On Wed, 22 Oct 2008 08:29:08 -0400, Neal Becker <[EMAIL PROTECTED]> wrote:
I have a class (actually implemented in c++ using boost::python). For an
instance of this class, 'r', I'd like to support len (r). I don't want to add
it to the c++ code, because this is a unique situation: this class should not
normally support len().
So I try:
r = ring_int (10)
r.__len__ = lambda: 10
This doesn't work:
len(r)
TypeError: object of type 'ring_int' has no len()
It appears that __len__ is being looked up only on the class dict, not the
instance dict? What's the correct way to do this? (Of course, I could just
use inheritance, but this is an opportunity for me to learn more about python)
I don't really know anything about the kind of classes that boost makes,
however the behavior you describe is like the behavior of new-style
classes:
>>> class x: pass
...
>>> y = x()
>>> y.__len__ = lambda: 10
>>> len(y)
10
>>> class x(object): pass
...
>>> y = x()
>>> y.__len__ = lambda: 10
>>> len(y)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: object of type 'x' has no len()
>>>
Special methods aren't looked up in the instance dict of instances of
new-style classes. Exactly what constitutes the set of "special methods"
is a bit undefined. Consider anything that CPython has to look up to
satisfy some other operation, such as len or +, to be a special method
and don't rely on _either_ it being looked up on the instance or it _not_
being looked up on the instance (ie, don't put a method there and expect it
not to be called). The methods which are actually considered special by
CPython can change and has in the past.
Jean-Paul
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