Numpy ndarrays are meant for all elements to have the same length. In this case, your second array doesn’t contain lists of the same length, so it ends up being a 1-D array of lists, as opposed to a “proper” 2-D array.

From the Numpy docs on N-dimensional arrays:

An ndarray is a (usually fixed-size) multidimensional container of items of the same type and size.

a = np.array([[1,2,3,4], [5,6,7,8], [9,10,11,12]]) a.shape # (3,4) a.ndim # 2 b = np.array([[1,2,3,4], [5,6,7,8], [9,10,11]]) b.shape # (3,) b.ndim # 1

This discussion may be useful.

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