Solve equation with sum and index using Sympy

List p needs to be converted into symbolic array before it can be indexed with symbolic value j.

from sympy.solvers import solve
from sympy import symbols, summation, Array

p = Array([0.2, 0.3, 0.3, 0.1, 0.1])
l, j = symbols('l j')
eq = summation(j * (1 - p[j]) / (l - j), (j, 0, 4))

s = solve(eq - 1, l) # [1.13175762143963 + 9.29204634892077e-30*I, 2.23358705810004 - 1.36185313905566e-29*I, 3.4387382449005 + 3.71056356734273e-30*I, 11.5959170755598 + 6.15921474293073e-31*I]

(assuming your p stands for 1 - q)