我尝试做一个简单的程序下载一个文件。我有一个问题的命令部分。这里是代码：

import os

#gather user input
print("hello! welcome to the website dowloader! paste in the url(including the http 
part) and type in the file name!)")
url = input("website url: ")
filename = input("the filename:")

#the command i want run. for example, if the url was "https://example.com" and the 
#filename was "example.html"
#then i would want the command run to be: 'curl https://example.com --output 
#example.html'
cmd = str("curl ", url," --output ", filename)
os.system(cmd)