o = open("output","a") #open for append
for line in open("file"):
   line = line.replace("someword","newword")
   o.write(line + "\n") 
o.close()

f = open("file")
o = open("output","a")
while 1:
  line = f.readline()
  if not line: break
  line = line.replace("someword","newword")
  o.write(line + "\n")
o.close()

import re
o = open("output","w")
data = open("file").read()
o.write( re.sub("someword","newword",data)  )
o.close()

import fileinput
for line in fileinput.FileInput("file",inplace=1):
   line = line.replace("blah","blahblah")
   print line

# read a text file, replace multiple words specified in a dictionary
# write the modified text back to a file
 
import re
 
def replace_words(text, word_dic):
    """
    take a text and replace words that match a key in a dictionary with
    the associated value, return the changed text
    """
    rc = re.compile('|'.join(map(re.escape, word_dic)))
    def translate(match):
        return word_dic[match.group(0)]
    return rc.sub(translate, text)
 
 
str1 = \
"""When we see a Space Shuttle sitting on its launch pad, there are two big
booster rockets attached to the sides of the main fuel tank. These are solid
rocket boosters, made by Thiokol at their factory in Utah. The engineers who
designed the solid rocket boosters might have preferred to make them a bit
fatter, but they had to be shipped by train from the factory to the launch
site.  The railroad line from the factory runs through a tunnel in the
mountains.  The boosters had to fit through that tunnel.  The tunnel is
slightly wider than the railroad track.  The width of the railroad track
came from the width of horse-drawn wagons in England, which were as wide
as two horses' behinds.  So, a major design feature of what is the world's
most advanced transportation system was determined over two thousand years
ago by the width of a horse's ass!
"""
 
test_file = "Mword1.txt"
# create a test file for this example
fout = open(test_file, "w")
fout.write(str1)
fout.close()
 
# read the file
fin = open(test_file, "r")
str2 = fin.read()
fin.close()
 
# the dictionary has target_word:replacement_word pairs
word_dic = {
'booster': 'rooster',
'rocket': 'pocket',
'solid': 'salted',
'tunnel': 'funnel',
'ship': 'slip'}
 
# call the function and get the changed text
str3 = replace_words(str2, word_dic)
 
# test
print str3
 
# write changed text back out
fout = open("Mword2.txt", "w")
fout.write(str3)
fout.close()