Explain the example better, small improvements to extending.rst

Author: tobinus

doc/extending.rst

@@ -1,8 +1,12 @@
 Adding new tags
 ===============
 
-Are there XML tags you want to use that aren't supported by PodGen? If so, you
-should be able to add them in using inheritance.
+Are there XML elements you want to use that aren't supported by PodGen? If so,
+you should be able to add them in using inheritance.
+
+.. warning::
+
+   This is an advanced topic.
 
 .. note::
 
@@ -11,47 +15,67 @@ should be able to add them in using inheritance.
 
 .. note::
 
-   Feel free to add a feature request to GitHub Issues if you think PodGen
-   should support a certain tag out of the box.
+   Feel free to add a feature request to `GitHub Issues`_ if you think PodGen
+   should support a certain element out of the box.
+
+.. _GitHub Issues: https://github.com/tobinus/python-podgen/issues
 
 
 Quick How-to
 ------------
 
-#. Create new class that extends Podcast.
+#. Create new class that extends :class:`.Podcast`.
 #. Add the new attribute.
-#. Override :meth:`.Podcast._create_rss`, call super()._create_rss(),
-   add the new tag to its result and return the new tree.
+#. Override :meth:`~.Podcast._create_rss`, call ``super()._create_rss()``,
+   add the new element to its result and return the new tree.
+
+You can do the same with :class:`.Episode`, if you replace
+:meth:`~.Podcast._create_rss` with :meth:`~Episode.rss_entry` above.
+
+There are plenty of small quirks you have to keep in mind. You are strongly
+encouraged to read the example below.
+
+Using namespaces
+^^^^^^^^^^^^^^^^
 
 If you'll use RSS elements from another namespace, you must make sure you
-update the _nsmap attribute of Podcast (you cannot define new namespaces from
-an episode!). It is a dictionary with the prefix as key and the
-URI for that namespace as value. To use a namespace, you must put the URI inside
-curly braces, with the tag name following right after (outside the braces).
-For example::
+update the :attr:`~.Podcast._nsmap` attribute of :class:`.Podcast`
+(you cannot define new namespaces from an episode!). It is a dictionary with the
+prefix as key and the URI for that namespace as value. To use a namespace, you
+must put the URI inside curly braces, with the tag name following right after
+(outside the braces). For example::
 
     "{%s}link" % self._nsmap['atom']  # This will render as atom:link
 
-The `lxml API documentation`_ is a pain to read, so just look at the source code
-for PodGen to figure out how to do things. The example below may help, too.
+The `lxml API documentation`_ is a pain to read, so just look at the `source code
+for PodGen`_ and the example below.
 
 .. _lxml API documentation: http://lxml.de/api/index.html
+.. _source code for PodGen: https://github.com/tobinus/python-podgen/blob/master/podgen/podcast.py
 
-You can do the same with Episode, if you replace _create_rss() with
-rss_entry() above.
-
-Example: Adding a ttl field
----------------------------
+Example: Adding a ttl element
+-----------------------------
 
 The examples here assume version 3 of Python is used.
 
+``ttl`` is an RSS element and stands for "time to live", and can only be an
+integer which indicates how many minutes the podcatcher can rely on its copy of
+the feed before refreshing (or something like that). There is confusion as to
+what it is supposed to mean (max refresh frequency? min refresh frequency?),
+which is why it is not included in PodGen. If you use it, you should treat it as
+the **recommended** update period (source: `RSS Best Practices`_).
+
+.. _RSS Best Practices: http://www.rssboard.org/rss-profile#element-channel-ttl
+
 Using traditional inheritance
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 ::
 
+    # The module used to create the XML tree and generate the XML
     from lxml import etree
 
+    # The class we will extend
     from podgen import Podcast
 
 
@@ -73,6 +97,9 @@ Using traditional inheritance
             # Call Podcast's constructor
             super().__init__(*args, **kwargs)
 
+            # If we were to use another namespace, we would add this here:
+            # self._nsmap['prefix'] = "URI"
+
         @property
         def ttl(self):
             """Your suggestion for how many minutes podcatchers should wait
@@ -83,42 +110,59 @@ Using traditional inheritance
             :type: :obj:`int`
             :RSS: ttl
             """
+            # By using @property and @ttl.setter, we encapsulate the ttl field
+            # so that we can check the value that is assigned to it.
+            # If you don't need this, you could just rename self.__ttl to
+            # self.ttl and remove those two methods.
             return self.__ttl
 
         @ttl.setter
         def ttl(self, ttl):
+            # Try to convert to int
             try:
                 ttl_int = int(ttl)
             except ValueError:
                 raise TypeError("ttl expects an integer, got %s" % ttl)
-
+            # Is this negative?
             if ttl_int < 0:
                 raise ValueError("Negative ttl values aren't accepted, got %s"
                                  % ttl_int)
+            # All checks passed
             self.__ttl = ttl_int
 
         def _create_rss(self):
+            # Let Podcast generate the lxml etree (adding the standard elements)
             rss = super()._create_rss()
+            # We must get the channel element, since we want to add subelements
+            # to it.
             channel = rss.find("channel")
+            # Only add the ttl element if it has been populated.
             if self.__ttl is not None:
+                # First create our new subelement of channel.
                 ttl = etree.SubElement(channel, 'ttl')
+                # If we were to use another namespace, we would instead do this:
+                # ttl = etree.SubElement(channel,
+                #                        '{%s}ttl' % self._nsmap['prefix'])
+
+                # Then, fill it with the ttl value
                 ttl.text = str(self.__ttl)
 
+            # Return the new etree, now with ttl
             return rss
 
     # How to use the new class (normally, you would put this somewhere else)
     myPodcast = PodcastWithTtl(name="Test", website="http://example.org",
                                explicit=False, description="Testing ttl")
-    myPodcast.ttl = 90
+    myPodcast.ttl = 90  # or set ttl=90 in the constructor
     print(myPodcast)
 
 
 Using mixins
 ^^^^^^^^^^^^
 
-To use mixins, you cannot make the class with the ttl functionality inherit
-Podcast. Instead, it must inherit nothing. Other than that, the code will be
-the same, so it doesn't make sense to repeat it here.
+To use mixins, you cannot make the class with the ``ttl`` functionality inherit
+:class:`.Podcast`. Instead, it must inherit nothing. Other than that, the code
+will be the same, so it doesn't make sense to repeat it here.
 
 ::
 
@@ -137,28 +181,29 @@ the same, so it doesn't make sense to repeat it here.
 
 Note the order of the mixins in the class declaration. You should read it as
 the path Python takes when looking for a method. First Python checks
-PodcastWithTtl, then TtlMixin, finally Podcast. This is also the order the
-methods are called when chained together using super(). If you had Podcast
-first, Podcast's _create_rss() method would be run first, and since it never
-calls super()._create_rss(), the TtlMixin's _create_rss would never be run.
-Therefore, you should always have Podcast last in that list.
+``PodcastWithTtl``, then ``TtlMixin`` and finally :class:`.Podcast`. This is
+also the order the methods are called when chained together using :func:`super`.
+If you had Podcast first, :meth:`.Podcast._create_rss` method would be run
+first, and since it never calls ``super()._create_rss()``, the ``TtlMixin``'s
+``_create_rss`` would never be run. Therefore, you should always have
+:class:`.Podcast` last in that list.
 
 Which approach is best?
 ^^^^^^^^^^^^^^^^^^^^^^^
 
 The advantage of mixins isn't really displayed here, but it will become
 apparent as you add more and more extensions. Say you define 5 different mixins,
-which all add exactly one more attribute to Podcast. If you used traditional
+which all add exactly one more element to :class:`.Podcast`. If you used traditional
 inheritance, you would have to make sure each of those 5 subclasses made up a
-tree. That is, class 1 would inherit Podcast. Class 2 would have to inherit
+tree. That is, class 1 would inherit :class:`.Podcast`. Class 2 would have to inherit
 class 1, class 3 would have to inherit class 2 and so on. If two of the classes
-had the same superclass, you would be screwed.
+had the same superclass, you could get screwed.
 
 By using mixins, you can put them together however you want. Perhaps for one
-podcast you only need ttl, while for another podcast you want to use the
-textInput element in addition to ttl, and another podcast requires the
-textInput element together with the comments element. Using traditional
-inheritance, you would have to duplicate code for textInput in two classes. Not
+podcast you only need ``ttl``, while for another podcast you want to use the
+``textInput`` element in addition to ``ttl``, and another podcast requires the
+``textInput`` element together with the ``comments`` element. Using traditional
+inheritance, you would have to duplicate code for ``textInput`` in two classes. Not
 so with mixins::
 
     class PodcastWithTtl(TtlMixin, Podcast):
@@ -174,6 +219,6 @@ so with mixins::
         def __init__(*args, **kwargs):
             super().__init__(*args, **kwargs)
 
-If the list of attributes you want to use varies between different podcasts,
+If the list of elements you want to use varies between different podcasts,
 mixins are the way to go. On the other hand, mixins are overkill if you are okay
-with one giant class with all the attributes you need.
+with one giant class with all the elements you need.