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add method to class dynamically?

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)

Oct 22 '08 #1
3 1435
Neal Becker 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)
Yes, they are only looked up on the class. Unfortunately, I don't find the
paragraph in the language-ref that says so.

BTW, could you stop setting the followup-to to a non-existing (at least for
a standard newsreader) gmane-newsgroup?

Diez
Oct 22 '08 #2
Diez B. Roggisch wrote:
BTW, could you stop setting the followup-to to a non-existing (at least
for a standard newsreader) gmane-newsgroup?
Is this any better?

Oct 22 '08 #3
Neal Becker wrote:
Diez B. Roggisch wrote:
>BTW, could you stop setting the followup-to to a non-existing (at least
for a standard newsreader) gmane-newsgroup?

Is this any better?
Yes, thanks.

Diez
Oct 22 '08 #4

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