Update chapter-06.rst

docs/chapter-06.rst

@@ -186,7 +186,7 @@ Here is an example of usage:
    @action.uses(T)
    def index(): return str(T('Hello world'))
 
-The string `hello world` will be translated based on the
+The string ``hello world`` will be translated based on the
 internationalization file in the specified “translations” folder that
 best matches the HTTP ``accept-language`` header.
 
@@ -215,7 +215,7 @@ action with a counter that counts “visits”.
        return str(T("You have been here {n} times").format(n=counter))
 
 
-If the `T` fixture is to be used from inside a template you may want to pass it to the template:
+If the ``T`` fixture is to be used from inside a template you may want to pass it to the template:
 
 .. code:: python
 
@@ -311,7 +311,7 @@ you have to redefine the select method for the T instance:
    T.on_request = lambda *_: T.local.__dict__.update(tag="it", language=T.languages["it"])
 
 This is to be done outside any action and will apply to all actions. Action will still need to declare 
-`action.uses(T)` else the behavior is undefined.
+``action.uses(T)`` else the behavior is undefined.
 
 
 
@@ -623,10 +623,10 @@ given condition. For example to enforce a workflow:
        return locals()
 
 Notice that the Condition fixtures takes a function as first argument
-which is called `on_request` and must evaluate to True or False.
+which is called ``on_request`` and must evaluate to True or False.
 
 Also notice that in the above example the Condition depends on the Session
-therefore it must be listed after `session` in `action.uses`.
+therefore it must be listed after ``session`` in ``action.uses``.
 
 If False, by default, the Condition fixture raises 404.
 It is possible to specify a different exception:
@@ -643,7 +643,7 @@ for example, to redirect to another page:
    Condition(cond, on_false=lambda: redirect(URL('step1')))
 
 You can use condition to check permissions. For example, if you
-are giving group memberships to users using `Tags` (it will be explained
+are giving group memberships to users using ``Tags`` (it will be explained
 later on the :ref:`Authorization using Tags` chapter), then you can
 require that users action have specific group membership:
 
@@ -855,13 +855,13 @@ A fixture is an object with the following minimal structure:
        def on_success(self, context): pass
        def on_error(self, context) pass
 
-For example in the DAL fixture case, `on_request` starts a transaction,
-`on_success` commits it, and `on_error` rolls it back.
+For example in the DAL fixture case, ``on_request`` starts a transaction,
+``on_success`` commits it, and ``on_error`` rolls it back.
 
-In the case of a template, `on_request` and `on_error` do nothing but
-`on_success` transforms the output.
+In the case of a template, ``on_request`` and ``on_error`` do nothing but
+``on_success`` transforms the output.
 
-In the case of `auth.user` fixtures, `on_request` does all the work of
+In the case of ``auth.user`` fixtures, ``on_request`` does all the work of
 determining if the user is logged in (from the dependent session fixture)
 and eventually preventing the request from accessing the inner layers.
 
@@ -873,24 +873,24 @@ Under normal circumstances above methods are executed in this order:
    request  -> A.on_request -> B.on_request -> C.on_request -> action
    response <- A.on_success <- B.on_success <- C.on_success <-
 
-i.e. the first fixture (A) is the first one to call `on_request`
-and the last one to call `on_success`. You can think of them as layers of
-an onion with the action (user code) at the center. `on_success` is called
-when entering a layer from the outside and `on_success` is called when
+i.e. the first fixture (A) is the first one to call ``on_request``
+and the last one to call ``on_success``. You can think of them as layers of
+an onion with the action (user code) at the center. ``on_success`` is called
+when entering a layer from the outside and ``on_success`` is called when
 exiting a layer from the inside (like WSGI middleware).
 
 If any point an exception is raised inner layers are not called
-and outer layers will call `on_error` instead of `on_success`.
+and outer layers will call ``on_error`` instead of ``on_success``.
 
 Context is a shared object which contains:
 
 - content['fixtures']: the list of all the fixtures for the action.
-- context['processed']: the list of fixtures that called `on_request` previously within the request.
+- context['processed']: the list of fixtures that called ``on_request`` previously within the request.
 - context['exception']: the exception raised by the action or any previous fixture logic (usually None)
 - context['output']: the action output.
 
-`on_success` and `on_error` can see the current `context['exception']` and
-transform it. They can see the current `context['output']` and transform it as well.
+``on_success`` and ``on_error`` can see the current ``context['exception']`` and
+transform it. They can see the current ``context['output']`` and transform it as well.
 
 For example here is a fixture that transforms the output text to upper case:
 
@@ -906,7 +906,7 @@ For example here is a fixture that transforms the output text to upper case:
    @action.uses(upper_case)
    def index(): return "hello world"
    
-Notice that this fixture assumes the `context['output']` is a string
+Notice that this fixture assumes the ``context['output']`` is a string
 and therefore it must come before the template.
 
 Here is a fixture that logs exceptions tracebacks to a file:
@@ -926,14 +926,14 @@ Here is a fixture that logs exceptions tracebacks to a file:
    @action.uses(errlog)
    def index(): return 1/0
 
-Fixtures also have a `__prerequisite__` attribute. If a fixture
+Fixtures also have a ``__prerequisite__`` attribute. If a fixture
 takes another fixture as an argument, its value must be appended
-to the list of `__prerequisites__`. This guarantees that they are
+to the list of ``__prerequisites__``. This guarantees that they are
 always executed in the proper order even if listed in the wrong order.
-It also makes it optional to declare prerequisite fixtures in `action.uses`.
+It also makes it optional to declare prerequisite fixtures in ``action.uses``.
 
-For example `Auth` depends on `db`, `session`, and `flash`. `db` and `session`
-are indeed arguments. `flash` is a special singleton fixture declared within `Auth`.
+For example ``Auth`` depends on ``db``, ``session``, and ``flash``. ``db`` and ``session``
+are indeed arguments. ``flash`` is a special singleton fixture declared within ``Auth``.
 This means that
 
 .. code:: python
@@ -949,7 +949,7 @@ is equivalent to
 Why are fixtures not simply functions that contain a try/except?
 
 We considered the option but there are some special exceptions that should
-not be considered errors but success (`py4web.HTTP`, `bottle.HTTResponse`)
+not be considered errors but success (``py4web.HTTP``, ``bottle.HTTResponse``)
 while other exceptions are errors. The actual logic can be complicated 
 and individual fixtures do not need to know these details.
 
@@ -961,6 +961,84 @@ Fixtures should not in general communicate with each other but nothing
 prevents one fixture to put data in the context and another fixture to
 retrieve that data.
 
+Fixtures with dependencies
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+If a fixture depends on another fixture, it needs to be passed that fixture in the initializer, 
+and the fixture must be listed in the ``__prerequisites__`` attribute.
+For example, suppose we want to create a fixture that grants access to a controller only
+to users whose email address is included in an ADMIN_EMAILS list. 
+We can write the following fixture:
+
+.. code:: python
+
+    class AdminAccess(Fixture):
+
+        def __init__(self, auth, admin_list, redirect_url=None):
+            super().__init__()
+            self.admin_list = admin_list
+            self.auth = auth
+            self.__prerequisites__ = [auth]
+            # One thing to note here is that the URL function can only be called in a 
+            # request context (while serving a request).  Thus, we cannot store in the fixture
+            # initialization the full URL to redirect, but only the path. 
+            self.redirect_url = redirect_url or 'index'
+            
+        def on_request(self, context):
+            if ((not self.auth.current_user) 
+                or self.auth.current_user.get('email') not in self.admin_list):
+                redirect(URL(self.redirect_url))
+                     
+        def on_error(self, context):
+            redirect(URL(self.redirect_url))
+
+The fixture can be created and used as follows:
+
+.. code:: python
+
+    admin_access = AdminAccess(auth, ['[email protected]',], 'index')
+
+    @action('/admin-only')
+    @action.uses('admin_only.html', admin_access)
+    def admin_only():
+        return dict()
+
+Using local storage
+~~~~~~~~~~~~~~~~~~~
+
+Fixtures can use a thread-local storage for data they need.  
+Here is an example: 
+
+.. code:: python
+
+    class LocalStorageDemo(Fixture):
+        
+       def __init__(self):
+           super().__init__()
+          
+       def on_request(self, context):
+           Fixture.local_initialize(self) 
+           # We can check whether the local storage is valid. 
+           print(f"is_valid: {self.is_valid()}")
+           content = str(uuid.uuid4())
+           print(f"Storing content: {content}")
+           self.local.my_content = content
+               
+       def on_success(self, context):
+           # The line below is used only to show that the thread-local object is in place.         
+           print(f"Retrieved: {self.local.my_content}")
+
+Notably, the initializer should contain the line: 
+
+.. code:: python
+
+    Fixture.local_initialize(self)
+
+in order to initialize the thread-local storage.
+Once this is done, the thread-local storage can be used to store and retrieve data
+using the the ``self.local`` object. 
+
+
 Multiple fixtures
 -----------------
 
@@ -1004,11 +1082,11 @@ is almost equivalent to
 
    @action.uses(A,B)
 
-but not quite. All fixtures declared in one `action.uses` share
-the same context while fixtures in different `action.uses` use
+but not quite. All fixtures declared in one ``action.uses`` share
+the same context while fixtures in different ``action.uses`` use
 different contexts and therefore they cannot communicate with each other.
 This may change in the future.
-For now we recommend using a single call to `action.uses`.
+For now we recommend using a single call to ``action.uses``.
 
 
 Caching and Memoize