AssertionError: View function mapping is overwriti

Question

Does anyone know why I can't overwrite an existing endpoint function if i have two url rules like this

app.add_url_rule('/',
                 view_func=Main.as_view('main'),
                 methods=["GET"])

app.add_url_rule('/<page>/',
                 view_func=Main.as_view('main'),
                 methods=["GET"])

Traceback:

Traceback (most recent call last): 
  File "demo.py", line 20, in <module> methods=["GET"]) 
  File ".../python2.6/site-packages/flask‌​/app.py", 
    line 62, in wrapper_func return f(self, *args, **kwargs) 
  File ".../python2.6/site-packages/flask‌​/app.py", 
    line 984, in add_url_rule 'existing endpoint function: %s' % endpoint)  
AssertionError: View function mapping is overwriting an existing endpoint 
    function: main

Answer 1

There is a fix for Flask issue #570 introduced recenty (flask 0.10) that causes this exception to be raised.

See https://github.com/mitsuhiko/flask/issues/796

So if you go to flask/app.py and comment out the 4 lines 948..951, this may help until the issue is resovled fully in a new version.

The diff of that change is here: http://github.com/mitsuhiko/flask/commit/661ee54bc2bc1ea0763ac9c226f8e14bb0beb5b1

Answer 2

I would just like to add to this a more 'template' type solution.

def func_name(f):
    def wrap(*args, **kwargs):
        if condition:
            pass
        else:
            whatever you want
        return f(*args, **kwargs)
    wrap.__name__ = f.__name__
    return wrap

Answer 3

Flask requires you to associate a single 'view function' with an 'endpoint'. You are calling Main.as_view('main') twice which creates two different functions (exactly the same functionality but different in memory signature). Short story, you should simply do

main_view_func = Main.as_view('main')

app.add_url_rule('/',
             view_func=main_view_func,
             methods=["GET"])

app.add_url_rule('/<page>/',
             view_func=main_view_func,
             methods=["GET"])

Answer 4

This same issue happen to me but with a different usage. When I tried to wrap an API function with 2 decorators:

1. @app.route()
2. My costume @exception_handler decorator

I got this same exception because I tried to wrap more than one function with those two decorators:

@app.route("/path1")
@exception_handler
def func1():
    pass

@app.route("/path2")
@exception_handler
def func2():
    pass

Specifically, it is caused by trying to register a few functions with the name wrapper:

def exception_handler(func):
  def wrapper(*args, **kwargs):
    try:
        return func(*args, **kwargs)
    except Exception as e:
        error_code = getattr(e, "code", 500)
        logger.exception("Service exception: %s", e)
        r = dict_to_json({"message": e.message, "matches": e.message, "error_code": error_code})
        return Response(r, status=error_code, mimetype='application/json')
  return wrapper

Changing the name of the function solved it for me (wrapper.func_name = func.func_name):

def exception_handler(func):
  def wrapper(*args, **kwargs):
    try:
        return func(*args, **kwargs)
    except Exception as e:
        error_code = getattr(e, "code", 500)
        logger.exception("Service exception: %s", e)
        r = dict_to_json({"message": e.message, "matches": e.message, "error_code": error_code})
        return Response(r, status=error_code, mimetype='application/json')
  # Renaming the function name:
  wrapper.func_name = func.func_name
  return wrapper

Then, decorating more than one endpoint worked.

Answer 5

Your view names need to be unique even if they are pointing to the same view method.

app.add_url_rule('/',
                 view_func=Main.as_view('main'),
                 methods = ['GET'])

app.add_url_rule('/<page>/',
                 view_func=Main.as_view('page'),
                 methods = ['GET'])

Answer 6

If you think you have unique endpoint names and still this error is given then probably you are facing issue. Same was the case with me.

This issue is with flask 0.10 in case you have same version then do following to get rid of this:

sudo pip uninstall flask
sudo pip install flask=0.9