[Python] Why does my Flask login link occasionally redirect to / instead of /auth/login?

I'm working on a Flask application that uses blueprints for modular route organization. I have a main blueprint that serves the index page, and an auth blueprint for authentication routes, including a /auth/login route. The link to the login page is set up in a Jinja2 template like this:

<a class="nav-link active" href="{{ url_for('auth.login') }}">Login</a>


Expected Behavior: When I click the "Login" link, it should reliably take me to /auth/login.

Issue: Occasionally, instead of redirecting to /auth/login, it takes me to / (root) or appends /? to the URL. The issue persists across different browsers and even in incognito mode, suggesting it’s not a caching issue.

Blueprint Setup in create_app: Here’s how I register the blueprints:

app.register_blueprint(main)
app.register_blueprint(auth, url_prefix='/auth')


Confirmed url_for Output: Printing url_for('auth.login') in my code always outputs the correct URL (/auth/login), so the path generation appears to be correct.

Request Logging with @app.before_request: I added a log_request_info function to print out the request path and method:

@app.before_request
def log_request_info():
print(f"Path: {request.path}, Method: {request.method}")


Sometimes, this logs the path as /, while other times it correctly logs /auth/login.

Checked for Redirect Loops: I reviewed my auth.login view to ensure there’s no conditional redirect to /.

Cleared Browser Cache: Tried clearing the cache in case of stale redirects, but the issue persists even in incognito mode.

Question: What might cause url_for('auth.login') to sometimes resolve to / instead of /auth/login? Could there be something in my setup or request handling that would redirect to / under certain conditions?

Edit 1: Here is my login route

auth: Blueprint = Blueprint('auth', __name__, template_folder='templates')


@auth.route('/login', methods=['GET', 'POST'])
def login() -> werResponse | str:
try:
form: LoginForm = LoginForm()
if request.method == 'POST' and form.validate_on_submit():
user: User = db.session.query(User).filter(User.nick == form.nick.data).first() # type: ignore[attr-defined]
if user and check_password_hash(user.password, form.password.data):
login_user(user)
return redirect(url_for('main.index'))
else:
flash('Wrong nick or password!')
except TemplateNotFound:
flash(f'Error{form.errors}')
finally:
return render_template('login.html', login=True, form=form)

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