How It Works

There are three main sections to Breathe: parser, finders and renderers. Briefly:

parser
Responsible for reading the doxygen xml output and creating objects representing the data. Found in breathe.parser.
finders
Responsible for finding reference objects within the output from the parser. Found in breathe.finder.
renderers
Responsible for producing reStructuredText nodes to represent the objects that the finders have found. The renderers generally descend through the object hierarchies rendering the objects, their children, their children’s children and so on. Found in breathe.renderer.

Parser

The parsers job is to parse the doxygen xml output and create a hierarchy of Python objects to represent the xml data.

Doxygen XML Output

The xml output from doxygen comes in multiple files. There is always an index.xml file which is a central reference point and contains a list of all the other files that have been generated by doxygen and an indication of what they contain.

For example, in examples/doxygen/func/xml directory, the index.xml file contains:

<?xml version='1.0' encoding='UTF-8' standalone='no'?>
<doxygenindex xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="index.xsd" version="1.7.2">
  <compound refid="class_test" kind="class"><name>Test</name>
    <member refid="class_test_1a45b6a232a4499f8ce37062eab5451570" kind="function"><name>member</name></member>
  </compound>
  <compound refid="func_8h" kind="file"><name>func.h</name>
  </compound>
</doxygenindex>

This suggests there is additional information about a class called Test which has a function called member. Additionally there is some more information about a file called func.h.

Now, the refid attribute on the compound xml nodes gives an indication of where the additional information can be found. So for the Test class, we should look in class_test.xml, which we get by simply appending .xml to the refid value, and for the func.h file we should look in func_8h.xml.

So the index.xml file is unique in its role and has its own structure which is defined in the index.xsd file which you will also find in the same directory. All the other files, the ones referenced by the index.xml file, follow another structure. This is described in compound.xsd file so we call these other files compound files. These are generally longer than the index.xml file and contain all the specific information you might expect from doxygen, including any documentation you added to your code as doxygen markup.

Have a look at examples/doxygen/func/xml/class_test.xml for a fairly short example.

Doing the Parsing

To get things up and running quickly, I have used the generateDS project to help create classes to parse the doxygen xml output. The script automatically creates the compound.py, compoundsuper.py, index.py and indexsuper.py files that you can see inside breathe/parser/doxygen.

So what is the difference between index.py and indexsuper.py, and compound.py and compoundsuper.py? These files allow us to separate the bulk of the automatically generated code from the code changes we might want to make. There are a large number of classes in the ...super.py files and each one has a basic derived class in the corresponding non-super files.

It is designed so that all the hard work done by the generated code is done in the ...super.py files and if we need to make changes we can do them in the derived classes in the non-super files and if we ever need to regenerate the code, we only regenerate the ...super.py files and so we don’t lose our changes in the process.

The end result is that for the parsing, we have written relatively little code, but have a large amount automatically generated for us. This has only been done once and it seems relatively unlikely that we’ll do it again. The entry points to the parsing code is the parse functions at the bottom of the breathe.parser.doxygen.compound and breathe.parser.doxygen.index.

I have never really examined the details of the parsing but you can see that there is a class for each node type you are likely to find in the xml files. I say “node type” instead of just “node” because different nodes can share the same type and there is one class per type. For example, there are detaileddescription nodes and briefdescription nodes which are both of type descriptionType. If we look in breathe.parser.doxygen.compoundsuper we see a descriptionType class and in breathe.parser.doxygen.compound we see a descriptionTypeSub class which is derived from descriptionType.

Our Changes

You’ll notice there are some classes in the non-super files that have some additional code in them. This tends to be adjusting the buildChildren member function in the derived class to extend or override the one in the automatically generated base class.

We have to do this sometimes as it seems the original code we generated with generateDS fails to construct the children of some classes. The generateDS scripts uses the descriptions in the .xsd files to determine what classes to generate and what nodes can be the children of other nodes. It is possible that the doxygen .xsd files contain levels of abstraction that the generateDS project did not cope with at the time I used it. It is possible that newer versions would handle it better but for the moment I’m content updating the derived classes to handle the cases I see missing.

Finders

The finder classes have a relatively small but important job of finding objects in the hierarchy generated by the parsers. For example, when a user specifies a particular class for the doxygenclass directive, we use the finder classes to go and find the object corresponding to that class.

In fact, if you look closely, it is the finders that use the parser entry points to parse the xml and then find the objects. The finders also use Filter objects to actually figure out if they have found what they are looking for.

The finder is given a hierarchy of filter objects which are designed to match at different levels of the XML hierarchy. Filters can also represent logical conditions such as ‘and’ and ‘or’.

More Details, Please

So initially, we create a finder to look at the root of the hierarchy: the doxygenTypeSub node. That finder, handily called DoxygenTypeSubItemFinder (you’ll notice a lot of that) looks through all the child compound nodes of the doxygenTypeSub node and tries a compound-level match against each of them and if something matches it creates a CompoundTypeSubItemFinder to look further.

In turn, that checks each of its member child nodes with a member-level match and if it finds one it creates a MemberTypeSubItemFinder (see the pattern?) and that does another check. The interesting part is, if that is successful, the CompoundTypeSubItemFinder finds the corresponding xml file that has more information in it (remember refid + .xml?) and parses that and creates another finder to start looking in there. This time it is a DoxygenTypeSubItemFinder from the breathe.finder.doxygen.compound module. And the search goes on until we find an object to return for rendering.

If the CompoundTypeSubItemFinder fails to find any deeper levels to match against then it returns itself as it must be the target we’re interested in.

As stated, the job of the finder is to find a single node for the renderers to starting rendering to reStructuredText. That is all the finder does.

Renderers

Finally, the bit that really does something we care about. Rendering is the art of turning whatever object we’ve found in the hierarchy into reStructuredText nodes. This almost invariably means most of its children as well.

Much like with the finder classes, we start off creating a renderer for a particular parser object and then it looks at its children and uses the renderer factory to create appropriate renderers for those objects and tells them to render and they look at their object’s children and create appropriate renderers for those and so on and so forth.

The node we start at is determined by the finder and ultimately by the user. The whole process is kicked off by the Builder class, though it doesn’t really do much. The aim of the renderers is to return a list of reStructuredText nodes which is passed back to Sphinx to render into whatever you’re final output format is.

There are two complicated bits here. All the different renderers and all the different reStructuredText nodes.

Different Renderers

Just like with the parsers, there is one renderer per node type. In fact there is one renderer class per parser class and they are named almost the same and are designed to match up. The renderers look at the data on the instance of the corresponding parser class that they have been given and grab the interesting bits and return reStructuredText nodes.

For reference on what there is to render, you can look at the parser class definitions or at the raw xml to see what attributes there are to render. Sometimes if something isn’t appearing in the final output, it is because the renderer isn’t returning an reStructuredText representation of it so the rendering code needs to be updated, and sometimes it is because the parser classes are not picking it up properly so both the parser and the renderer code needs to be updated.

Given a little bit of time, you get used to chasing through the xml nodes, the parser classes and the corresponding renderers to figure out where all the information is ending up.

reStructuredText Nodes

We use the reStructuredText API as provided by the fabulous docutils project and extended by Sphinx itself. For the most part, they are fairly straight forward and they are certainly well named.

Unfortunately there are a lot of nodes and only certain ways of combining them. It is also not always clear what arguments their constructs take. Whilst I’m sure it would be possible to figure it out with time and the appropriate source code, the use of them is not something I’ve found very well documented and my code largely operates on a basis of trial and error.

One day I’m sure I’ll be enlightened, until then expect fairly naive code.