Steven D'Aprano wrote:
On Fri, 24 Jul 2009 15:03:29 -0400, Roy Smith wrote:
In article <mailman.3674.1248461573.8015.python-l...@python.org>,
Terry Reedy <tjre...@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.
[st...@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
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