To modify what I said, Idle only auto-expands literal tabs entered into an editor window with the Tab key.  When one is doing such entry, all key sequences are subject to interception, so this is nothing special.

Tabs chars put in a string with '\t' or '\x09' are not and should not be modified (except by explicit string methods.  The issue here and in #23220 is what happens when strings are written to a file object with print or file.write.