if __name__ == "__main__" 的作用

作用：当模块被直接运行时，以下代码块将被运行，当模块是被导入时，代码块不被运行。

例子：

# 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

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")

如果你执行one.py文件，

python one.py
会输出：

top-level in one.py
one.py is being run directly
如果你执行two.py文件，

python two.py
会输出：

top-level in one.py
one.py is being imported into another module
top-level in two.py
func() in one.py
two.py is being run directly

总结：当模块导入时if __name__ == "__main__" 代码块不被执行，写if __name__ == "__main__"是为了防止模块导入，导致又重新运行了一遍导入的模块的代码。