In this example, we will learn how to determine the prime number in Python.

### Source Code

```
#! /usr/bin/env python3
# -*- coding: utf-8 -*-
# Enter a number and cast to integer
num = int(input("Please enter a number: "))
# Prime number must be greater than 1
if num > 1:
# Set for loop from 2 to num-1 and check whether there is a factor, if found, then it is not prime number.
for i in range(2, num):
if (num % i) == 0:
print(num, " is not prime number")
print(i, "*", num // i, "is", num)
break
else:
print(num, "is prime number")
else:
print(num, "is not prime number")
```

Output:
```
Please enter a number: 8
8 is not prime number
2 * 4 is 8
Please enter a number: 5
5 is prime number
```

### 4. Think More

The method above is in low efficiency. We know that if a number can be factorized, then the two numbers* a* and *b* obtained during the decomposition must have this situation: *a* is less than or equal to **sqrt(n) **and *b*** **is greater than or equal to **sqrt(n)**. Based on this, the codes above can be optimized. We just need traverse to **sqrt(n)** and stop there. This is because if no divisor is found on the left side of **sqrt(n)**, we can not find it on the right side as well.