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'])