Numpy: arr[…,0,:] works. But how do I store the data contained in the slice command (…, 0, :)?

问题

In Numpy (and Python in general, I suppose), how does one store a slice-index, such as (...,0,:), in order to pass it around and apply it to various arrays? It would be nice to, say, be able to pass a slice-index to and from functions.

回答1:

Python creates special objects out of the slice syntax, but only inside the square brackets for indexing. You can either create those objects by hand (in this case, (...,0,:) is (Ellipsis, 0, slice(None, None, None)), or you can create a little helper object:

class ExtendedSliceMaker(object):
    def __getitem__(self, idx):
        return idx

>>> ExtendedSliceMaker()[...,0,:]
(Ellipsis, 0, slice(None, None, None))

回答2:

Use s_ in NumPy:

In [1]: np.s_[...,0,:]
Out[1]: (Ellipsis, 0, slice(None, None, None))

回答3:

The equivalent to (...,0,:) should be...

>>> myslice = (..., 0, slice(None, None))
>>> myslice
(Ellipsis, 0, slice(None, None, None))

回答4:

The neat thing about Python is that you can actually make a class to inspect how these things are represented. Python uses the magic method __getitem__ to handle indexing operations, so we'll make a class that overloads this to show us what was passed in, instantiate the class, and "index in" to the instance:

class foo:
  def __getitem__(self, index): print index

foo()[...,0,:]

And our result is:

(Ellipsis, 0, slice(None, None, None))

Ellipsis and slice are builtins, and we can read their documentation:

help(Ellipsis)
help(slice)

回答5:

I think you want to just do myslice = slice(1,2) to for example define a slice that will return the 2nd element (i.e. myarray[myslice] == myarray[1:2])

来源：https://stackoverflow.com/questions/6795657/numpy-arr-0-works-but-how-do-i-store-the-data-contained-in-the-slice-co

标签

python

indexing

numpy

slice