|
| 1 | +Adding new tags |
| 2 | +=============== |
| 3 | + |
| 4 | +Are there XML elements you want to use that aren't supported by PodGen? If so, |
| 5 | +you should be able to add them in using inheritance. |
| 6 | + |
| 7 | +.. warning:: |
| 8 | + |
| 9 | + This is an advanced topic. |
| 10 | + |
| 11 | +.. note:: |
| 12 | + |
| 13 | + There hasn't been a focus on making it easy to extend PodGen. |
| 14 | + Future versions may provide better support for this. |
| 15 | + |
| 16 | +.. note:: |
| 17 | + |
| 18 | + Feel free to add a feature request to `GitHub Issues`_ if you think PodGen |
| 19 | + should support a certain element out of the box. |
| 20 | + |
| 21 | +.. _GitHub Issues: https://github.com/tobinus/python-podgen/issues |
| 22 | + |
| 23 | + |
| 24 | +Quick How-to |
| 25 | +------------ |
| 26 | + |
| 27 | +#. Create new class that extends :class:`.Podcast`. |
| 28 | +#. Add the new attribute. |
| 29 | +#. Override :meth:`~.Podcast._create_rss`, call ``super()._create_rss()``, |
| 30 | + add the new element to its result and return the new tree. |
| 31 | + |
| 32 | +You can do the same with :class:`.Episode`, if you replace |
| 33 | +:meth:`~.Podcast._create_rss` with :meth:`~Episode.rss_entry` above. |
| 34 | + |
| 35 | +There are plenty of small quirks you have to keep in mind. You are strongly |
| 36 | +encouraged to read the example below. |
| 37 | + |
| 38 | +Using namespaces |
| 39 | +^^^^^^^^^^^^^^^^ |
| 40 | + |
| 41 | +If you'll use RSS elements from another namespace, you must make sure you |
| 42 | +update the :attr:`~.Podcast._nsmap` attribute of :class:`.Podcast` |
| 43 | +(you cannot define new namespaces from an episode!). It is a dictionary with the |
| 44 | +prefix as key and the URI for that namespace as value. To use a namespace, you |
| 45 | +must put the URI inside curly braces, with the tag name following right after |
| 46 | +(outside the braces). For example:: |
| 47 | + |
| 48 | + "{%s}link" % self._nsmap['atom'] # This will render as atom:link |
| 49 | + |
| 50 | +The `lxml API documentation`_ is a pain to read, so just look at the `source code |
| 51 | +for PodGen`_ and the example below. |
| 52 | + |
| 53 | +.. _lxml API documentation: http://lxml.de/api/index.html |
| 54 | +.. _source code for PodGen: https://github.com/tobinus/python-podgen/blob/master/podgen/podcast.py |
| 55 | + |
| 56 | +Example: Adding a ttl element |
| 57 | +----------------------------- |
| 58 | + |
| 59 | +The examples here assume version 3 of Python is used. |
| 60 | + |
| 61 | +``ttl`` is an RSS element and stands for "time to live", and can only be an |
| 62 | +integer which indicates how many minutes the podcatcher can rely on its copy of |
| 63 | +the feed before refreshing (or something like that). There is confusion as to |
| 64 | +what it is supposed to mean (max refresh frequency? min refresh frequency?), |
| 65 | +which is why it is not included in PodGen. If you use it, you should treat it as |
| 66 | +the **recommended** update period (source: `RSS Best Practices`_). |
| 67 | + |
| 68 | +.. _RSS Best Practices: http://www.rssboard.org/rss-profile#element-channel-ttl |
| 69 | + |
| 70 | +Using traditional inheritance |
| 71 | +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ |
| 72 | + |
| 73 | +:: |
| 74 | + |
| 75 | + # The module used to create the XML tree and generate the XML |
| 76 | + from lxml import etree |
| 77 | + |
| 78 | + # The class we will extend |
| 79 | + from podgen import Podcast |
| 80 | + |
| 81 | + |
| 82 | + class PodcastWithTtl(Podcast): |
| 83 | + """This is an extension of Podcast, which supports ttl. |
| 84 | + |
| 85 | + You gain access to ttl by creating a new instance of this class instead |
| 86 | + of Podcast. |
| 87 | + """ |
| 88 | + def __init__(self, *args, **kwargs): |
| 89 | + # Initialize the ttl value |
| 90 | + self.__ttl = None |
| 91 | + |
| 92 | + # Has the user passed in ttl value as a keyword? |
| 93 | + if 'ttl' in kwargs: |
| 94 | + self.ttl = kwargs['ttl'] |
| 95 | + kwargs.pop('ttl') # avoid TypeError from super() |
| 96 | + |
| 97 | + # Call Podcast's constructor |
| 98 | + super().__init__(*args, **kwargs) |
| 99 | + |
| 100 | + # If we were to use another namespace, we would add this here: |
| 101 | + # self._nsmap['prefix'] = "URI" |
| 102 | + |
| 103 | + @property |
| 104 | + def ttl(self): |
| 105 | + """Your suggestion for how many minutes podcatchers should wait |
| 106 | + before refreshing the feed. |
| 107 | + |
| 108 | + ttl stands for "time to live". |
| 109 | + |
| 110 | + :type: :obj:`int` |
| 111 | + :RSS: ttl |
| 112 | + """ |
| 113 | + # By using @property and @ttl.setter, we encapsulate the ttl field |
| 114 | + # so that we can check the value that is assigned to it. |
| 115 | + # If you don't need this, you could just rename self.__ttl to |
| 116 | + # self.ttl and remove those two methods. |
| 117 | + return self.__ttl |
| 118 | + |
| 119 | + @ttl.setter |
| 120 | + def ttl(self, ttl): |
| 121 | + # Try to convert to int |
| 122 | + try: |
| 123 | + ttl_int = int(ttl) |
| 124 | + except ValueError: |
| 125 | + raise TypeError("ttl expects an integer, got %s" % ttl) |
| 126 | + # Is this negative? |
| 127 | + if ttl_int < 0: |
| 128 | + raise ValueError("Negative ttl values aren't accepted, got %s" |
| 129 | + % ttl_int) |
| 130 | + # All checks passed |
| 131 | + self.__ttl = ttl_int |
| 132 | + |
| 133 | + def _create_rss(self): |
| 134 | + # Let Podcast generate the lxml etree (adding the standard elements) |
| 135 | + rss = super()._create_rss() |
| 136 | + # We must get the channel element, since we want to add subelements |
| 137 | + # to it. |
| 138 | + channel = rss.find("channel") |
| 139 | + # Only add the ttl element if it has been populated. |
| 140 | + if self.__ttl is not None: |
| 141 | + # First create our new subelement of channel. |
| 142 | + ttl = etree.SubElement(channel, 'ttl') |
| 143 | + # If we were to use another namespace, we would instead do this: |
| 144 | + # ttl = etree.SubElement(channel, |
| 145 | + # '{%s}ttl' % self._nsmap['prefix']) |
| 146 | + |
| 147 | + # Then, fill it with the ttl value |
| 148 | + ttl.text = str(self.__ttl) |
| 149 | + |
| 150 | + # Return the new etree, now with ttl |
| 151 | + return rss |
| 152 | + |
| 153 | + # How to use the new class (normally, you would put this somewhere else) |
| 154 | + myPodcast = PodcastWithTtl(name="Test", website="http://example.org", |
| 155 | + explicit=False, description="Testing ttl") |
| 156 | + myPodcast.ttl = 90 # or set ttl=90 in the constructor |
| 157 | + print(myPodcast) |
| 158 | + |
| 159 | + |
| 160 | +Using mixins |
| 161 | +^^^^^^^^^^^^ |
| 162 | + |
| 163 | +To use mixins, you cannot make the class with the ``ttl`` functionality inherit |
| 164 | +:class:`.Podcast`. Instead, it must inherit nothing. Other than that, the code |
| 165 | +will be the same, so it doesn't make sense to repeat it here. |
| 166 | + |
| 167 | +:: |
| 168 | + |
| 169 | + class TtlMixin(object): |
| 170 | + # ... |
| 171 | + |
| 172 | + # How to use the new mixin |
| 173 | + class PodcastWithTtl(TtlMixin, Podcast): |
| 174 | + def __init__(*args, **kwargs): |
| 175 | + super().__init__(*args, **kwargs) |
| 176 | + |
| 177 | + myPodcast = PodcastWithTtl(name="Test", website="http://example.org", |
| 178 | + explicit=False, description="Testing ttl") |
| 179 | + myPodcast.ttl = 90 |
| 180 | + print(myPodcast) |
| 181 | + |
| 182 | +Note the order of the mixins in the class declaration. You should read it as |
| 183 | +the path Python takes when looking for a method. First Python checks |
| 184 | +``PodcastWithTtl``, then ``TtlMixin`` and finally :class:`.Podcast`. This is |
| 185 | +also the order the methods are called when chained together using :func:`super`. |
| 186 | +If you had Podcast first, :meth:`.Podcast._create_rss` method would be run |
| 187 | +first, and since it never calls ``super()._create_rss()``, the ``TtlMixin``'s |
| 188 | +``_create_rss`` would never be run. Therefore, you should always have |
| 189 | +:class:`.Podcast` last in that list. |
| 190 | + |
| 191 | +Which approach is best? |
| 192 | +^^^^^^^^^^^^^^^^^^^^^^^ |
| 193 | + |
| 194 | +The advantage of mixins isn't really displayed here, but it will become |
| 195 | +apparent as you add more and more extensions. Say you define 5 different mixins, |
| 196 | +which all add exactly one more element to :class:`.Podcast`. If you used traditional |
| 197 | +inheritance, you would have to make sure each of those 5 subclasses made up a |
| 198 | +tree. That is, class 1 would inherit :class:`.Podcast`. Class 2 would have to inherit |
| 199 | +class 1, class 3 would have to inherit class 2 and so on. If two of the classes |
| 200 | +had the same superclass, you could get screwed. |
| 201 | + |
| 202 | +By using mixins, you can put them together however you want. Perhaps for one |
| 203 | +podcast you only need ``ttl``, while for another podcast you want to use the |
| 204 | +``textInput`` element in addition to ``ttl``, and another podcast requires the |
| 205 | +``textInput`` element together with the ``comments`` element. Using traditional |
| 206 | +inheritance, you would have to duplicate code for ``textInput`` in two classes. Not |
| 207 | +so with mixins:: |
| 208 | + |
| 209 | + class PodcastWithTtl(TtlMixin, Podcast): |
| 210 | + def __init__(*args, **kwargs): |
| 211 | + super().__init__(*args, **kwargs) |
| 212 | + |
| 213 | + class PodcastWithTtlAndTextInput(TtlMixin, TextInputMixin, Podcast): |
| 214 | + def __init__(*args, **kwargs): |
| 215 | + super().__init__(*args, **kwargs) |
| 216 | + |
| 217 | + class PodcastWithTextInputAndComments(TextInputMixin, CommentsMixin, |
| 218 | + Podcast): |
| 219 | + def __init__(*args, **kwargs): |
| 220 | + super().__init__(*args, **kwargs) |
| 221 | + |
| 222 | +If the list of elements you want to use varies between different podcasts, |
| 223 | +mixins are the way to go. On the other hand, mixins are overkill if you are okay |
| 224 | +with one giant class with all the elements you need. |
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