flask内置session原理

def wsgi_app(self, environ, start_response):
        # 获取请求相关数据，并进行封装和加工
        ctx = self.request_context(environ)
        # 将请求消息推送到堆栈中，并执行 open_session方法
        ctx.push()
        error = None
        try:
            try:
                response = self.full_dispatch_request()
            except Exception as e:
                error = e
                response = self.make_response(self.handle_exception(e))
            return response(environ, start_response)
        finally:
            if self.should_ignore_error(error):
                error = None
            ctx.auto_pop(error)

    def __call__(self, environ, start_response):
        """Shortcut for :attr:`wsgi_app`."""
        return self.wsgi_app(environ, start_response)

 def push(self):
        top = _request_ctx_stack.top
        if top is not None and top.preserved:
            top.pop(top._preserved_exc)

        # Before we push the request context we have to ensure that there
        # is an application context.
        app_ctx = _app_ctx_stack.top
        if app_ctx is None or app_ctx.app != self.app:
            app_ctx = self.app.app_context()
            app_ctx.push()
            self._implicit_app_ctx_stack.append(app_ctx)
        else:
            self._implicit_app_ctx_stack.append(None)

        if hasattr(sys, ‘exc_clear‘):
            sys.exc_clear()

        _request_ctx_stack.push(self)

        # 调用Flask对象的open_session方法
        self.session = self.app.open_session(self.request)
        if self.session is None:
            self.session = self.app.make_null_session()

 def open_session(self, request):
        """Creates or opens a new session.  Default implementation stores all
        session data in a signed cookie.  This requires that the
        :attr:`secret_key` is set.  Instead of overriding this method
        we recommend replacing the :class:`session_interface`.

        :param request: an instance of :attr:`request_class`.
        """
        # self指的是Flask对象，session_interface默认值为SecureCookieSessionInterface()
        return self.session_interface.open_session(self, request)

class SecureCookieSessionInterface(SessionInterface):
    """The default session interface that stores sessions in signed cookies
    through the :mod:`itsdangerous` module.
    """
    #: the salt that should be applied on top of the secret key for the
    #: signing of cookie based sessions.
    salt = ‘cookie-session‘
    #: the hash function to use for the signature.  The default is sha1
    digest_method = staticmethod(hashlib.sha1)
    #: the name of the itsdangerous supported key derivation.  The default
    #: is hmac.
    key_derivation = ‘hmac‘
    #: A python serializer for the payload.  The default is a compact
    #: JSON derived serializer with support for some extra Python types
    #: such as datetime objects or tuples.
    serializer = session_json_serializer
    session_class = SecureCookieSession

    def get_signing_serializer(self, app):
        if not app.secret_key:
            return None
        signer_kwargs = dict(
            key_derivation=self.key_derivation,
            digest_method=self.digest_method
        )
        return URLSafeTimedSerializer(app.secret_key, salt=self.salt,
                                      serializer=self.serializer,
                                      signer_kwargs=signer_kwargs)

    def open_session(self, app, request):
        # 获取加密相关的类，必须设置app.secret_key，不然s就是None
        s = self.get_signing_serializer(app)

        if s is None:
            return None
        # 去Cookie中获取 session 对应的值（该值默认是加密之后的session的值，也可以改造成随机字符串）
        val = request.cookies.get(app.session_cookie_name)
        if not val:
            # 未获取到值，则创建一个空字典（就是flask中用到的session）
            return self.session_class()
        max_age = total_seconds(app.permanent_session_lifetime)
        try:
            data = s.loads(val, max_age=max_age)
            # 如果获取到值，则将值放入字典中（就是flask中用到的session）
            return self.session_class(data)
        except BadSignature:
            # 解密失败，则创建一个空字典（就是flask中用到的session）
            return self.session_class()

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

    #: some session backends can tell you if a session is new, but that is
    #: not necessarily guaranteed.  Use with caution.  The default mixin
    #: implementation just hardcodes ``False`` in.
    new = False

    #: for some backends this will always be ``True``, but some backends will
    #: default this to false and detect changes in the dictionary for as
    #: long as changes do not happen on mutable structures in the session.
    #: The default mixin implementation just hardcodes ``True`` in.
    modified = True

class UpdateDictMixin(object):

    """Makes dicts call `self.on_update` on modifications.

    .. versionadded:: 0.5

    :private:
    """

    on_update = None

    def calls_update(name):
        def oncall(self, *args, **kw):
            rv = getattr(super(UpdateDictMixin, self), name)(*args, **kw)
            if self.on_update is not None:
                self.on_update(self)
            return rv
        oncall.__name__ = name
        return oncall

    def setdefault(self, key, default=None):
        modified = key not in self
        rv = super(UpdateDictMixin, self).setdefault(key, default)
        if modified and self.on_update is not None:
            self.on_update(self)
        return rv

    def pop(self, key, default=_missing):
        modified = key in self
        if default is _missing:
            rv = super(UpdateDictMixin, self).pop(key)
        else:
            rv = super(UpdateDictMixin, self).pop(key, default)
        if modified and self.on_update is not None:
            self.on_update(self)
        return rv

    __setitem__ = calls_update(‘__setitem__‘)
    __delitem__ = calls_update(‘__delitem__‘)
    clear = calls_update(‘clear‘)
    popitem = calls_update(‘popitem‘)
    update = calls_update(‘update‘)
    del calls_update


class CallbackDict(UpdateDictMixin, dict):

    """A dict that calls a function passed every time something is changed.
    The function is passed the dict instance.
    """

    def __init__(self, initial=None, on_update=None):
        dict.__init__(self, initial or ())
        self.on_update = on_update

    def __repr__(self):
        return ‘<%s %s>‘ % (
            self.__class__.__name__,
            dict.__repr__(self)
        )


class SecureCookieSession(CallbackDict, SessionMixin):
    """Base class for sessions based on signed cookies."""

    def __init__(self, initial=None):
        def on_update(self):
            self.modified = True
        CallbackDict.__init__(self, initial, on_update)
        self.modified = False

class MyDict(dict):
    def __init__(self, initial):
        dict.__init__(self, initial)


session = MyDict({‘k1‘: 123})

print(session, type(session)) # {‘k1‘: 123} <class ‘__main__.MyDict‘>


session[‘k2‘] = ‘v2‘
print(session)

class SecureCookieSessionInterface(SessionInterface):
    """The default session interface that stores sessions in signed cookies
    through the :mod:`itsdangerous` module.
    """
    #: the salt that should be applied on top of the secret key for the
    #: signing of cookie based sessions.
    salt = ‘cookie-session‘
    #: the hash function to use for the signature.  The default is sha1
    digest_method = staticmethod(hashlib.sha1)
    #: the name of the itsdangerous supported key derivation.  The default
    #: is hmac.
    key_derivation = ‘hmac‘
    #: A python serializer for the payload.  The default is a compact
    #: JSON derived serializer with support for some extra Python types
    #: such as datetime objects or tuples.
    serializer = session_json_serializer
    session_class = SecureCookieSession

    def get_signing_serializer(self, app):
        if not app.secret_key:
            return None
        signer_kwargs = dict(
            key_derivation=self.key_derivation,
            digest_method=self.digest_method
        )
        return URLSafeTimedSerializer(app.secret_key, salt=self.salt,
                                      serializer=self.serializer,
                                      signer_kwargs=signer_kwargs)

    def open_session(self, app, request):
        s = self.get_signing_serializer(app)
        if s is None:
            return None
        val = request.cookies.get(app.session_cookie_name)

        if not val:
            return self.session_class()
        max_age = total_seconds(app.permanent_session_lifetime)
        try:
            data = s.loads(val, max_age=max_age)
            return self.session_class(data)
        except BadSignature:
            return self.session_class()

    def save_session(self, app, session, response):
        domain = self.get_cookie_domain(app)
        path = self.get_cookie_path(app)

        # Delete case.  If there is no session we bail early.
        # If the session was modified to be empty we remove the
        # whole cookie.
        if not session:
            if session.modified:
                response.delete_cookie(app.session_cookie_name,
                                       domain=domain, path=path)
            return

        # Modification case.  There are upsides and downsides to
        # emitting a set-cookie header each request.  The behavior
        # is controlled by the :meth:`should_set_cookie` method
        # which performs a quick check to figure out if the cookie
        # should be set or not.  This is controlled by the
        # SESSION_REFRESH_EACH_REQUEST config flag as well as
        # the permanent flag on the session itself.
        if not self.should_set_cookie(app, session):
            return

        httponly = self.get_cookie_httponly(app)
        secure = self.get_cookie_secure(app)
        expires = self.get_expiration_time(app, session)
        val = self.get_signing_serializer(app).dumps(dict(session))
        response.set_cookie(app.session_cookie_name, val,
                            expires=expires, httponly=httponly,
                            domain=domain, path=path, secure=secure)