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')