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我们有时候在一个页面存储了一些信息，然后需要在另一个页面使用，这个时候就需要使用flash，其实从应用需求角度来看session也能完成上面的需求，其实flash就是对session的封装，它的底层实现其实也是session。

首先我们看下flash 和 get_flashed_message这两个函数的源码

def flash(message, category="message"):
    """Flashes a message to the next request. In order to remove the
 flashed message from the session and to display it to the user,
 the template has to call :func:`get_flashed_messages`.

 .. versionchanged:: 0.3
 `category` parameter added.

 :param message: the message to be flashed.
 :param category: the category for the message. The following values
 are recommended: ``'message'`` for any kind of message,
 ``'error'`` for errors, ``'info'`` for information
 messages and ``'warning'`` for warnings. However any
 kind of string can be used as category.
 """
    # Original implementation:
    #
    # session.setdefault('_flashes', []).append((category, message))
    #
    # This assumed that changes made to mutable structures in the session are
    # always in sync with the session object, which is not true for session
    # implementations that use external storage for keeping their keys/values.
    flashes = session.get("_flashes", [])
    flashes.append((category, message))
    session["_flashes"] = flashes
    message_flashed.send(
        current_app._get_current_object(), message=message, category=category
    )


def get_flashed_messages(with_categories=False, category_filter=()):
    """Pulls all flashed messages from the session and returns them.
 Further calls in the same request to the function will return
 the same messages. By default just the messages are returned,
 but when `with_categories` is set to ``True``, the return value will
 be a list of tuples in the form ``(category, message)`` instead.

 Filter the flashed messages to one or more categories by providing those
 categories in `category_filter`. This allows rendering categories in
 separate html blocks. The `with_categories` and `category_filter`
 arguments are distinct:

 * `with_categories` controls whether categories are returned with message
 text (``True`` gives a tuple, where ``False`` gives just the message text).
 * `category_filter` filters the messages down to only those matching the
 provided categories.

 See :ref:`message-flashing-pattern` for examples.

 .. versionchanged:: 0.3
 `with_categories` parameter added.

 .. versionchanged:: 0.9
 `category_filter` parameter added.

 :param with_categories: set to ``True`` to also receive categories.
 :param category_filter: whitelist of categories to limit return values
 """
    flashes = _request_ctx_stack.top.flashes
    if flashes is None:
        _request_ctx_stack.top.flashes = flashes = (
            session.pop("_flashes") if "_flashes" in session else []
        )
    if category_filter:
        flashes = list(filter(lambda f: f[0] in category_filter, flashes))
    if not with_categories:
        return [x[1] for x in flashes]
    return flashes

从上面我可以再次认证flash就是利用session实现的。

下面讲解下如何使用它。

1、首先也是先引入它

from flask imoport Flask,flash

2、在需要的地方写入flash

@app.route('/login', methods=['GET', 'POST'])
def login():
    error = None
    if request.method == 'POST':
        if request.form['username'] != app.config['USERNAME']:
            error = 'Invalid username'
        elif request.form['password'] != app.config['PASSWORD']:
            error = 'Invalid password'
        else:
            session['logged_in'] = True
            flash('You were logged in')
            return redirect(url_for('index'))
    return render_template('login.html', error=error)
  

3、在需要读取的地方进行读取

{% for message in get_flashed_messages() %}
    <div class=flash>{{ message }}</div>
 {% endfor %}

说明点：flash和它的名字一样，是闪现，意思就是我们的消息只会显示一次，当我们再次刷新也面的时候，它就不存在了，而正是这点，它经常被用来显示一些提示消息，比如登陆之后，显示欢迎信息等