Re: python bug when subclassing list? - Python

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 )
>
> OR
>
> Vector( [1, 2, 3] )
>
>
>
>
>
> so I have this:
>
>
>
> class Vector(list):
>
> def __new__( cls, *a ):
>
> try:
>
> print a
>
> return list.__new__(cls, a)
>
> except:
>
> 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?
>
Vector(1, 2, 3) fails for exactly the same reasons as list:

>>> list(1, 2, 3)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: list() takes at most 1 argument (3 given)

So the behavior you want cannot be inherited from list, since list
doesn't implement that behavior!

As toy our assertion that you can subclass tuple that way, I am inclined
to doubt it because of this:

>>> tuple(1, 2, 3)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: tuple() takes at most 1 argument (3 given)

regards
Steve
--
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