BitBuffer class for converting variable-length bit strings to Base64 on the fly

bitbuffer.py

class BitBuffer:
    def __init__(self):
        self.buf = bytearray()
        self.accum = 0
        self.nbits = 0
        self.column = 0

    def Append(self, pattern):
        data, dbits = pattern
        while self.nbits + dbits >= 6:
            # We can emit a complete base64 character to represent a chunk of 6 bits.
            grab = 6 - self.nbits
            mask = (1 << grab) - 1
            self.accum = (self.accum << grab) | (mask & (data >> (dbits - grab)))
            dbits -= grab
            self.buf.append(b'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/'[self.accum])
            self.accum = 0
            self.nbits = 0
            self.column += 1
            if self.column == 80:
                self.buf.append(ord('\n'))
                self.column = 0

        # Transfer any residual data bits into the accumulator.
        if dbits >= 0:
            mask = (1 << dbits) - 1
            self.accum = (self.accum << dbits) | (mask & data)
            self.nbits += dbits

    def Format(self):
        # Flush any remaining bits left in the accumulator.
        if self.nbits > 0:
            self.Append((0, (6 - self.nbits)))

        # Always end on a newline.
        if self.column > 0:
            self.buf.append(ord('\n'))
            self.column = 0

        # Convert the bytes to utf-8 text.
        return self.buf.decode('utf-8')