Just try the code below:
As I see you have inserted 'r+' or this command open the file in read mode so you are not able to write into it, so you have to open file in write mode 'w' if you want to overwrite the file contents and write new data, otherwise you can append data to file by using 'a'
I hope this will help ;)
f = open('testfile.txt', 'w')# just put 'w' if you want to write to the file
x = f.readlines() #this command will read file lines
y = int(x)+1
print y
z = str(y) #making data as string to avoid buffer error
f.write(z)
f.close()