Memoizing IO Stream

memo_stream.py

class MemoizingStream(object):
    """
    Write always to end, read from separate position.
    """

    def __init__(self, fd):
        print 'create memo'
        self.memory = StringIO()
        self.source_fd = fd
        self.finished = False

    def read(self, n=-1):
        buf = self.memory.read(n)

        if not self.finished and (not buf or (n and n > len(buf))):
            # Read from source
            rest_n = n - len(buf) if n and n > 0 else -1
            new_buf = self.source_fd.read(rest_n)
            if not new_buf:
                self.finished = True
            else:
                buf += new_buf
                self.write(new_buf)
                self.memory.seek(len(new_buf), 1)

        return buf

    def write(self, s):
        read_pos = self.memory.tell()
        self.memory.seek(0, 2)
        self.memory.write(s)
        self.memory.seek(read_pos)

    def seek(self, pos, mode=0):
        if mode == 2:
            raise TypeError("Can't seek to the end of MemoizingStream")
        self.memory.seek(pos, mode)

    def tell(self):
        self.memory.tell()