Calculating value of pi using prime numbers

pi_from_primes.py

from operator import mul

def sievePrimes(maximum):
	primes = dict.fromkeys(range(3,maximum+1,2),True)
	sieveList = [2]
	for num in sorted(primes):
		if primes[num] == True:
			sieveList.append(num)
			j = num ** 2
			while j < maximum:
				if j in primes:
					primes[j] = False
				j += num
	return(sieveList)

y = sievePrimes(100000)
y.remove(2) # 2 isn't in the list

y = map(lambda x: (1.0+1.0/x) if (x-1)%4==0 else (1.0-1.0/x),y)
two_over_pi = (reduce(mul, y,1))

print(["2/(PI) is approximately " , two_over_pi])
print(2.0/two_over_pi)