Subzero: Print defaults as appropriate in python script help text.

pydir/build-pnacl-ir.py

 #!/usr/bin/env python2
 
 import argparse
 import os
 import tempfile
 from utils import shellcmd
 from utils import FindBaseNaCl
 
 if __name__ == '__main__':
     argparser = argparse.ArgumentParser()
     argparser.add_argument('cfile', nargs='+', type=str,
         help='C file(s) to convert')
     argparser.add_argument('--dir', nargs='?', type=str, default='.',
-                           help='Output directory')
+                           help='Output directory. Default "%(default)s".')
     argparser.add_argument('--disable-verify', action='store_true')
     args = argparser.parse_args()
 
     nacl_root = FindBaseNaCl()
     # Prepend bin to $PATH.
     os.environ['PATH'] = (
         nacl_root + '/toolchain/linux_x86/pnacl_newlib/bin' + os.pathsep +
         os.pathsep + os.environ['PATH'])
 
     tempdir = tempfile.mkdtemp()
 
     for cname in args.cfile:
         basename = os.path.splitext(cname)[0]
         llname = os.path.join(tempdir, basename + '.ll')
         pnaclname = basename + '.pnacl.ll'
         pnaclname = os.path.join(args.dir, pnaclname)
 
         shellcmd('pnacl-clang -O2 -c {0} -o {1}'.format(cname, llname))
         shellcmd('pnacl-opt ' +
                  '-pnacl-abi-simplify-preopt -pnacl-abi-simplify-postopt' +
                  ('' if args.disable_verify else
                   ' -verify-pnaclabi-module -verify-pnaclabi-functions') +
                  ' -pnaclabi-allow-debug-metadata'
                  ' {0} -S -o {1}'.format(llname, pnaclname))