In NumPy, I can get the size (in bytes) of a particular data type by:
datatype(...).itemsize
or:
datatype(...).nbytes
For example:
np.float32(5).itemsize # 4
np.float32(5).nbytes # 4
I have two questions. First, is there a way to get this information without creating an instance of the datatype? Second, what's the difference between itemsize and nbytes?
You need an instance of the dtype to get the itemsize, but you shouldn't need an instance of the ndarray. (As will become clear in a second, nbytes is a property of the array, not the dtype.)
E.g.
np.dtype(float).itemsize # 8
np.dtype(np.float32).itemsize # 4
np.dtype('|S10').itemsize # 10
As far as the difference between itemsize and nbytes, nbytes is just x.itemsize * x.size.
E.g.
np.arange(100).itemsize # 8
np.arange(100).nbytes # 800