Answering to myself, sorry. memoryview() does return the right answer of whether the object supports the buffer interface, *however* it doesn't mean the len() will be right. For example, take an array.array of ints:

>>> memoryview(array.array("I", [1,2,3]))
<memory at 0x1cf5720>
>>> len(array.array("I", [1,2,3]))
3
>>> len(memoryview(array.array("I", [1,2,3])))
3
>>> len(bytes(array.array("I", [1,2,3])))
12

len() returns 3 but the number of bytes written out by sendall() will really be 12...

*However*, the right len can be calculated using the memoryview:

>>> m = memoryview(array.array("I", [1,2,3]))
>>> len(m) * m.itemsize
12

(without actually converting to a bytes object, so all this is cheap even for very large buffers)