Indirectly Changing Variables in Python

Question

I'm currently working through the following FEniCS tutorial program Heat Equation Tutorial. In the program, the following block of code appears initializing the variables L, linear_form and b.

L = (u_n + dt * f) * v * ufl.dx
linear_form = fem.form(L)
b = create_vector(linear_form)

later in the program, we have the following block of code -

for i in range(num_steps):
    t += dt

    # Update the right hand side reusing the initial vector
    with b.localForm() as loc_b:
        loc_b.set(0)
    assemble_vector(b, linear_form)
.
.
.
# Update solution at previous time step (u_n)
    u_n.x.array[:] = uh.x.array

I'm not sure how, but in every iteration of the for loop, the variable L is updated (presumably due to u_n being updated. My question is the following - how does this occur? Given a variable x defined in terms of another, y, under what circumstances can changing y cause x to change as well?

I know this is probably a very basic question, I'm a mathematician and new to Python.

As a side question, if anyone can refer me to a section in a text where this is treated more thoroughly, please send it my way