Simple BitField in python

bitfield.py

class BitMember(object):
  def __init__(self, size, offset=0):
    self.size = size
    self.offset = offset

  @property
  def mask(self):
    return ~((~0) << (self.offset + self.size)) & ((~0) << self.offset)


class BitField(object):
  def __init__(self):
    self.value = 0
    self._offset = 0

  def __setattr__(self, name, value):
    __dict__ = super(BitField, self).__getattribute__('__dict__')
    obj = __dict__.get(name, None)
    if isinstance(obj, BitMember):
      offset = self.__dict__[name].offset
      mask = self.__dict__[name].mask

      # Clear the current value
      self.value = self.value & ~mask

      # Update with the new value
      self.value |= (value << offset) & mask
    else:
      if isinstance(value, BitMember):
        value = BitMember(value.size, offset=self._offset)
        self._offset += value.size
      super(BitField, self).__setattr__(name, value)

  def __getattribute__(self, name):
    __dict__ = super(BitField, self).__getattribute__('__dict__')
    obj = __dict__.get(name, None)
    if isinstance(obj, BitMember):
      offset = self.__dict__[name].offset
      mask = self.__dict__[name].mask
      return (self.value & mask) >> offset

    return super(BitField, self).__getattribute__(name)

  def describe(self):
    """
    Return a string describing the BitField. For example:

    ```
    class MyVec(BitVector):
      def __init__(self):
        super(MyVec, self).__init__()
        self.x = BitMember(0,3)
        self.y = BitMember(3,1)
        self.z = BitMember(4,10)
    ```

    ```
    print MyVec().describe()
    ```

    ```
    0000000000 0 000
              c b   a

      off size field
      --- ---- -----
    a:   0    3 x
    b:   3    1 y
    c:   4   10 z
    ```
    """

    __dict__ = super(BitField, self).__getattribute__('__dict__')
    fields = []
    for name, obj in __dict__.items():
      if isinstance(obj, BitMember):
        fields.append((obj.offset, name, obj))

    fields = sorted(fields, reverse=True)

    line0 = []
    line1 = []
    legend = []
    keys = 'abcdefghijklmnopqrstuvwxyz'
    for idx, (_, name, obj) in enumerate(fields):
      key = keys[len(fields) - idx - 1]
      line0.append('0' * obj.size)
      line1.append('{}{}'.format(' ' * (obj.size - 1), key))
      legend.append((key, obj, name))

    out = io.StringIO()
    out.write(u' '.join(line0))
    out.write(u'\n')
    out.write(u' '.join(line1))
    out.write(u'\n\n')
    out.write(u'   off size field\n')
    out.write(u'   --- ---- -----\n')
    for key, obj, name in legend[::-1]:
      out.write(u'{:1s}: {:3d} {:4d} {}\n'.format(
          key, obj.offset, obj.size, name))

    return out.getvalue()