average of filelines

def average_file(filename):
      mtr=open(filename,"r")
      mylist=mtr.readlines()
      summ=0
      try:
        for it in mylist:
            summ+=int(it)
        fin=summ/len(mylist)
      except ValueError:
        mtr.close()
        return "Error reading file!" 
      mtr.close()
      return fin