Allow use of ellipsis in scipy.sparse slicing

[jnothman]

It should be legal to use an Ellipsis to slice sparse matrices so that they may be used alongside ndarrays.

>>> import numpy as np
>>> m = np.matrix([[0,1], [2,3]])
>>> import scipy.sparse as sp
>>> s = sp.csr_matrix(m)
>>> m[..., 1]
matrix([[1],
        [3]])
>>> s[..., 1]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/scipy/sparse/csr.py", line 249, in __getitem__
    P = extractor(row, self.shape[0])        #[[1,2],j] or [[1,2],1:2]
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/scipy/sparse/csr.py", line 209, in extractor
    indices = asindices(indices)
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/scipy/sparse/csr.py", line 191, in asindices
    raise IndexError('invalid index')
IndexError: invalid index

[cowlicks]

There is currently a PR here for fancy indexing in Sparse matrices. I'll look into it asap.

[cowlicks]

So... Since sparse matrices are limited to 2D, the Ellipsis is exactly the same as a :?

[jnothman]

Yes. It's an uninteresting case, but compatibility is required.

[pv]

Same as :, except that x[:,:,...] and x[:,...,:], x[...,:,:] are also allowed for 2D