python bug when subclassing list?
Arnaud Delobelle
arnodel at googlemail.com
Thu Nov 6 15:05:52 EST 2008
Hamish McKenzie <hamish at valvesoftware.com> writes:
> 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?
You're doing something wrong!
When you call Vector.__new__(cls, *a), this creates a new list object
(call it L) and then calls Vector.__init__(L, *a) automatically, falling
back to list.__init__(L, *a) (which won't work when a has more than one
element). In fact list initialisation is done in list.__init__, not in
list.__new__ (as opposed to tuple initialisation because tuples are
immutable).
A solution:
>>> class Vector(list):
... def __init__(self, *args):
... try:
... list.__init__(self, *args)
... except TypeError:
... list.__init__(self, args)
...
>>> Vector([1,2,3])
[1, 2, 3]
>>> Vector(1,2,3)
[1, 2, 3]
>>> Vector(1)
[1]
>>> Vector()
[]
HTH
--
Arnaud
More information about the Python-list
mailing list