How to make a multidimension numpy array with a varying row size?

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We are now almost 7 years after the question was asked, and your code 

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

executed in numpy 1.12.0, python 3.5, doesn’t produce any error and
cells contains:

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

You access your cells elements as cells[0][2] # (=2) .

An alternative to tom10’s solution if you want to build your list of numpy arrays on the fly as new elements (i.e. arrays) become available is to use append:

d = []                 # initialize an empty list
a = np.arange(3)       # array([0, 1, 2])
d.append(a)            # [array([0, 1, 2])]
b = np.arange(3,-1,-1) #array([3, 2, 1, 0])
d.append(b)            #[array([0, 1, 2]), array([3, 2, 1, 0])]

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