How to use tempfile.NamedTemporaryFile() in Python

I want to use tempfile.NamedTemporaryFile() to write some contents into it and then open that file. I have written following code:

tf = tempfile.NamedTemporaryFile()
tfName = tf.name
tf.seek(0)
tf.write(contents)
tf.flush()

but I am unable to open this file and see its contents in Notepad or similar application. Is there any way to achieve this? Why can't I do something like:

os.system('start notepad.exe ' + tfName)

at the end.

I don't want to save the file permanently on my system. I just want the contents to be opened as a text in Notepad or similar application and delete the file when I close that application.