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.wr         


        

Answer 1

A variation on kdtrv's answer. This version keeps the existing file contents, and offers a write_lines method that preserves line order.

class Prepender(object):
    def __init__(self,
                 file_path,
                ):
        # Read in the existing file, so we can write it back later
        with open(file_path, mode='r') as f:
            self.__write_queue = f.readlines()

        self.__open_file = open(file_path, mode='w')

    def write_line(self, line):
        self.__write_queue.insert(0,
                                  "%s\n" % line,
                                 )

    def write_lines(self, lines):
        lines.reverse()
        for line in lines:
            self.write_line(line)

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

    def __enter__(self):
        return self

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


with Prepender('test_d.out') as f:
    # Must write individual lines in reverse order
    f.write_line('This will be line 3')
    f.write_line('This will be line 2')
    f.write_line('This will be line 1')

with Prepender('test_d.out') as f:
    # Or, use write_lines instead - that maintains order.
    f.write_lines(
        ['This will be line 1',
         'This will be line 2',
         'This will be line 3',
        ]
    )