Flask view return error "View function did not return a response"

The following does not return a response:

@app.route('/hello', methods=['GET', 'POST'])
def hello():
    hello_world()

You mean to say...

@app.route('/hello', methods=['GET', 'POST'])
def hello():
    return hello_world()

Note the addition of return in this fixed function.

No matter what code executes in a view function, the view must return a value that Flask recognizes as a response. If the function doesn't return anything, that's equivalent to returning None, which is not a valid response.

In addition to omitting a return statement completely, another common error is to only return a response in some cases. If your view has different behavior based on an if or a try/except, you need to ensure that every branch returns a response.

This incorrect example doesn't return a response on GET requests, it needs a return statement after the if:

@app.route("/hello", methods=["GET", "POST"])
def hello():
    if request.method == "POST":
        return hello_world()

    # missing return statement here

This correct example returns a response on success and failure (and logs the failure for debugging):

@app.route("/hello")
def hello():
    try:
        return database_hello()
    except DatabaseError as e:
        app.logger.exception(e)
        return "Can't say hello."