num_reads = 7
with open('data.txt') as read_file:
    with open('new_data.txt', 'w') as write_file:

        while (True):
            lines = []
            try:       # expect errors if the number of lines in the file are not a multiplication of num_reads
                for i in range(num_reads):
                    lines.append(next(read_file))  # when the file finishes an exception occurs here

                #do sutff with the lines (exactly num_reads number of lines)
                processed = " ".join(list(map(lambda x: x.replace("n", ''), lines)))
                write_file.write(processed + 'n')

            except StopIteration:     # here we process the (possibly) insufficent last lines
                #do stuff with the lines (less that  num_reads)
                processed = " ".join(list(map(lambda x: x.replace("n", ''), lines)))
                write_file.write(processed + 'n')
                break

line1
line2
line3
line4
line5
line7
line8
line9

line1 line2 line3 line4 line5 line7
line8 line9

while (....) {
        foo(7);
    }
    foo(3);

def read_n_lines(infile, n):
        lines = []
        try:
            for _ in range(n):
                lines.append(next(infile))
        except StopIteration:
            pass
        return lines

while True:
        lines = read_n_lines(infile, 7)
        if len(lines) == 0:
            break
        process_lines(lines)