I want to use os.path.join to gather my file path.
But there have some trouble.
Please help me.

```py
print(sys.argv[1]) #print hw2
path = os.path.join(sys.argv[1], '{:s}.txt')
print(path) #print {:s}.txt

```

Why the result is not hw2/{:s}.txt?
I have asked the other people and it can work for them.
My python version is 3.6.8.