Why does my Flask app redirect one route even though I haven't configured it to?

Question

I'm trying to write a POST route in Flask, but everytime I test the route it redirects with a 301 code the same url but as a GET request. I don't want it to, because I need the POST body.

I don't see why Flask does this however.

What's further confusing is that in the same flask application there is another route that does not redirect.

I have also tried only setting "POST" as an allowed method, but then the route is still redirected, and the response is a 405 method not allowed, which is logical because I have set that GET is not an allowed method. But why does it redirect at all and to itself no less?

Access logs:

<redacted ip> - - [14/Feb/2019:11:32:39 +0000] "POST /service HTTP/1.1" 301 194 "-" "python-requests/2.21.0"
<redacted ip> - - [14/Feb/2019:11:32:43 +0000] "GET /service HTTP/1.1" 200 8 "-" "python-requests/2.21.0"

App code, please note that two routes that respectively do not and do redirect:

def create_app(test_config=None):
    # create and configure the app
    app = Flask(__name__, instance_relative_config=True)

    if test_config is None:
        # load the instance config, if it exists, when not testing
        app.config.from_pyfile('config.py', silent=True)
    else:
        # load the test config if passed in
        app.config.from_mapping(test_config)

    # ensure the instance folder exists
    try:
        os.makedirs(app.instance_path)
    except OSError:
        pass

    @app.route('/path/does/not/redirect')
    def notredirected():
        url_for('static', filename='style.css')
        return render_template('template.html')

    @app.route('/service', strict_slashes=True, methods=["GET", "POST"])
    def service():
        return "Arrived!"

    return app

app = create_app()

My expected result would be that it would be that POST /service would return 200 and Arrived! as its output.

EDIT: If I change the route signature to: @app.route('/service', strict_slashes=True, methods=["POST"]) , it still redirects to a GET request and then returns a 405, because there is no GET route for /service.

<redacted> - - [14/Feb/2019:13:05:27 +0000] "POST /service HTTP/1.1" 301 194 "-" "python-requests/2.21.0"
<redacted> - - [14/Feb/2019:13:05:27 +0000] "GET /service HTTP/1.1" 405 178 "-" "python-requests/2.21.0"

Answer 1

Your service route has been configured to handle both GET and POST requests , but your service route doesn't distinguish between incoming GET and POST requests . By default if you don't specify the supported methods on your route, flask will default to supporting a GET request . However, with no check on the incoming request your service route doesn't know how to handle the incoming request, since both GET and POST are supported, so it attempts a redirect in order to process the request . A simple conditional like the following: if flask.request.method == 'POST': can be used to distinguish between the two types of requests. With that being said, maybe you could try out something like the following:

@app.route('/service', methods=['GET', 'POST']) 
def service():
    if request.method == "GET":
        msg = "GET Request from service route"
        return jsonify({"msg":msg})
    else: # Handle POST Request
        # get JSON payload from POST request
        req_data = request.get_json()

        # Handle data as appropriate

        msg = "POST Request from service route handled"
        return jsonify({"msg": msg})

Answer 2

The problem was that the server redirected all non-https requests to the https variant, which I was unaware of. Since this is not problematic behaviour for us, the solution was specifying in the clients to use https.