在python编译器读取源文件的时候会执行它找到的所有代码，而在执行之前会根据当前运行的模块是否为主程序而定义变量__name__的值为__main__还是模块名。因此，该判断语句为真的时候，说明当前运行的脚本为主程序，而非主程序所引用的一个模块。这在当你想要运行一些只有在将模块当做程序运行时而非当做模块引用时才执行的命令，只要将它们放到if __name__ == "__main__:"判断语句之后就可以了。
具体举个例子方便理解：
# file one.py
def func():
print("func() in one.py")
print("top-level in one.py")
if __name__ == "__main__":
print("one.py is being run directly")
else:
print("one.py is being imported into another module")# file two.py
import one # start executing one.py
print("top-level in two.py")
one.func()
if __name__ == "__main__":
print("two.py is being run directly")
else:
print("two.py is being imported into another module")当运行python one.py，输出:
top-level in one.py
one.py is being run directly当运行python two.py,输出:
top-level in one.py
one.py is being imported into another module
top-level in one.py
func() in one.py
two.py is being run directly