python bug when subclassing list?
ethan at stoneleaf.us
Wed Nov 12 01:33:20 CET 2008
Hamish McKenzie wrote:
> I want to write a Vector class and it makes the most sense to just
> subclass list. I also want to be able to instantiate a vector using either:
> Vector( 1, 2, 3 )
> Vector( [1, 2, 3] )
> so I have this:
> class Vector(list):
> def __new__( cls, *a ):
> print a
> return list.__new__(cls, a)
> print 'broken'
> return list.__new__(cls, list(a))
> doing Vector( 1, 2, 3 ) on this class results in a TypeError – which
> doesn’t seem to get caught by the try block (ie “broken” never gets
> printed, and it never tries to
> I can do pretty much the exact same code but inheriting from tuple
> instead of list and it works fine.
> is this a python bug? or am I doing something wrong?
I am not sure of the proper way to fix this issue, but the problem you
have is that lists do not have a __new__ method:
<built-in method __new__ of *type object* at 0x1E1D6A78>
<slot wrapper '__init__' of *'list' objects*>
Changing the __new__ to __init__ at least gets your code to run, but
then you have this:
--> vector.Vector(1, 2, 3)
(1, 2, 3)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "vector.py", line 15, in __init__
return list.__new__(cls, list(a))
TypeError: list.__new__(X): X is not a type object (Vector)
Good luck in your journey!
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