In the flask:flashing documentation, I encounter the following situation.
from flask import Flask, flash, redirect, render_template, request, url_for
app = Flask(__name__)
app.secret_key = b'_5#y2L"F4Q8znxec]/'
@app.route('/')
def index():
return render_template('index.html')
@app.route('/login', methods=['GET', 'POST'])
def login():
error = None
if request.method == 'POST':
if request.form['username'] != 'admin' or request.form['password'] != 'secret':
error = 'Invalid credentials'
else:
flash('You were successfully logged in')
return redirect(url_for('index'))
return render_template('login.html', error=error)
And here is the Jinja2 used HTML file.
<!doctype html>
<title>My Application</title>
{% with messages = get_flashed_messages() %}
{% if messages %}
<ul class=flashes>
{% for message in messages %}
<li>{{ message }}</li>
{% endfor %}
</ul>
{% endif %}
{% endwith %}
{% block body %}{% endblock %}
So I wonder that even if there is no function passed in to the HTML file by using return statement in Python file, Jinja2 can read get_flashed_messages() function, which is a function under the flask module. How is this possible?
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Answer
Messages to be flashed get appended to session, a global object that jinja2 has access to along with what you return in your route functions.
From the flask github repo flask/src/flask/helpers.py:
from .globals import session
def flash(message: str, category: str = "message") -> None:
...
flashes = session.get("_flashes", [])
flashes.append((category, message))
session["_flashes"] = flashes
...
flask‘s source code is available on github and it is well worth the exploration.