computing pi value using python, division always zero

I am trying to use python to compute an approximation of (pi) using unlimited series.

stopping condition is for accuracy = 10**(-9)

accuracy is abs(exact_pi-computed_pi)

my code:

from math import sqrt

exact_pi=3.14159265358979323846
accuracy=10**(-9)
computed_pi=float(sqrt(12))
k=1

while abs(float(computed_pi)-float(exact_pi))>accuracy:
   
    num1=float((-3)**(-k))
    num2=float((2*k+1))
    computed_pi=float(computed_pi)+ float(num1/num2)
    k=k+1

The result of float(num1/num2) after some iterations for k=100 for example it outputs 0 so it gets into an infinite loop. can someone help?

PI FORMULA

>Solution :

As @quamrana noticed you have missed sum operation.

# your code before while

while abs(computed_pi - exact_pi) > accuracy:
    multiplier = sum((-3)**(-i)/(2 * i + 1) for i in range(k))
    computed_pi = 12**0.5 * multiplier
    k += 1

or slightly optimized version (thanks again to quamrana)

multipler = 0
while abs(computed_pi - exact_pi) > accuracy:
    multiplier += (-3)**(-k) / (2 * k + 1)
    computed_pi = 12**0.5 * multiplier
    k += 1