'The ``_BaseDiff`` class does not implement a ``_diff`` method and
should not be instantiated directly. Instead instantiate the
appropriate subclass of ``_BaseDiff`` for the objects being compared
(for example, use `HeaderDiff` to compare two `Header` objects.'

def __init__(self, a, b):

    self.a = a
    self.b = b
    self._fileobj = None
    self._indent = 0
    self._diff()
