Memoryview objects provide a structured view over a memory area, meaning
the length, indexing and slicing operations respect the itemsize:

>>> import array
>>> a = array.array('i', [1,2,3])
>>> m = memoryview(a)
>>> len(a)
3
>>> m.itemsize
4
>>> m.format
'i'

However, in some cases, you want the memoryview to behave as a chunk of
pure bytes regardless of the original object *and without making a
copy*. Therefore, it would be handy to be able to change the format of
the memoryview, or ask for a new memoryview with another format.

An example of use could be:
>>> a = array.array('i', [1,2,3])
>>> m = memoryview(a).with_format('B')
>>> len(a), m.itemsize, m.format
(12, 1, 'B')