<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def collect_output(mr_out_dir, out_file=None):
 """ Return all mapreduce output in ``mr_out_dir``. Append the output to ``out_file`` if provided. Otherwise, return the result as a single string (it is the caller's responsibility to ensure that the amount of data retrieved fits into memory). """

    if out_file is None:
        output = []
        for fn in iter_mr_out_files(mr_out_dir):
            with hdfs.open(fn, "rt") as f:
                output.append(f.read())
        return "".join(output)
    else:
        block_size = 16777216
        with open(out_file, 'a') as o:
            for fn in iter_mr_out_files(mr_out_dir):
                with hdfs.open(fn) as f:
                    data = f.read(block_size)
                    while len(data) > 0:
                        o.write(data)
                        data = f.read(block_size)