Variable inside ei function. Python can't calculate : My code is attached below. 
Python is unable to calculate the value of N_next because it's inside the Ei function. How can I circumvent this?
Exact error is "Can't assign function to call".

    import math
    import numpy as np
    import matplotlib
    import matplotlib.pyplot as plt
    
    t_f =100
    N_0 = 10
    t = []
    N_t = [N_0,]
    r = 2.5
    K = 1000
    
    for i in range(0,100):
        ei(r*N_next/K) = i*e^r + ei(r/K * N_t[i])
        N_t.append(N_next)
        t.append(i)
        
    plt.plot(t,N_t)
    plt.show