This looks quite complicated but that's only because a lot of the lines can be condensed. The script opens the file, erases the contents and then allows the user to write in new data. Used with other scripts, it's quite a powerful few lines of code.

from sys import argv

script, filename = argv

print "We are going to erase %r." % filename
print "If you don't want that, press ctrl-c."
print "Otherwise, hit Return."

raw_input("?")

print "Opening the file..."
target = open(filename, 'w')

print "Truncating the file...Goodbye"
target.truncate()

print "Now I am going to ask you for three lines."

line1 = raw_input("Line 1")
line2 = raw_input("Line 2")
line3 = raw_input("line 3")

print "I am going to write these to the file."

target.write(line1)
target.write("\n")

target.write(line2)
target.write("\n")

target.write(line3)
target.write("\n")

print "And finally, we close it."
target.close()