Adding new tags¶

Are there XML elements you want to use that aren’t supported by pod2gen? If so, you should be able to add them in using inheritance.

Note

There hasn’t been a focus on making it easy to extend pod2gen. Future versions may provide better support for this.

Note

Feel free to add a feature request to GitLab Issues if you think pod2gen should support a certain element out of the box.

Quick How-to¶

Create new class that extends Podcast.
Add the new attribute.
Override 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 Episode, if you replace Podcast._create_rss() with 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:

"{%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 pod2gen and the example below.

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 pod2gen. If you use it, you should treat it as the recommended update period (source: RSS Best Practices).

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 pod2gen import Podcast


class PodcastWithTtl(Podcast):
    """This is an extension of Podcast, which supports ttl.

    You gain access to ttl by creating a new instance of this class instead
    of Podcast.
    """
    def __init__(self, *args, **kwargs):
        # Initialize the ttl value
        self.__ttl = None

        # Call Podcast's constructor (this will set ttl using setattr if
        # given as argument to the constructor, hence why self.__ttl is
        # defined before we do this)
        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
        before refreshing the feed.

        ttl stands for "time to live".

        :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)
if __name__ == '__main__':
    myPodcast = PodcastWithTtl(name="Test", website="http://example.org",
                               explicit=False, description="Testing ttl")
    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.

class TtlMixin(object):
    # ...

# How to use the new mixin
class PodcastWithTtl(TtlMixin, Podcast):
    def __init__(*args, **kwargs):
        super().__init__(*args, **kwargs)

myPodcast = PodcastWithTtl(name="Test", website="http://example.org",
                           explicit=False, description="Testing ttl")
myPodcast.ttl = 90
print(myPodcast)

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 and finally Podcast. This is also the order the methods are called when chained together using super(). If you had Podcast first, 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 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 element to 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 class 1, class 3 would have to inherit class 2 and so on. If two of the classes 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 so with mixins:

class PodcastWithTtl(TtlMixin, Podcast):
    def __init__(*args, **kwargs):
        super().__init__(*args, **kwargs)

class PodcastWithTtlAndTextInput(TtlMixin, TextInputMixin, Podcast):
    def __init__(*args, **kwargs):
        super().__init__(*args, **kwargs)

class PodcastWithTextInputAndComments(TextInputMixin, CommentsMixin,
                                      Podcast):
    def __init__(*args, **kwargs):
        super().__init__(*args, **kwargs)

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 elements you need.