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

[Steven D'Aprano]

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.

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?




-- 
Steven