removing nested iffs

[Peter Otten]

Josh Benner wrote:

> I'm writing a function to create a string that gets longer iff an argument
> is defined.  In there a more elegant way than nesting all those ifs?
> 
> def format_rsync_src_string(args, server="RSYNC"):
>     """ Format an rsync source directory string. """
>     if args.server is None:
>         raise CopyNightlyError("No rsync server provided.")
>     src = "{0}::".format(args.server)
>     if args.project not None:
>         src += "{0}/".format(args.project)
>         if args.version not None:
>                 src += "{0}/".format(args.version)
>                 if args.build not None:
>                     src += "Build {0}".format(args.build)
>     return src

How about 

def format_pairs(pairs):
    for template, value in pairs:
        if value is None:
            break
        yield template.format(value)

def format_rsync_src_string(args, server="RSYNC"):
    """ Format an rsync source directory string. """
    if args.server is None:
        raise CopyNightlyError("No rsync server provided.")

    return "".join(format_pairs([
        ("{0}::", args.server),
        ("{0}/", args.project),
        ("{0}/", args.version),
        ("Build {0}", args.build)]))