python bug when subclassing list?

Ethan Furman ethan at
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 )
> 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?
> thanks,
> -h.


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:

--> list
<type 'list'>

--> list.__new__
<built-in method __new__ of *type object* at 0x1E1D6A78>

--> list.__init__
<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 "", 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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