Question

I would like to create a two dimensional numpy array of arrays that has a different number of elements on each row.

Trying

cells = numpy.array([[0,1,2,3], [2,3,4]])

gives an error

ValueError: setting an array element with a sequence.

Answer 1

While Numpy knows about arrays of arbitrary objects, it’s optimized for homogeneous arrays of numbers with fixed dimensions. If you really need arrays of arrays, better use a nested list. But depending on the intended use of your data, different data structures might be even better, e.g. a masked array if you have some invalid data points.

If you really want flexible Numpy arrays, use something like this:

numpy.array([[0,1,2,3], [2,3,4]], dtype=object)

However this will create a one-dimensional array that stores references to lists, which means that you will lose most of the benefits of Numpy (vector processing, locality, slicing, etc.).