'Pre-load the stdlibs zone, if able.
"progress_cb" (optional) is a callable that is called as
follows to show the progress of the update:
progress_cb(<desc>, <value>)
where <desc> is a short string describing the current step
and <value> is an integer between 0 and 100 indicating the
level of completeness.
Use `.can_preload()\' to determine if able to pre-load.'

def preload(self, progress_cb=None):

    import which
    import process
    log.debug('preloading    stdlibs    zone')
    if progress_cb:
        try:
            progress_cb('Preloading    stdlibs...', None)
        except:
            log.exception('error    in    progress_cb    (ignoring)')
    preload_zip = self._get_preload_zip()
    unzip_exe = which.which('unzip')
    cmd = ('"%s"    -q    -d    "%s"    "%s"' % (unzip_exe, dirname(self.base_dir), preload_zip))
    p = process.ProcessOpen(cmd, stdin=None)
    (stdout, stderr) = p.communicate()
    retval = p.wait()
    if retval:
        raise OSError(("error    running    '%s'" % cmd))
