Slicing wrapped numpy arrays

[Robert Kern]

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
    ...:

 From the output of that you should be able to figure out how to process each of 
the various inputs that you want to handle.

In [2]: s = Sliceable()

In [3]: s[0]
0

In [4]: s[-1]
-1

In [5]: s[100]
100

In [6]: s[1:10]
slice(1, 10, None)

In [7]: s[1:10:2]
slice(1, 10, 2)

In [8]: s[1:10:-2]
slice(1, 10, -2)

In [9]: s[:10:2,10]
(slice(None, 10, 2), 10)

In [10]: s[:10:2,10:100:10]
(slice(None, 10, 2), slice(10, 100, 10))

In [11]: s[:10:2,10::10]
(slice(None, 10, 2), slice(10, None, 10))

In [12]: s[::2,10::10]
(slice(None, None, 2), slice(10, None, 10))

-- 
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