Python, tell us a little about speed?

Question

Python

I'm testing something here, explain why the speed varies so much (errors on average + -0.1 sec)
1. Average replenishment time 3.3 - 3.5

string = "123"
result = 0
for i in range(10000000):
 result = int(string)

2.2.6 - 2.8

string = "123"
result = 0
def main():
    for i in range(10000000):
        result = int(string)
main()

3.2.4 - 2.6

string = "123"
result = 0
def main():
    for i in range(10000000):
        result = int(string)
if __name__ == '__main__':
    main()

I'm still a noob, can you tell me a little about it?