need reading file in binary mode

[Sergey P. Vazulia]

this part of my code:

        f = file(work_dir + filename,'r')
        n = int(totalSize/recordLenth)
        i = 0
        while i < n:
            buf = f.read(recordLenth);

sometime (when find something like \0A\00\00 in data) returm less bytes then
file have.

Q: how-to read all data from binary file with constant_length portions?