Euler Method for Lotka Volterra going wrong?

Question

I am trying to do the Lotka-Volterra model using the Euler method but I am not getting the right graph at all. This is on Python.

I have tried changing the tstep and arrays which has improved it. I have noticed I cannot make my time step bigger than 0.1 and my array has to be small hence 50.

import math
import numpy as np
import matplotlib.pyplot as plt

alpha=3
beta=0.5
gamma=0.4
delta=3
tstep=0.05

t=np.zeros(50)
P=np.zeros(50)
N=np.zeros(50)
N[0]=2
P[0]=1

for i in range(0,49):
    deltaN= (alpha * N[i] - beta * N[i] * P[i]) * tstep
    deltaP= (gamma * N[i] * P[i] - delta * P[i]) * tstep
    t[i+1]= t[i] + tstep
    N[i+1]=N[i]+deltaN
    P[i+1]=P[i]+deltaP



print t
print"N=", N
print"P=", P    

plt.plot(t,N)
plt.plot(t,P)

My graph does not oscillate like the Lotka-Volterra model is supposed to. The line only oscillates once.