The Sourceforge tracker is kaputt so I'm sending it here, in part
because the effbot says it's interesting.
I can derive from list and override __getitem__
>>> class Spam(list):
... def __getitem__(self, i):
... print i
... return 2
...
>>> Spam()[1]
1
2
>>> Spam()[9]
9
2
>>>
Now consider the following
>>> class Spam(list):
... def __getitem__(self, i):
... print "Asking for", i
... if i == 0: return "zero"
... if i == 1: return "one"
... raise IndexError, i
...
>>> Spam()[1]
Asking for 1
'one'
>>>
Spiffy! For my next trick....
>>> "".join(Spam())
''
>>>
The relevant code in stringobject uses PySequence_Fast_GET_ITEM(seq, i),
which likely doesn't know about my derived __getitem__.
p = PyString_AS_STRING(res);
for (i = 0; i < seqlen; ++i) {
size_t n;
item = PySequence_Fast_GET_ITEM(seq, i);
n = PyString_GET_SIZE(item);
memcpy(p, PyString_AS_STRING(item), n);
p += n;
if (i < seqlen - 1) {
memcpy(p, sep, seplen);
p += seplen;
}
}
The Unicode join has the same problem
>>> class Spam(list):
... def __getitem__(self, i):
... print "Asking for", i
... if i == 0: return "zero"
... if i == 1: return u"one"
... raise IndexError, i
...
>>> "".join(Spam())
''
While if my class is not derived from list, everything is copacetic.
>>> class Spam:
... def __getitem__(self, i):
... print "Asking for", i
... if i == 0: return "zero"
... if i == 1: return u"one"
... raise IndexError, i
...
>>> "".join(Spam())
Asking for 0
Asking for 1
Asking for 2
u'zeroone'
>>>
Ditto for deriving from object.
>>> class Spam(object):
... def __getitem__(self, i):
... print "Asking for", i
... if i == 0: return "zero"
... if i == 1: return "one"
... raise IndexError, i
...
>>> "".join(Spam())
Asking for 0
Asking for 1
Asking for 2
'zeroone'
>>>
Andrew
[EMAIL PROTECTED]
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