The prime number in mathematics is a number greater than 1 that cannot be exactly divided by any whole number other than itself and 1.

**How to Check If a Number is Prime in Python**

To **check** if a **number** is **prime** in **Python**, use a **for loop** and the** % operator** that checks if the number is divisible by any other number from 2 to given_num-1. It is prime if the number is not divisible by any of these numbers.

```
def is_prime(num: int) -> bool:
# Check if num is less than 2, which is not prime
if num < 2:
return False
# Check if num is divisible by any number from 2 to num-1
for i in range(2, num):
if num % i == 0:
return False
# If the loop completes without finding a divisor, num is prime
return True
num = 19
if is_number_prime(num):
print(f"{num} is a prime number.")
else:
print(f"{num} is not a prime number.")
```

**Output**

`19 is a prime number.`

You can see that 19 is a prime number. That’s why the **is_number_prime()** function returns **True**.

You can then use the **is_number_prime()** function to check if a number is prime by calling it and passing it as an argument.

Let’s check for the **“21”** number. The 21 is not a prime number. Let’s check by the program.

```
num = 21
if is_number_prime(num):
print(f"{num} is a prime number.")
else:
print(f"{num} is not a prime number.")
```

**Output**

`21 is not a prime number.`

That’s it. We successfully checked if the given number is prime programmatically in Python.

Remember that this approach is simple and easy to understand, but it can be slow for larger numbers because it checks all numbers from 2 to num-1.

There are more efficient algorithms for checking if a number is prime, such as the Sieve of Eratosthenes, which can be used for higher performance.

That’s it for this tutorial.

**Further reading**

Find prime numbers in Python range

Krunal Lathiya is a Software Engineer with over eight years of experience. He has developed a strong foundation in computer science principles and a passion for problem-solving. In addition, Krunal has excellent knowledge of Data Science and Machine Learning, and he is an expert in R Language. Krunal has experience with various programming languages and technologies, including PHP, Python, and JavaScript. He is comfortable working in front-end and back-end development.