Add new page in documentation for Flask migration issues

doc/extensions/flask-migration.rst

@@ -0,0 +1,131 @@
+==============================
+Migration from Pylons to Flask
+==============================
+
+On CKAN 2.6, work started to migrate from the Pylons web framework to a more
+modern alternative, `Flask <http://flask.pocoo.org/>`_. This will be a gradual
+process spanning multiple CKAN versions, where both the Pylons app and the
+Flask app will live side by side with their own controllers or blueprints
+which handle the incoming requests. The idea is that any other lower level code,
+like templates, logic actions and authorization are shared between them as much
+as possible. You can learn more about the approach followed and the work
+already done on this page in the CKAN wiki:
+
+https://github.com/ckan/ckan/wiki/Migration-from-Pylons-to-Flask
+
+This page lists changes and deprecations that both core and extensions
+developers should be aware of going forward, as well as common exceptions and
+how to fix them.
+
+-----------------------------------------------------------------------
+Always import methods and objects from the plugins toolkit if available
+-----------------------------------------------------------------------
+
+This is a :ref:`good practice in general <use the plugins toolkit>` when
+writing extensions but in the context of the Flask migration it becomes
+specially important with these methods and objects::
+
+    from ckan.plugins.toolkit import url_for, redirect_to, request, config
+
+    url_for()
+    redirect_to()
+    request
+    config
+
+The reason is that these are actually wrappers provided by CKAN that will proxy
+the call to the relevant Pylons or Flask underlying object or method depending
+on who is handling the request. For instance in the ``config`` case, if you use
+``pylons.config`` directly from your extension changes in configuration will
+only be applied to the Pylons application, and the Flask application will be
+misconfigured.
+
+.. note:: ``conf`` was added to the plugins toolkit on CKAN 2.6. If your
+    extension needs to target CKAN versions lower and greater than CKAN 2.6 you
+    can use `ckantoolkit <https://github.com/ckan/ckantoolkit>`, a separate
+    package that provides wrappers for cross-version CKAN compatibility::
+
+        from ckantoolkit import config
+
+
+------------------------------------------------------------------------------
+Wrap ``url_for`` calls in tests with a test request context (``RuntimeError``)
+------------------------------------------------------------------------------
+
+Starting from CKAN 2.8 you might get the following exception when running the
+tests::
+
+    RuntimeError: Attempted to generate a URL without the application context being
+    pushed. This has to be executed when application context is available.
+
+Users familiar with Flask may recognize this exception. Basically the Flask
+router (called internally by ``ckan.lib.helpers.url_for``) requires you to be
+in the context on a web request when calling it (You can learn more about this
+in the `Flask documentation <http://flask.pocoo.org/docs/testing/>`_). What this
+means is that a test like this will raise the above exception::
+
+    from ckan.plugins.toolkit import url_for    
+    from ckan.tests.helpers import FunctionalTestBase
+
+    class TestSearch(FunctionalTestBase):
+
+        def test_search_page_works(self):
+
+            app = self._get_test_app()
+            url = url_for(controller='package', action='search')              
+            res = app.get(url)
+
+To fix it, we need to wrap the ``url_for`` call with a test request context::
+
+    from ckan.plugins.toolkit import url_for    
+    from ckan.tests.helpers import FunctionalTestBase
+
+    class TestSearch(FunctionalTestBase):
+
+        def test_search_page_works(self):
+
+            app = self._get_test_app()
+            with app.flask_app.test_request_context():
+                url = url_for(controller='package', action='search')              
+            res = app.get(url)
+
+If you are not extending ``FunctionalTestBase`` you can get an instance of the
+test app client in order to create the test request context::
+
+    from ckan.plugins.toolkit import url_for
+    from ckan.tests.helpers import _get_test_app
+
+    class TestSearch(object):
+        def test_search_page_works(self):
+
+            app = _get_test_app()
+            with app.flask_app.test_request_context():
+                url = url_for(controller='package', action='search')              
+            res = app.get(url)
+
+Note that the call to ``url_for`` might not be explicitly done in the test
+itself but rather internally. For instance consider the following example::
+
+
+    from ckan.tests.helpers import FunctionalTestBase
+
+    class TestDatastore(FunctionalTestBase):
+
+        def test_create_datastore_only_view(self):
+            # ...
+            # datastore_create will call ``url_for`` internally (or trigger
+            # something that calls it) so we need a Flask test context
+            with self.app.flask_app.test_request_context():
+                result = helpers.call_action('datastore_create', **data)
+
+Or this one::
+
+    import ckan.lib.dictization.model_dictize as model_dictize
+    from ckan.tests.helpers import _get_test_app
+
+    class TestDictize(object):
+
+        def test_resource_dictize(self):
+            # Internally resource_dictize calls ``url_for`` so we need a test context
+            app = helpers._get_test_app()
+            with app.flask_app.test_request_context():
+                resource_dict = model_dictize.resource_dictize(...)

doc/extensions/index.rst

@@ -39,3 +39,4 @@ features by developing your own CKAN extensions.
    plugins-toolkit
    validators
    translating-extensions
+   flask-migration