or the designer 
of isinstance did so here too.  Fortunately.  Practicality beats 
'purity', especially misguided purity.

> What would be the point of using a list? You're never going to sort it, 
> or append items to it, or otherwise mutate it. You build it, pass it to a 
> function which doesn't modify it in any fashion, then it gets garbage 
> collected.
> 
> 
>> I suppose a set would be even more right, but (I'm
>> pretty sure) isinstance() predates sets.
> 
> Yes.
> 
> [steve at sylar ~]$ python1.5
> Python 1.5.2 (#1, Apr  1 2009, 22:55:54)  [GCC 4.1.2 20070925 (Red Hat 
> 4.1.2-27)] on linux2
> Copyright 1991-1995 Stichting Mathematisch Centrum, Amsterdam
>>>> isinstance
> <built-in function isinstance>
>>>> set
> Traceback (innermost last):
>   File "<stdin>", line 1, in ?
> NameError: set
> 
> 
> 
>> I'm curious why a tuple was chosen.
> 
> Tuples are smaller and faster to build than lists -- they're the most 
> lightweight sequence type in Python. You don't need all the extra 
> functionality of lists, so why go to the time and effort of building a 
> list?

In fact, constant tuples can be and now are compiled as constants:

 >>> dis(compile('(1,2,3)','','eval'))
   1           0 LOAD_CONST               3 ((1, 2, 3))
               3 RETURN_VALUE
 >>> dis(compile('[1,2,3]','','eval'))
   1           0 LOAD_CONST               0 (1)
               3 LOAD_CONST               1 (2)
               6 LOAD_CONST               2 (3)
               9 BUILD_LIST               3
              12 RETURN_VALUE

Internally, even a frozenset is more complicated than a tuple since it 
still needs a hash table, which is overkill for something that will be 
linearly scanned exactly once.

tjr