gistfile1.sh

_update_prompt () {
    if [ -z "$_dumb_prompt" ]; then

        # if $_dumb_prompt isn't set, do something potentially expensive, e.g.:
        git status --porcelain | perl -ne 'exit(1) if /^ /; exit(2) if /^[?]/'

        case "$?" in
             # handle all the normal cases
             ...
             # but also add a case for exit due to SIGINT
             "130" ) _dumb_prompt=1 ;;
        esac
    else
        # in this case the user asked the prompt to be dumbed down
       ...
    fi
}

# helper commands to explicitly change the setting:

dumb_prompt () {
    _dumb_prompt=1
}

smart_prompt () {
    unset _dumb_prompt
}