A file stores text outside your variables. These exercises run in an isolated virtual filesystem: the files live only while the Python session runs and disappear when you reload the page. /tmp is a folder inside that virtual filesystem, not a real folder on your computer.

open() is a built-in function that opens a file and returns a file object. Its common signature is:

• file — a string with the path of the file to open, such as "/tmp/notes.txt". Here the name means the path, not the file object that open() returns.
• mode — a string that says what you want to do with the file. Default: "r" (read).

The real open() has more optional parameters, including encoding. These exercises use the defaults. You can inspect them with help(open).

The modes you need in this chapter:

• "r" — read. The file must already exist.
• "w" — write. Creates the file, or empties it if it already exists.
• "a" — append. Adds text at the end; creates the file if it does not exist.

The file object has methods:

• write(text) writes exactly the string you pass. It does not add a newline, so write "\n" yourself when you want one.
• close() takes no arguments. Call it when you finish, or let with close the file for you (lesson 3 of this chapter).

After this code, /tmp/demo.txt contains the text Hello followed by a newline.

Coming from JavaScript? Calling open() with "w" is similar to fs.writeFileSync: it creates or replaces the file. In Python you write through the returned file object and then call close().