def if_name_=="_main_". What does this mean?

Question

print("before import")
import math

print("before functionA")
def functionA():
print("Function A")

print("before functionB")
def functionB():
print("Function B {}".format(math.sqrt(100)))

print("before __name__ guard")
if __name__ == '__main__':
functionA()
functionB()
print("after __name__ guard")

Answer 1

When the program is runing by itself and it's not imported as a module __name__=='__main__'
(It's actually two underlines)
But when it's imported as a module, __name__ is module's filename.

It's can be so useful when you want to write a code that can run by itself and you want use its functions in other codes, too.

Answer 2

Thanks