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