If there were an easy way to do interpolation I might well want to teach it before any of str() or string concatenation.
I'm afraid your students would end up appending a character c to a string s by writing
s = "$s$c".sub()
Not exactly good style.
And I would probably treat it in preference to the magic and special "," operator of the print statement.
I object to this insinuation.
I prefer to teach something that is generally useful like $ rather than something which they may have to unlearn like "," -- unlearn to the extent that they will naturally expect that commas in other contexts will do whitespace-generating concatenation and they hardly ever will.
(a) You're making this argument up. I don't believe for a second that you've observed this mistake in an actual student.
(b) I expect that students never even *think* about the space between printed items -- it's entirely natural.
(c) Commas are designed to "disappear" in our interpretation of things, and they do. The comma has so many uses where whitespace generation is just not one of the things you could possibly think about that I find it hard to take this argument serious.
--Guido van Rossum (home page: http://www.python.org/%7Eguido/)