_ _name_ _是一个内置变量，当运行程序自己的是时候，变量值为_ _main_ _;当被其他程序当作模块导入时，值为文件名;这个一般被配合if else语句使用;写在程序最下面，函数定义，变量定义写在最上面。由于python是解释性脚本程序，依据从上到下执行，由以上的做法巧合的让python出现主函数入口，且可屏蔽被导入模块里边的主函数运行，即，另一个程序里边的函数只能被调用。例如：
# file green.py
def fun():
print("fun() in green.py")
if __name__ == "__main__":
print("green.py is being run directly")
else:
print("green.py is being imported into another module")
# file bule.py
import green
green.fun()
if __name__ == "__main__":
print("bule.py is being run directly")
else:
print("bule.py is being imported into another module")
如果你执行green.py文件，python green.py
输出：green.py is being run directly
如果你执行bule.py文件，python bule.py
输出：green.py is being imported into another module
fun() in green.py
bule.py is being run directly