• Don't use is to compare if two objects are equal, use == instead:

In [110]: [] is []
Out[110]: False

In [111]: []==[]
Out[111]: True

• Your g is a 1D numpy array, and g[j] gets you an element, whilst curr_g might be an array. To reproduce your error:

In [117]: l=np.arange(3)
     ...: l[0]=[1,2] #assigning an element of a numpy array to as a python list
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-117-817e72d77bd0> in <module>()
      1 l=np.arange(3)
----> 2 l[0]=[1,2]

ValueError: setting an array element with a sequence.

• np.array([no_classes, no_features, no_classes]) gives you a 1D array of length 3. If you want to initialize g as a 3D array of shape (no_classes, no_features, no_classes), use np.zeros, e.g.:

In [124]: np.zeros((2,3,2))
Out[124]: 
array([[[ 0.,  0.],
        [ 0.,  0.],
        [ 0.,  0.]],

       [[ 0.,  0.],
        [ 0.,  0.],
        [ 0.,  0.]]])

• To check if a subarray of g is unassigned(namely, all are 0s here IIUC), use np.allclose:

In [128]: l=np.zeros((2,3,2))

In [129]: np.allclose(l[0], 0)
Out[129]: True