write() at beginning of file?

Question

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

Answer 1

Take a look at this question. There are some solutions there.

Though I would probably go that same way Daniel and MAK suggest -- maybe make a lil' class to make things a little more flexible and explicit:

class Prepender:

    def __init__(self, fname, mode='w'):
        self.__write_queue = []
        self.__f = open(fname, mode)

    def write(self, s):
        self.__write_queue.insert(0, s)

    def close(self):
        self.__exit__(None, None, None)

    def __enter__(self):
        return self

    def __exit__(self, type, value, traceback):
        if self.__write_queue: 
            self.__f.writelines(self.__write_queue)
        self.__f.close()

with Prepender('test_d.out') as f:
    f.write('string 1\n')
    f.write('string 2\n')
    f.write('string 3\n')

Answer 2

You could throw a f.seek(0) between each write (or write a wrapper function that does it for you), but there's no simple built in way of doing this.

EDIT: this doesn't work, even if you put a f.flush() in there it will continually overwrite. You may just have to queue up the writes and reverse the order yourself.

So instead of

f.write("string 1")
f.write("string 2")
f.write("string 3")

Maybe do something like:

writeList = []
writeList.append("string 1\n")
writeList.append("string 2\n")
writeList.append("string 3\n")
writeList.reverse()
f.writelines(writeList)