human
I am working on the file "2.3.transport_through_barrier.ipynb".
The first task is:
The next one is slightly modified: the onsite value is now no longer a constant but a value function.
Kwant’s value functions are used when there are parameters to the Hamiltonian.  Kwant will call them only when a calculation is requested, at the last possible moment.  The onsite function always take one arguments (the site), and an arbitrary number of parameters that are supplied by the user.  Here, there is only one such parameter: V.
Can you write Python code for it?
assistant
#Here is the Python code you requested.

def make_wire_with_flat_potential(W=10, L=2, t=1):
    def onsite(s, V):
        return (4 - V) * t

    # Construct the scattering region.
    sr = kwant.Builder()
    sr[(lat(x, y) for x in range(L) for y in range(W))] = onsite
    sr[lat.neighbors()] = -t

    # Build and attach lead from both sides.
    lead = make_lead_x(W, t)
    sr.attach_lead(lead)
    sr.attach_lead(lead.reversed())

    return sr