old_contents: import argparse
  import multiprocessing
  
  from tox import hookimpl
  
  
  @hookimpl
  def tox_addoption(parser):
      def positive_integer(value):
          ivalue = int(value)
          if ivalue <= 0:
              raise argparse.ArgumentTypeError(
                  "%s is an invalid positive int value" % value)
          return ivalue
  
      try:
          num_proc = multiprocessing.cpu_count()
      except Exception:
          num_proc = 2
      parser.add_argument(
          "-n", "--num",
          type=positive_integer,
          action="store",
          default=num_proc,
          dest="numproc",
          help="set the number of concurrent processes "
               "(default %s)." % num_proc)
  
new_contents: import argparse
  import multiprocessing
  
  from tox import hookimpl
  
  
  @hookimpl
  def tox_addoption(parser):
      def positive_integer(value):
          ivalue = int(value)
          if ivalue <= 0:
              raise argparse.ArgumentTypeError("{} must be greater 0".format(value))
          return ivalue
  
      try:
          num_proc = multiprocessing.cpu_count()
      except Exception:
          num_proc = 2
      parser.add_argument(
          "-n",
          "--num",
          type=positive_integer,
          action="store",
          default=num_proc,
          dest="numproc",
          help="set the number of concurrent processes " "(default {}).".format(num_proc),
      )
  
subject: make error msg shorter but clearer, tidy up the code