一个有效的方法来制造一个大的随机的bytearray

I need to create a large bytearry of a specific size but the size is not known prior to run time. The bytes need to be fairly random. The bytearray size may be as small as a few KBs but as large as a several MB. I do not want to iterate byte-by-byte. This is too slow -- I need performance similar to numpy.random. However, I do not have the numpy module available for this project. Is there something part of a standard python install that will do this? Or do i need to I need to create a large bytearry of a specific