fileObject.writelines( sequence )

#!/usr/bin/python'

# Open a file in witre mode
fo = open("foo.txt", "rw+")
print "Name of the file: ", fo.name

# Assuming file has following 5 lines
# This is 1st line
# This is 2nd line
# This is 3rd line
# This is 4th line
# This is 5th line

seq = ["This is 6th linen", "This is 7th line"]
# Write sequence of lines at the end of the file.
fo.seek(0, 2)
line = fo.writelines( seq )

# Now read complete file from beginning.
fo.seek(0,0)
for index in range(7):
  line = fo.next()
  print "Line No %d - %s" % (index, line)

# Close opend file
fo.close()

Name of the file: foo.txt
Line No 0 - This is 1st line

Line No 1 - This is 2nd line

Line No 2 - This is 3rd line

Line No 3 - This is 4th line

Line No 4 - This is 5th line

Line No 5 - This is 6th line

Line No 6 - This is 7th line