question about using the python interface

I seem to be having trouble using the python interface to do something.

I want to be able to displace an atom and then query the forces that are generated.

So I create a ctype thing using gather_atoms
x0ctype = lmp.gather_atoms("x",1,3)

["lmp" is my instance of lammps]

As I understand it, this array now has x,y,z for the first atom, then x,y,z for the second, and so forth

Then I displace an atoms by modifying x0ctype
For example,
x0ctype[0] += h

then I use scatter_atoms to put the entire array back into LAMMPS' memory locations:

[I have set
atom_modify map hash
which I understand is necessary for this to work.]

Then I execute a single step

lmp.command("run 0 post no")

To query the forces and make an array which corresponds to the atoms as contained in x0ctype,
I do this:

    fx = lmp.extract_variable("fx","all",1)
    fy = lmp.extract_variable("fy","all",1)
    fz = lmp.extract_variable("fz","all",1)
    fs = np.array([ [fx[i],fy[i],fz[i]] for i in range(len(fz)) ]).flatten()

(np is "numpy")

This does not seem to be working. I seem to be doing something fundamentally in error.

Any suggestions would be appreciated.

Best regards,
Murray Daw