params={'param1':1, 'param2':2}
fi=open("a.txt")
fo=open("a_processed.txt","w")
for line in fi:
    for k,v in params.iteritems():
        fo.write(line.replace(k,str(v)))
fo.close()
fi.close()

class Param(object):
   def __init__(self):
      ##   parameter dictionary with the key=string to search for,
      ##   and value=replacement value
      self.params_dic={"param1":"3",
                       "param2":"2",
                       "param3":"1" } 
 
 
   def process_file(self, data_in):   
      output_list = []
      for rec in data_in:
         r_list = rec.split("=")
         test_s = r_list[1].strip()
         if test_s in self.params_dic:     ## found
            output_list.append("%s=%s\n" % \
                                        (r_list[0], self.params_dic[test_s]))
         else:                               ## not found
            output_list.append(rec)
 
      print "final output ="
      for rec in output_list:
         print rec,
 
P = Param()
input_file_list = ['a =1\n', 'b=2\n', 'c= param1\n', 'd=4\n', 'e=param3\n']
P.process_file(input_file_list)