write() at beginning of file?

I'm doing it like this now, but I want it to write at the beginning of the file instead.

f = open('out.txt', 'a') # or 'w'?
f.write("string 1")
f.write("string 2")
f.write("string 3")
f.close()

so that the contents of out.txt will be:

string 3
string 2
string 1

and not (like this code does):

string 1
string 2
string 3