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.