if __name__ == "__main__"

Often script can be executed independently and can be imported as a module by another script. Since importing a script runs this script, it is often necessary to specify that some strings should not be executed when importing.

In previous example there were two scripts: check_ip_function.py and get_correct_ip.py. And when starting get_correct_ip.py, print() from check_ip_function.py was displayed.

Python has a special technique that specifies that a code must not be executed at import: all lines that are in if __name__ == '__main__' block are not executed at import.

Variable __name__ is a special variable that will be equal to "__main__" only if file is run as the main program and is set equal to module name when importing the module. That is, if __name__ == '__main__' condition checks whether a file was run directly.

As a rule, if __name__ == '__main__' block includes all function calls and information output on the standard output stream. That is, in check_ip_function.py script this block conytains everything except import and return_correct_ip function:

import ipaddress


def check_ip(ip):
    try:
        ipaddress.ip_address(ip)
        return True
    except ValueError as err:
        return False


if __name__ == '__main__':
    ip1 = '10.1.1.1'
    ip2 = '10.1.1'

    print('Cheking IP...')
    print(ip1, check_ip(ip1))
    print(ip2, check_ip(ip2))

Result of script execution:

$ python check_ip_function.py
Cheking IP...
10.1.1.1 True
10.1.1 False

When you start check_ip_function.py script directly, all lines are executed, because __name__ variable in this case is equal to '__main__'.

Script get_correct_ip.py remains unchanged

from check_ip_function import check_ip


def return_correct_ip(ip_addresses):
    correct = []
    for ip in ip_addresses:
        if check_ip(ip):
            correct.append(ip)
    return correct


print('Checking list of IP addresses')
ip_list = ['10.1.1.1', '8.8.8.8', '2.2.2']
correct = return_correct_ip(ip_list)
print(correct)

Execution of get_correct_ip.py script:

$ python get_correct_ip.py
Checking list of IP addresses
['10.1.1.1', '8.8.8.8']

Now the output contains only information from script getcorrect__ip.py.

In general, it is better to write all code that calls functions and outputs something to the standard output stream inside if __name__ == '__main__' block.

Warning

Starting with Section 11, for tests to work correctly you have to always write a function call in task file within if __name__ == '__main__' block. The absence of this block will not cause errors in all tasks, but it will still avoid problems.