How to read and write text file in python

list1=file("filename.txt").readlines() #don't forget the parenthesis
set1=set(liste1) #cast it to a set

set1=[elem.strip() for elem in set1] #strip all the elements of the set by removing trailing characters such us /t or /n

To find the differences with another set :
set3=set1.difference(set2)

To save into a file
fLog=tFile.open('filename.log','w')

First I add line return to each elem of the set
set4=[elem+"\n" for elem in set3]

Then I write the whole set into the file:

fLog.writelines(set4)
fLog.close()