I guess the code below in this example may be incorrect, though it does not hurt the conclusions significantly. I would appreciate it if someone could double check it. Thanks.
We update the variables.
U[1:-1, 1:-1], V[1:-1, 1:-1] = \
Uc + dt * (D*deltaU) + R*(Vc - Uc),\
Vc + dt * (D*deltaV) + R*(Uc - Vc)
My suggestion:
We update the variables.
U[1:-1, 1:-1], V[1:-1, 1:-1] = \
Uc + dt * (D*deltaU + R*(Vc - Uc)),\
Vc + dt * (D*deltaV + R*(Uc - Vc))