import os import fileinput def file_insert(fname,linenos=[],strings=[]):   """   Insert several strings to lines with linenos repectively.    The elements in linenos must be in increasing order and len(strings)   must be equal to or less than len(linenos).    The extra lines ( if len(linenos)> len(strings)) will be inserted   with blank line.   """   if os.path.exists(fname):     lineno = 0     i = 0     for line in fileinput.input(fname,inplace=1):       # inplace must be set to 1       # it will redirect stdout to the input file       lineno += 1       line = line.strip()       if i<len(linenos) and linenos[i]==lineno:         if i>=len(strings):           print "/n",line         else:           print strings[i]           print line         i += 1       else:         print line file_insert('a.txt',[1,4,5],['insert1','insert4'])