How to create a tridiagonal array in Python?

Hello, I'd like to know how to write a tridiagonal NxN array. I have three vectors that will be updated in a loop.

I'm working on something similar to this:

Note: I would just like to know about zeros and what Python parameters I could use to adjust this. Well, I have two codes here, the first one I wrote in Fortran and it's working fine. The second is what I tried to write in Python.

Fortran:

do i=2,n-1
  do j=2,n-1
    if (i.eq.j) then

      D(i,j+1)=-u_med(i+1)/(delta_r(i)*delta_r(i+1))

      t1 =u_med(i+1)/(delta_r(i)*delta_r(i))

      t2 = u_med(i)/(delta_r(i)*delta_r(i))

      D(i,j)= t1 + t2 + V(i)

      D(i+1,j)=-u_med(i+1)/(delta_r(i)*delta_r(i+1))

    end if

  end do
end do

Python:

for i in range(2,n):
  for j in range(2,n):
    if i == j:
      D[i][j+1] = - u_med[i+1]/(delta_r[i]*delta_r[i+1])
      t1 = u_med[i+1]/(delta_r[i]*delta_r[i])
      t2 = u_med[i]/(delta_r[i]*delta_r[i])
      D[i][j]= t1 + t2 + V[i]
      D[i+1][j]= - u_med[i+1]/(delta_r[i]*delta_r[i+1])

t1 = u_med[2]/(delta_r[1]*delta_r[1])
t2 = 0
D[1][1]= t1 + t2 + V[1]
D[1][2]= - u_med[2]/(delta_r[1]*delta_r[2])
D[2,1]= - u_med[2]/(delta_r[2]*delta_r[1])
t1 = 0
t2 = u_med[n]/(delta_r[n]*delta_r[n])

D[n][n]= t1 + t2 + V[n]

That gives the error:

  

D [i] [j + 1] = - u_med [i + 1] / (delta_r [i] * delta_r [i + 1]) ValueError: setting   an array element with a sequence.

Definitions of u_med , delta_r , r , n and D in Python:

u_med = np.zeros((n,2))
for i in range(2,n):
  tta1 = r[i]*u[i]
  tta2 = r[i-1]*u[i-1]
  u_med[i] = 0.5*(tta1 + tta2)/(r[i] - r[i-1])

u_med[1] = u_med[2]

delta_r = np.zeros((n-1,2))
for i in range(2,n-1):
  ft1 = r[i+1]*r[i+1]
  ft2 = r[i-1]*r[i-1]
  ft3 = 2*r[i]*(r[i+1] - r[i-1])
  delta_r[i] = math.sqrt(0.125*abs(ft1 - ft2 + ft3))

ri=0
n1 = 51

r1 = ri
r2 = 250
hr1 = (r2-r1)/(n1-1)

r = np.zeros((n1,1))
for i in range(n1):
  r[i] = r1 + i*hr1

u = np.zeros((n+1,1))
for i in range(1,n+1):
  i = 1

D = npm.zeros((n,n))

I suspect the error is in:

D = npm.zeros((n,n))

But how could I rewrite it?