Calling `flash` after `get_flashed_messages` fails

[Andreas Klöckner]

In a single request, do this:

• call get_flashed_messages()
• call flash(...)
• call get_flashed_messages() again, observe that it returns the empty list, despite a message just having been flashed.

After the first call to get_flashed_messages(), request_ctx.flashes is [], which appears to send this code in get_flashed_messages off the rails:

flask/src/flask/helpers.py

Lines 364 to 367 in 98a7f9f

 flashes = request_ctx.flashes 
 if flashes is None: 
     flashes = session.pop("_flashes") if "_flashes" in session else [] 
     request_ctx.flashes = flashes 

Environment:

• Python version: 3.11
• Flask version: 3.0.2

[David Lord]

What does "off the rails" mean? Did an error happen? Be sure to provide a minimal reproducible example and the full traceback when reporting an issue.

[Andreas Klöckner]

Sure, fair:

from flask import Flask, flash, get_flashed_messages

app = Flask(__name__)
app.secret_key = "abcdef"

@app.route("/")
def hello_world():
    get_flashed_messages()
    flash("hi")
    assert get_flashed_messages()

Run with

flask --app=flask_repro run 

go to http://127.0.0.1:5000/ and see

Traceback (most recent call last):
  File "/home/andreas/src/env-3.12/lib/python3.12/site-packages/flask/app.py", line 1455, in wsgi_app
    response = self.full_dispatch_request()
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/andreas/src/env-3.12/lib/python3.12/site-packages/flask/app.py", line 869, in full_dispatch_request
    rv = self.handle_user_exception(e)
         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/andreas/src/env-3.12/lib/python3.12/site-packages/flask/app.py", line 867, in full_dispatch_request
    rv = self.dispatch_request()
         ^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/andreas/src/env-3.12/lib/python3.12/site-packages/flask/app.py", line 852, in dispatch_request
    return self.ensure_sync(self.view_functions[rule.endpoint])(**view_args)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/andreas/tmp/flask_repro.py", line 10, in hello_world
    assert get_flashed_messages()
AssertionError

What does "off the rails" mean?

"Off the rails" means that flashes is not None there, and so the code does not look in the session (assuming that's what it's supposed to do).

[David Lord]

This is documented: https://flask.palletsprojects.com/en/3.0.x/api/#flask.get_flashed_messages

Further calls in the same request to the function will return the same messages.

The intended use is documented as well: https://flask.palletsprojects.com/en/3.0.x/patterns/flashing/

The flashing system basically makes it possible to record a message at the end of a request and access it next request and only next request.

You're intended to put messages into one request, then get them out in the next request. Getting them out, putting them in, and getting them out again in the same request is not the intended use case.