In article <[EMAIL PROTECTED]>,
[EMAIL PROTECTED] (Bengt Richter) wrote:
> On 14 Mar 2005 17:43:53 -0800, [EMAIL PROTECTED] wrote:
>
> >
> >Why doesn't this work?
> >
> >>>> def foo(lst):
> >... class baz(object):
> >... def __getitem__(cls, idx): return cls.lst[idx]
> >... __getitem__=classmethod(__getitem__)
> >... baz.lst = lst
> >... return baz
> >...
> >>>> f = foo([1,2,3])
> >>>> f[0]
> >Traceback (most recent call last):
> > File "<stdin>", line 1, in ?
> >TypeError: unsubscriptable object
> >>>> f.__getitem__(0)
> >1
> >>>>
> >
> >
> >I thought x[y] and x.__getitem__(y) were supposed to always be
> >synonymous.
> Yes, but what was your "x"?
> Note:
>
> >>> def foo(lst):
> ... class baz(object):
> ... def __getitem__(cls, idx): return cls.lst[idx]
> ... __getitem__ = classmethod(__getitem__)
> ... baz.lst = lst
> ... return baz
> ...
> >>> f = foo([1,2,3])
> >>> f
> <class '__main__.baz'>
>
> Your "x" was the baz *class*, and the baz *class* is not subscriptable
> *itself*,
But why not? __getitem__ is a class method, not an instance method.
(You can treat that as a rhetorical question. I know the answer already
from other postings in this thread.)
> BTW, I see a class factory, but no "(meta)"class per se.
That's because I took out extraneous code to not distract from the
problem I was having, and in the process took out all the useful stuff
:-)
What I'm really trying to do is to create enumerated types such that if:
e1 = enum(lst) and v = e1(x)
then
(x in lst) and (e1[v] == x)
In other words, I want to map values onto their representations using
the () operator, and representations onto their values using the []
operator. That requires me to define the [] operator on the enumerated
classes themselves. So my actual code was:
def enum(vals):
class enum(object):
def __init__(self, val):
try:
self.val = type(self).vals.index(val)
except:
raise TypeError, "%s is not a valid %s" % (val, type(self))
def __getitem__(self, index): return self.vals.index(index)
__getitem__=classmethod(__getitem__)
enum.vals = vals
return enum
(Actually, what I'm really trying to do is create a whole hierarchy of
static type descriptors and automatically generate database schema from
those descriptors, but that's a story for another day.)
Thanks for all the responses.
rg
--
http://mail.python.org/mailman/listinfo/python-list
