[Tutor] problem when subclassing tuple
sigurd at 12move.de
Sat Jan 24 17:21:13 EST 2004
On 24 Jan 2004, Gregor Lingl <- glingl at aon.at wrote:
> I'm trying to define a fancy subclass of tuple: ;-)
> class Vec(tuple):
> def __init__(self, v):
> and some more methods ...
> Works fine.
> Actually I want my Vec class objects to be constructed
> like this:
> a = Vec(1,2,3)
Maybe I don't understand what you want to achieve exactly but what about
overriding __new__ instead of __init__? (I had the idea when I read
def __new__(cls, x,y,z):
return tuple.__new__(cls, [x,y,z])
> But when I tried to overwrite the constructor of tuple
> with a new one with a different number of parameters, I failed:
> So does someone know a way how to define a subclass of
> tuple so its objects can be constructed the way
> mentioned above?
Perhaps like the above solution.
> Hints as well as solutions are highly appreciated,
> pointers to appropriate sections of the docs also.
,----[ descintro.html ]
| * If you want to change the constructor's signature, you often have to override both
| __new__ and __init__ to accept the new signature. However, most built-in types ignore
| the arguments to the method they don't use; in particular, the immutable types (int,
| long, float, complex, str, unicode, and tuple) have a dummy __init__, while the
| mutable types (dict, list, file, and also super, classmethod, staticmethod, and
| property) have a dummy __new__. The built-in type 'object' has a dummy __new__ and a
| dummy __init__ (which the others inherit). The built-in type 'type' is special in many
| respects; see the section on metaclasses.
This writing about the dummy init method seems to describe your problem.
If that won't help perhaps we have to dig in metaclasses; would be
interesting I didn't explore them myself yet.
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