code: def random_useragent(filename=True):
      
      import random
  
      default_ua = 'urlfetch/%s' % __version__
  
      if isinstance(filename, basestring):
          filenames = [filename]
      else:
          filenames = []
  
      if filename and UAFILE:
          filenames.append(UAFILE)
  
      for filename in filenames:
          try:
              st = os.stat(filename)
              if stat.S_ISREG(st.st_mode) and os.access(filename, os.R_OK):
                  break
          except:
              pass
      else:
          return default_ua
  
      with open(filename, 'rb') as f:
          filesize = st.st_size
          pos = 0
          r = random.Random()
  
          # try getting a valid line for no more than 3 times
          for i in range(3):
  
              pos += r.randint(0, filesize)
              pos %= filesize
              f.seek(pos)
  
              # in case we are in middle of a line
              f.readline()
  
              line = f.readline()
              if not line:
                  if f.tell() == filesize:
                      # end of file
                      f.seek(0)
                      line = f.readline()
  
              line = line.strip()
              if line and line[0] != '#':
                  return line
  
      return default_ua
docs: Returns a User-Agent string randomly from file.
  
      :arg string filename: (Optional) Path to the file from which a random
          useragent is generated. By default it's ``True``, a file shipped
          with this module will be used.
      :returns: An user-agent string.