necessary and sufficient

From man python on OS X 10.5 (and probably before):

INTERACTIVE INPUT EDITING AND HISTORY SUBSTITUTION
       The Python inteterpreter supports editing of the current input line and
       history substitution, similar to facilities found in the Korn shell and
       the  GNU  Bash shell.  However, rather than being implemented using the
       GNU Readline library, this Python interpreter  uses  the BSD  EditLine
       library editline(3) with a GNU Readline emulation layer.

       The  readline  module  provides the access to the EditLine library, but
       there are a few major differences compared to a traditional implementa-
       tion  using  the  Readline  library.   The command language used in the
       preference files is that of EditLine, as described in editrc(5) and not
       that   used  by  the  Readline  library. This  also  means that  the
       parse_and_bind() routines uses EditLine commands.  And  the preference
       file itself is ~/.editrc instead of ~/.inputrc.

       For  example,  the rlcompleter module, which defines a completion func-
       tion for the  readline  modules,  works correctly  with the  EditLine
       libraries, but needs to be initialized somewhat differently:

              import rlcompleter
              import readline
              readline.parse_and_bind ("bind ^I rl_complete")

       For vi mode, one needs:

              readline.parse_and_bind ("bind -v")

Well, that explains why Python doesn’t play nice with my readline stuff.