When I write the following code I get garbage for an output. It is just a simple program to find prime numbers. It works when the first for loops range only goes up to 1000 but once the range becomes large the program fail's to output meaningful data

output = open("output.dat", 'w')
for i in range(2, 10000):
    prime = 1
for j in range(2, i-1): 
    if i%j == 0:
        prime = 0
        j = i-1
if prime == 1:
    output.write(str(i) + " " )
output.close()
print "writing finished"