Python printing statements for a biased coin flip?What does the last 2 lines mean?

Question

What is the purpose of the last two lines of this code?

from random import random

def flip(bias):
    """
    Flip a coin once.
    `bias` is the likelihood of the result being heads, 0. <= bias <= 1.
    Returns True for heads or False for tails
    """
    return random() < bias
   
def main():
    bias = float(input("What bias do your coins have? "))
    count = {False: 0, True: 0}
    for i in range(1, 4):
        toss = flip(bias)
        count[toss] += 1
        print("Coin flip {} has a value of heads: {}".format(i, toss))
    print("Final result: {} heads, {} tails".format(count[True], count[False]))

if __name__ == "__main__":
    main()

Answer 1

It's for importation reasons.

If I wanted to use that function in another program for example, those two lines would not allow me to do so. You can check here for more information. Quoting the accepted answer:

One of the reasons for doing this is that sometimes you write a module (a .py file) where it can be executed directly. Alternatively, it can also be imported and used in another module. By doing the main check, you can have that code only execute when you want to run the module as a program and not have it execute when someone just wants to import your module and call your functions themselves.

In other words, the function must be called in the file it was placed in. If that was in one.py for example and imported into two.py, I can run the function in one.py but not in two.py.

Answer 2

Python is a scripting language. Unlike, say, C++, Python doesn't treat main() as anything special. So we want to run main() if we're running the file as a script.

if __name__ == "__main__":
    main()

If the file is being run as a script, __name__ is guaranteed to be equal to "__main__", so this will run the main function. If the file is being loaded as a module, __name__ will be the module name, so this won't run.

This is a fairly common Python idiom that you'll see a lot, so when you see if __name__ == "__main__", you'll just learn to recognize it.

Answer 3

If the file you are executing is this particular python file, then __name__ == "__main__" is true. If this file is imported and another file is run, then __name__ == "__main__" will not be true, so the code after name == "main" will not be run.