Martin Manns wrote: > On Sun, 13 Jan 2008 16:03:16 -0600 > Robert Kern <[EMAIL PROTECTED]> wrote: > >> Martin Manns wrote: >>> Hi, >>> >>> I have created a class that wraps a numpy array of custom objects. I >>> would like to be able to slice respective objects (without copying >>> the array if possible). >>> >>> I have browsed the doc and found some hints at __getitem__. >>> However, I still do not grasp how to do it. How do I implement >>> __getitem__ correctly? >>> mymap[10:20,15:20,:] # This line should work afterwards >> The first thing you should do is simply implement a very basic, >> nonfunctional version just to see what objects come in: >> >> In [1]: class Sliceable(object): >> ...: def __getitem__(self, arg): >> ...: print arg >> ...: > > I did that and got here: > >> (slice(None, None, 2), slice(10, None, 10)) > > However, I still do not see how I get a Map object that employs a > slice of the array without creating a new Map object.
That's because creating a new Map object is the right thing to do. Instead of making Map.__init__() generate the map array from the dimensions, it should just take a preconstructed map array. You can use a @classmethod or just a factory function to provide an alternate constructor which builds a Map from the dimensions. When designing classes, I usually try to do as little computation as possible in an __init__(). Doing a lot of stuff there limits the ways you can build the object later. For some classes, this doesn't matter a whole lot, but for data structures that can be sliced and concatenated or otherwise transformed, you really want that flexibility. -- 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 -- http://mail.python.org/mailman/listinfo/python-list
