python - Size of data type using NumPy

Question

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?

Answer 1

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.

print np.dtype(float).itemsize
print np.dtype(np.float32).itemsize
print np.dtype('|S10').itemsize

As far as the difference between itemsize and nbytes, nbytes is just x.itemsize * x.size.

E.g.

In [16]: print np.arange(100).itemsize
8

In [17]: print np.arange(100).nbytes
800