memmap_example.py

import numpy as np
from numpy.lib.format import open_memmap

shape = (1, 20, 600, 2160)
dtype = np.float32

# Open a memory mapped numpy formatted file for a temporary variable
data = open_memmap('temp.npy', dtype = dtype, mode = 'w+', shape = shape)
data[:] = np.random.rand(*shape)
data.flush() # force write disk

test = np.array(data) # copy to memory for checking below

print(data)
del data # release memory mapped file

# load temporary variable as memory mappe filed (no data is loaded into memory)
data2 = np.load('temp.npy', mmap_mode='r')
np.testing.assert_array_equal(test, data2) # read and compare as necessary

# create a second output file, also memory mapped and store result of computation
data3 = open_memmap('out.npy', dtype = dtype, mode = 'w+', shape = shape)
data3[:] = data2*40. + data2**4
del data3 # flushes to disk

# load using memory map (no data is loaded yet)
data4 = np.load('out.npy')

# assert that contents match computed values above
np.testing.assert_array_equal((data2*40. + data2**4), data4)