How to signal "not implemented yet"?

[Roy Smith]

Is there some standard way to signal "not implemented yet" in
unfinished code?  When I'm coding, I'll often only flesh out one side
of a branch, or delay writing some method until later.  It would be
nice to be able to identify these right in the code to make sure they
don't get forgotten about.

I know I can just use "assert (False)", but that really means "this
can never happen", as opposed to "this can certainly happen, but I
havn't gotten around to dealing with it yet".  An assert with a "not
implemented yet" comment would be better than the naked assert, but
the comment won't be exposed in automated testing.

How do other people do this?