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This is one of most annoying things in Python to me. When you want accept any bytes-like object in you bytes-processing function you need special case empty input.
def foo(data):
 data = memoryview(data)
 if data:
 data = data.cast('B')
 else:
 data = b''
You can't use just memoryview(data).cast('B') because it doesn't work for empty data.
>>> memoryview(b'').cast('b')
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
TypeError: memoryview: cannot cast view with zeros in shape or strides
It would be very nice to allow cast() for empty views.

This actually sounds like a bug to me. A view over the empty bytestring is a one-dimensional zero-length view, it isn't a special kind of entity.

Cool. Therefore I can hope on fixing this in maintained releases?

> Cool. Therefore I can hope on fixing this in maintained releases?
Provided someone makes a patch :-)

Here is a patch. I can't tell whether it's right for sure, since the whole buffer thing has become so complicated.

I agree that this cast should work. Perhaps disallowing zero strides
is enough -- I have to look at this more closely though.

Ok, I think the main reason for disallowing zeros in view->shape here
was that casts are undefined if also the "shape" argument is given:
x = memoryview(b'')
x.cast('d', shape=[1])
Now, this case *is* already caught at a later stage, since there isn't
enough space for the cast. Nevertheless, the code is tricky, so I'd
prefer to be conservative and catch shape arguments earlier.
I left a suggestion in Rietveld. I would commit it myself, but I'm
moving and my infrastructure is a mess. It would be great if one
of you could take this one.
I'll try to review the general case for ndim > 1 later, but that's
not particularly important right now.

New changeset b08e092df155 by Antoine Pitrou in branch '3.3':
Issue #19014: memoryview.cast() is now allowed on zero-length views.
http://hg.python.org/cpython/rev/b08e092df155
New changeset 1e13a58c1b92 by Antoine Pitrou in branch 'default':
Issue #19014: memoryview.cast() is now allowed on zero-length views.
http://hg.python.org/cpython/rev/1e13a58c1b92 

Applied Stefan's suggestion. Thanks for the review :)

Thank you Antoine.