* Ernest Adrogué:
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.
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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.
I'd go for it anyway, because the programmer who subclasses needs to understand the base class. And if, say, that class has a custom _init method, and it's documented that that what's a subclass should override, then, OK. No problem.
So what is there to do? Any suggestion?
An alternative can be to simply check for argument value None in the constructor, and if so, don't do anything.
Cheers & hth., - Alf -- http://mail.python.org/mailman/listinfo/python-list
