how do I write a scliceable class?

Ernest Adrogué eadrogue at
Sat Feb 13 13:51:09 CET 2010

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
			# 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?



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