decibyte decibyte - 16 days ago 12
Python Question

Django Rest Framework: `get_serializer_class` called several times, with wrong value of request method

Using a

ModelViewSet
, is it normal for
get_serializer_class
to be called multiple times for a single request, when accessing the browsable API? And that the value of
self.method.request
changes between each call?

I've created a small test project to show the behaviour. In
project/example/views.py
there's a
ThingViewSet
with a custom
get_serializer_class
, which prints the current request method.

If you start the server and navigate to http://127.0.0.1:8000/things/1/, the output will be something like:

./manage.py runserver
Performing system checks...

System check identified no issues (0 silenced).
May 19, 2015 - 08:51:34
Django version 1.8.1, using settings 'project.settings'
Starting development server at http://127.0.0.1:8000/
Quit the server with CONTROL-C.
Method is: GET
Method is: PUT
Method is: PATCH
Method is: PUT
[19/May/2015 08:51:40]"GET /things/1/ HTTP/1.1" 200 11679


Clearly,
get_serializer_class
is called 4 times, with different values (
GET
,
PUT
,
PATCH
,
PUT
), although only a single
GET
request is performed.

Strange thing is, that this doesn't happen if you request it as JSON:

./manage.py runserver
Performing system checks...

System check identified no issues (0 silenced).
May 19, 2015 - 10:25:57
Django version 1.8.1, using settings 'project.settings'
Starting development server at http://127.0.0.1:8000/
Quit the server with CONTROL-C.
Method is: GET
[19/May/2015 10:26:22]"GET /things/?format=json HTTP/1.1" 200 49


And the problem is that the request method in the last call to
get_serializer_class
of the browsable API is
PUT
(which is obviously wrong for a
GET
request) and then we end up using the wrong serializer for the request, given that different serializers are returned for different request methods, which we do in our real life project (e.g. for read and write operations).

Can anybody shed some light on what is going on? Why is
get_serializer_class
called several times for the browsable API, with wrong method values?

Answer

The reason why you are seeing get_serializer_class being called multiple times is because you are using the browsable API. If you test it out without using the browsable API, for example by forcing the JSON renderer (?format=json or an Accept header), you will only see it called one.

The browsable API generates the forms that are displayed based on the serializer, so get_serializer_class is called once for each form and possible request type.

So while the first request, a GET makes sense for the original serializer that is used to handle the response data (the specific object, in this case), the next three are custom to the browsable API. These are the calls that happen, in the following order, to get_serializer which you are seeing

  1. The raw PUT form (for entering any request body).
  2. The raw PATCH form.
  3. The full PUT form (contains the instance data by default).

The method is being changed with the override_method with function which emulates the request method being overridden, which would normally happen in a POST request that needed a different method.