Numpy array exclude some elements

I'm always using boolean masks for such things, you could consider:

# Mask every sixth row
mask = (np.arange(images.shape[0]) % 6) != 0

# Only use the not masked images
training_images = images[mask]

The validation set would then be every masked element:

validation_images = images[~mask]

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Mathematical operations on numpy arrays work element wise, so taking the modulo (%) will be executed on each element and returns another array with the same shape. The != 0 works also element-wise and compares if the modulo is not zero. So the mask is just an array containing False where the value is not an int * 6 and True where it is.