一个 flask 问题

打算看 flask 的源码，然后看到这个
class SessionMixin(object):
"""Expands a basic dictionary with an accessors that are expected
by Flask extensions and users for the session.
"""

def _get_permanent(self):
    return self.get('_permanent', False)

def _set_permanent(self, value):
    self['_permanent'] = bool(value)

#: this reflects the ``'_permanent'`` key in the dict.
permanent = property(_get_permanent, _set_permanent)
del _get_permanent, _set_permanent

为什么我创建了一个 SessionMixin 类之后， permanet 打印不出来，报这个错误
AttributeError: 'SessionMixin' object has no attribute 'get'