Cheapest way to get a numpy array into C-contiguous order?

Question

The following produces a C-contiguous numpy array:

import numpy

a = numpy.ones((1024,1024,5))

Now if I slice it, the result may not longer

Answer 1

As things stand, any attempt to coerce the slice bn to C contiguous order is going to create a copy.

If you don't want to change the shapes you're starting with (and don't need a itself in C order), one possible solution is to start with the array a in Fortran order:

>>> a = numpy.ones((1024, 1024, 5), order='f')

The slices are also then F-contiguous:

>>> bn = a[:, :, 0]
>>> bn.flags
  C_CONTIGUOUS : False
  F_CONTIGUOUS : True
  OWNDATA : False
  ...

This means that the transpose of the slice bn will be in C order and transposing does not create a copy:

>>> bn.T.flags
  C_CONTIGUOUS : True
  F_CONTIGUOUS : False
  OWNDATA : False
  ...

And you can then hash the slice:

>>> hashlib.sha1(bn.T).hexdigest()
'01dfa447dafe16b9a2972ce05c79410e6a96840e'