Use Numpy Multidimensional Array Slicing Without Using the [slice,slice] Syntax?

Question

Is there are a way to use Numpy's multidimensional array slicing without using the [slice, slice] syntax?

I need to be able to use it from normal function calls, but I haven't found a way to use the slice object to do it.

I cannot use the syntax [(slice,slice)] for my program because [] is special syntax outside of regular function calls. The language I am using is Hy, a Lisp for Python, and it does not support this syntax. More importantly, it shouldn't support this syntax. Numpy, however, doesn't seem to support multidimensional slicing without using the [] syntax.

What's tripping me up is that the mix of C and Python in the Numpy source makes it difficult to discern how the [slice,slice] is implemented. It may not even be possible to circumvent this syntax.

EDIT:

The answer provided below by @Joe Kington allows one to slice Numpy matrices like so:

x = np.array([list(range(5)) for x in list(range(5))]) x.getitem(slice(1,4)) array([[0, 1, 2, 3, 4], [0, 1, 2, 3, 4], [0, 1, 2, 3, 4]]) x.getitem(tuple([slice(1,4),slice(1,4)])) array([[1, 2, 3], [1, 2, 3], [1, 2, 3]])

Could you please clarify what you are trying to achieve, what you have tried so far and how exactly it failed? What constitutes an abnormal function call? — jonrsharpe
– jonrsharpe, Commented Mar 20, 2014 at 14:53
[bla, blah] is just [(blah, blah)] I bet there are many questions about this... — seberg
– seberg, Commented Mar 20, 2014 at 19:25
I have edited the question to further clarify. Please let me know if it remains unclear. — calben
– calben, Commented Mar 21, 2014 at 17:20
this question may be a better fit for the numpy github, but i'm surprised this hasn't been encountered before and thought it might be best if instead done in a more public forum like stackoverflow in case someone in the future needs clarification — calben
– calben, Commented Mar 21, 2014 at 17:40
It sounds like you want arr.__getitem__(tuple_of_slice_objects), and __setitem__ for assignment. (Incidentally, your edits helped make the question much more clear. I think it's a good question.) — Joe Kington
– Joe Kington, Commented Mar 21, 2014 at 17:44

Joe Kington · Accepted Answer · 2014-03-21 18:14:23Z

8

From your description, it seems like you're asking what function calls are used to implement slicing and slice assignment.

Python uses the "special" methods __getitem__ and __setitem__ to implement and/or allow customization of how slicing works. Any class that implements these can be sliced. There's actually nothing numpy-specific about this.

In other words

x = arr[4:10, 9:15, ::-1]
x[0] = 100

is translated into

x = arr.__getitem__((slice(4, 6), slice(9, 10), slice(None, None, -1)))
x.__setitem__(0, 100)

For example:

class Foo(object):
    def __getitem__(self, index):
        print 'Getting', index
    def __setitem__(self, index, val):
        print 'Setting', index, 'to', val

f = Foo()
print 'Getting...'
f[:]
f[4:10, ::-1, ...]

print 'Equivalently:'
f.__getitem__(slice(None))
f.__getitem__((slice(4, 10), slice(None, None, -1), Ellipsis))

print 'Setting...'
f[0] = 1
f[5:10, 100] = 2
f[...] = 100

print 'Equivalently:'
f.__setitem__(0, 1)
f.__setitem__((slice(5,10), 100), 2)
f.__setitem__(Ellipsis, 100)

Also, it can be handy to know about numpy.index_exp (or equivalently, np.s_). It's nothing fancy -- it just translates slicing into the equivalent tuple, etc. It's quite similar to our Foo class above. For example:

In [1]: np.index_exp[10:4, ::-1, ...]
Out[1]: (slice(10, 4, None), slice(None, None, -1), Ellipsis)

edited Mar 21, 2014 at 18:14

answered Mar 21, 2014 at 17:59

Joe Kington

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2 Comments

calben Over a year ago

fantastic! that was unexpectedly straightforward, thanks so much! i'm going to edit my question to give a summary of this solution's use in numpy

Alexey Tigarev Over a year ago

Nice explanation about how slicing works under the hood and how slice objects are represented.

Ben Echols · Accepted Answer · 2014-03-20 15:17:21Z

1

I suspect you are trying to pass the slice through as a parameter?

def do_slice(sl, mystring):
    return mystring[sl]

sl = slice(0,2)
mystr = "Hello"

print do_slice(sl, mystr)

answered Mar 20, 2014 at 15:17

Ben Echols

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