How to make a condition to terminate appending?

I am writing a code to plot several projectile trajectories of various theta values in Python.

theta = np.arange(np.pi/6, np.pi/3)
t = np.linspace(0,2,num=100)
while y0>=0:
    for i in theta:
        x = []
        y = []
        for k in t:
                x0= v_0*np.cos(i)*k
                y0= v_0*np.sin(i)*k - 1/2*g*(k**2)
                x.append(x0)
                x.append(y0)

After forming the arrays and putting in the necessary conditions for projectile, I have used a while loop to put the terminating instruction in the program. I think, I am missing a crucial point. Thanks!

I think you want your terminating condition inside your inner-most loop. See below, where I also defined a couple of missing constants (v_0, g) and fixed one x to y. also printing the results

theta = np.arange(np.pi/6, np.pi/3)
t = np.linspace(0,2,num=100)
v_0 = 1
g=10

for i in theta:
    x = []
    y = []
    for k in t:
        x0= v_0*np.cos(i)*k
        y0= v_0*np.sin(i)*k - 1/2*g*(k**2)
        x.append(x0)
        y.append(y0)
        if y0 < 0: # the main change here. Stop looping when y_0 below zero
            break
    print(f'theta:{i}')
    print(f'x:{x}')    
    print(f'y:{y}')

produces

theta:0.5235987755982988
x:[0.0, 0.017495462702715934, 0.03499092540543187, 0.052486388108147805, 0.06998185081086374, 0.08747731351357968]
y:[0.0, 0.008060401999795939, 0.012039587797163551, 0.011937557392102841, 0.007754310784613805, -0.0005101520253035577]

Plotting it (y vs x), looks reasonable

It is also worth noting that your definition of theta = np.arange(np.pi/6, np.pi/3) looks rather strange, what are you trying to achieve here?