pallets/flask · error · TypeError

The view function for {request.endpoint!r} did not return a

Error message

The view function for {request.endpoint!r} did not return a valid response. The function either returned None or ended without a return statement.

What it means

Raised in `Flask.make_response()` (src/flask/app.py:1307) when the view's return value (after tuple unpacking) is None. A WSGI response needs a body, and the most common cause is a view that fell off the end without a `return` statement — Python implicitly returns None — so Flask names the offending endpoint to make the fix obvious.

Source

Thrown at src/flask/app.py:1307

            if len_rv == 3:
                rv, status, headers = rv  # type: ignore[misc]
            # decide if a 2-tuple has status or headers
            elif len_rv == 2:
                if isinstance(rv[1], (Headers, dict, tuple, list)):
                    rv, headers = rv  # pyright: ignore
                else:
                    rv, status = rv  # type: ignore[assignment,misc]
            # other sized tuples are not allowed
            else:
                raise TypeError(
                    "The view function did not return a valid response tuple."
                    " The tuple must have the form (body, status, headers),"
                    " (body, status), or (body, headers)."
                )

        # the body must not be None
        if rv is None:
            raise TypeError(
                f"The view function for {request.endpoint!r} did not"
                " return a valid response. The function either returned"
                " None or ended without a return statement."
            )

        # make sure the body is an instance of the response class
        if not isinstance(rv, self.response_class):
            if isinstance(rv, (str, bytes, bytearray)) or isinstance(rv, cabc.Iterator):
                # let the response class set the status and headers instead of
                # waiting to do it manually, so that the class can handle any
                # special logic
                rv = self.response_class(
                    rv,  # pyright: ignore
                    status=status,
                    headers=headers,  # type: ignore[arg-type]
                )
                status = headers = None
            elif isinstance(rv, (dict, list)):

View on GitHub ↗ (pinned to 6a2f545bfd)

Solutions

1. Add a `return` for every code path in the view named in the error message — search it for branches that end without returning.
2. Check custom decorators wrapping the view: the wrapper must `return func(*args, **kwargs)`.
3. Make sure side-effect calls like `render_template()` or `redirect()` are returned, not just evaluated.
4. If a route legitimately has no body, `return ('', 204)` instead of nothing.

Example fix

# before
@app.route("/submit", methods=["GET", "POST"])
def submit():
    if request.method == "POST":
        return redirect(url_for("done"))
    render_template("form.html")  # not returned!

# after
@app.route("/submit", methods=["GET", "POST"])
def submit():
    if request.method == "POST":
        return redirect(url_for("done"))
    return render_template("form.html")

# lint/test guard: every code path must return
# ruff/pylint flag inconsistent-return-statements (R1710)
def view():
    if cond:
        return render_template('a.html')
    return render_template('b.html')  # no implicit None fall-through

When it happens

Trigger: A view function with a missing `return` (e.g. only a code path for POST returns, GET falls through); `return None` explicitly; `return None, 200` or `(None, 200, headers)` — the None body is caught after tuple unpacking; a decorator or `before_request`-style wrapper that swallows the view's return value.

Common situations: Conditional branches where only one branch returns (`if form.validate(): return redirect(...)` with no else); views that call `render_template(...)` without returning it; custom decorators that forget `return f(*args, **kwargs)`; error handlers that log but return nothing; async views where a wrapper dropped the awaited result.

Related errors

• The view function did not return a valid response tuple. The
• {e} The view function did not return a valid response. The r
• The view function did not return a valid response. The retur
• Working outside of application context. Attempted to use fu
• Working outside of request context. Attempted to use functi

AI-assisted analysis of pallets/flask@6a2f545bfd (2026-07-31). Data as JSON: /data/errors/86949444004012ae.json. Report an issue: GitHub ↗.