Flask flash message cause Session is unavailable

Question

Use exact code from official docs:

@app.route('/', methods=['GET', 'POST'])
def upload_file():
 if request.method == 'POST':
 ...
 if file.filename == '':
 flash('No selected file')
 return redirect(request.url)
 if file and allowed_file(file.filename):
 ..
 flash('File was successfully uploaded')
 return redirect(url_for('uploaded_file', filename=filename))
 return render_template('home.html')

Without flash() everything works ok. But when adding it causes error The session is unavailable because no secret. Below is a trace:

> 127.0.0.1 - - [18/Feb/2019 10:16:21] "POST / HTTP/1.1" 500 -
> 127.0.0.1 - - [18/Feb/2019 10:17:16] "GET / HTTP/1.1" 200 - [2019年02月18日 10:17:23,999] ERROR in app: Exception on / [POST]
> Traceback (most recent call last): File
> "/home/dvperv/PycharmProjects/astros/venv/lib/python3.6/site-packages/flask/app.py",
> line 2292, in wsgi_app
> response = self.full_dispatch_request() File "/home/dvperv/PycharmProjects/astros/venv/lib/python3.6/site-packages/flask/app.py",
> line 1815, in full_dispatch_request
> rv = self.handle_user_exception(e) File "/home/dvperv/PycharmProjects/astros/venv/lib/python3.6/site-packages/flask/app.py",
> line 1718, in handle_user_exception
> reraise(exc_type, exc_value, tb) File "/home/dvperv/PycharmProjects/astros/venv/lib/python3.6/site-packages/flask/_compat.py",
> line 35, in reraise
> raise value File "/home/dvperv/PycharmProjects/astros/venv/lib/python3.6/site-packages/flask/app.py",
> line 1813, in full_dispatch_request
> rv = self.dispatch_request() File "/home/dvperv/PycharmProjects/astros/venv/lib/python3.6/site-packages/flask/app.py",
> line 1799, in dispatch_request
> return self.view_functions[rule.endpoint](**req.view_args) File "/home/dvperv/PycharmProjects/astros/app.py", line 39, in upload_file
> flash('No file part') File "/home/dvperv/PycharmProjects/astros/venv/lib/python3.6/site-packages/flask/helpers.py",
> line 410, in flash
> session['_flashes'] = flashes File "/home/dvperv/PycharmProjects/astros/venv/lib/python3.6/site-packages/werkzeug/local.py",
> line 350, in __setitem__
> self._get_current_object()[key] = value File "/home/dvperv/PycharmProjects/astros/venv/lib/python3.6/site-packages/flask/sessions.py",
> line 101, in _fail
> raise RuntimeError('The session is unavailable because no secret ' RuntimeError: The session is unavailable because no secret key was
> set. Set the secret_key on the application to something unique and
> secret.
> 127.0.0.1 - - [18/Feb/2019 10:17:24] "POST / HTTP/1.1" 500 -

I have read several posts about this error, but they all relates to a Flask-Session extension, I do not use it.

I also tried to add app.secret_key = 'super secret key' in the application, w/o success.

Shall I use Flask Session and make appropriate configuration or there is a way of just using plain Flask flash() which somehow works with sessions itself behind the scene.

App config:

app = Flask(__name__)
UPLOAD_FOLDER = ..
ALLOWED_EXTENSIONS = ..
app.config['UPLOAD_FOLDER'] = UPLOAD_FOLDER
app.config['MAX_CONTENT_LENGTH'] = 16 * 1024 * 1024
if __name__ == '__main__':
app.run()

• user8060120: Possible duplicate of secret key not set in flask session, using the Flask-Session extension
• Dmitry: I am not using this extension, as I said in my question
• Danila Ganchar: @Dmitry Did you try to set secret key using config(not app property)? app.config['SECRET_KEY'] = ...
• Dmitry: No, as I believe, they should work identically. Shall I try?
• Danila Ganchar: @Dmitry yes they should work identically. anyway you need to find why setting do not work. Do you use 1 config? Does the problem repeat locally?

Answer 1

It's being stated very clearly in the stack trace that the secret key is not set. Be sure to set the secret key at the right place, for instance here:

app = Flask(__name__)
app.secret_key = 'asrtarstaursdlarsn'
UPLOAD_FOLDER = ..

Answer 2

The attached trace indicates that you have not added a secret key. May be you added the secret key later but the application was not synced with updated code.

I am sharing a basic example of using plain flash message in Flask.

app.py:

from flask import Flask, render_template, flash
app = Flask(__name__)
app.secret_key = 'super secret'
@app.route('/')
def index():
 flash("Flashing a dummy message")
 return render_template("flash_example.html")
@app.route('/another')
def show_page_without_flash():
 return render_template("flash_example.html")
if __name__ == '__main__':
 app.run(debug=True)

flash_example.html:

<!DOCTYPE html>
<html>
 <head><title>Flash example</title></head>
 <body>
 {% with messages = get_flashed_messages() %}
 {% if messages %}
 <ul class=flashes>
 {% for message in messages %}
 <li>{{ message }}</li>
 {% endfor %}
 </ul>
 {% endif %}
 {% endwith %}
 <p>Dummy paragraph</p>
 </body>
</html>

Route with a flash message(/):

flash message example

Route without a flash message(/another):

enter image description here

Answer 3

app = Flask(__name__)
app.config['SECRET_KEY'] = 'anystringthatyoulike'

add a secret key