From: Robert Kern on
On 2010-02-13 06:51 , Ernest Adrogu� wrote:
> Hello everybody,
> I'm designing a container class that supports slicing.
> The problem is that I don't really know how to do it.
> class MyClass(object):
> def __init__(self, input_data):
> self._data = transform_input(input_data)
> def __getitem__(self, key):
> if isinstance(key, slice):
> # return a slice of self
> pass
> else:
> # return a scalar value
> return self._data[key]
> The question is how to return a slice of self.
> First I need to create a new instance... but how? I can't
> use MyClass(self._data[key]) because the __init__ method
> expects a different kind of input data.
> Another option is
> out = MyClass.__new__(MyClass)
> out._data = self._data[key]
> return out
> But then the __init__ method is not called, which is
> undesirable because subclasses of this class might need
> to set some custom settings in their __init__ method.
> So what is there to do? Any suggestion?

I highly recommend making __init__() constructors do as little computation as
possible. You can add other constructors using @classmethod that do convenient

Robert Kern

"I have come to believe that the whole world is an enigma, a harmless enigma
that is made terrible by our own mad attempt to interpret it as though it had
an underlying truth."
-- Umberto Eco