pallets/flask · error · TypeError

Cookies are disabled. Create a client with 'use_cookies=True

Error message

Cookies are disabled. Create a client with 'use_cookies=True'.

What it means

Raised as a TypeError by `FlaskClient.session_transaction()` when the test client was created with cookies disabled (`self._cookies is None`). Session transactions work by reading the session cookie, letting you mutate the session, then writing the resulting Set-Cookie back into the client's cookie jar — none of which is possible without cookie support.

Source

Thrown at src/flask/testing.py:156

    ) -> t.Iterator[SessionMixin]:
        """When used in combination with a ``with`` statement this opens a
        session transaction.  This can be used to modify the session that
        the test client uses.  Once the ``with`` block is left the session is
        stored back.

        ::

            with client.session_transaction() as session:
                session['value'] = 42

        Internally this is implemented by going through a temporary test
        request context and since session handling could depend on
        request variables this function accepts the same arguments as
        :meth:`~flask.Flask.test_request_context` which are directly
        passed through.
        """
        if self._cookies is None:
            raise TypeError(
                "Cookies are disabled. Create a client with 'use_cookies=True'."
            )

        app = self.application
        ctx = app.test_request_context(*args, **kwargs)
        self._add_cookies_to_wsgi(ctx.request.environ)

        with ctx:
            sess = app.session_interface.open_session(app, ctx.request)

        if sess is None:
            raise RuntimeError("Session backend did not open a session.")

        yield sess
        resp = app.response_class()

        if app.session_interface.is_null_session(sess):
            return

View on GitHub ↗ (pinned to 6a2f545bfd)

Solutions

1. Create the client with cookies enabled (the default): `client = app.test_client()` or explicitly `app.test_client(use_cookies=True)`.
2. Use a separate cookie-enabled client fixture for tests that touch the session, keeping the cookie-less one for pure API tests.

Example fix

# before
client = app.test_client(use_cookies=False)
with client.session_transaction() as sess:
    sess["user_id"] = 1
# after
client = app.test_client()
with client.session_transaction() as sess:
    sess["user_id"] = 1

client = app.test_client(use_cookies=True)
# only then use client.session_transaction()

def cookies_enabled(client) -> bool:
    return getattr(client, '_cookies', None) is not None

try:
    with client.session_transaction() as sess:
        sess['user_id'] = 1
except TypeError:
    # client was created with use_cookies=False
    raise

When it happens

Trigger: Calling `with client.session_transaction() as sess:` on a client constructed via `app.test_client(use_cookies=False)` or `FlaskClient(app, use_cookies=False)`.

Common situations: Test suites that disable cookies globally for stateless API testing, then later add a test needing session manipulation; shared pytest fixtures where `use_cookies=False` was set for one test class and reused elsewhere.

Related errors

• Session backend did not open a session.
• The session is unavailable because no secret key was set. S
• Cannot pop this context ({self!r}), it is not pushed.
• Tag '{key}' is already registered.
• Cannot nest client invocations

AI-assisted analysis of pallets/flask@6a2f545bfd (2026-07-31). Data as JSON: /data/errors/081d89dcc4e312ce.json. Report an issue: GitHub ↗.