[libxml2.wiki] Create Python bindings

[Nick Wellnhofer <nwellnhof src gnome org>]

commit c79b959c6db45d994fbfbcf1dd352055803101aa
Author: Nick Wellnhofer <wellnhofer aevum de>
Date:   Sat Feb 12 18:06:22 2022 +0000

    Create Python bindings

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+The distribution includes a set of Python bindings, which are guaranteed to be maintained as part of the 
library in the future, though the Python interface have not yet reached the completeness of the C API.
+
+Note that some of the Python purist dislike the default set of Python bindings, rather than complaining I 
suggest they have a look at [lxml the more pythonic bindings for libxml2 and libxslt](http://lxml.de/) and 
[check the mailing-list](http://lxml.de/mailinglist/).
+
+[Stephane Bidoul](mailto:stephane bidoul softwareag com) maintains [a Windows port of the Python 
bindings](http://users.skynet.be/sbi/libxml-python/).
+
+Note to people interested in building bindings, the API is formalized as [an XML API description 
file](http://xmlsoft.org/libxml2-api.xml) which allows to automate a large part of the Python bindings, this 
includes function descriptions, enums, structures, typedefs, etc... The Python script used to build the 
bindings is python/generator.py in the source distribution.
+
+To install the Python bindings there are 2 options:
+
+* If you use an RPM based distribution, simply install the [libxml2-python 
RPM](http://rpmfind.net/linux/rpm2html/search.php?query=libxml2-python) (and if needed the [libxslt-python 
RPM](http://rpmfind.net/linux/rpm2html/search.php?query=libxslt-python)).
+* Otherwise use the [libxml2-python module distribution](ftp://xmlsoft.org/libxml2/python/) corresponding to 
your installed version of libxml2 and libxslt. Note that to install it you will need both libxml2 and libxslt 
installed and run "python [setup.py](http://setup.py) build install" in the module tree.
+
+The distribution includes a set of examples and regression tests for the python bindings in the 
`python/tests` directory. Here are some excerpts from those tests:
+
+### tst.py
+
+This is a basic test of the file interface and DOM navigation:
+
+```plaintext
+import libxml2, sys
+
+doc = libxml2.parseFile("tst.xml")
+if doc.name != "tst.xml":
+    print "doc.name failed"
+    sys.exit(1)
+root = doc.children
+if root.name != "doc":
+    print "root.name failed"
+    sys.exit(1)
+child = root.children
+if child.name != "foo":
+    print "child.name failed"
+    sys.exit(1)
+doc.freeDoc()
+```
+
+The Python module is called libxml2; parseFile is the equivalent of xmlParseFile (most of the bindings are 
automatically generated, and the xml prefix is removed and the casing convention are kept). All node seen at 
the binding level share the same subset of accessors:
+
+* `name` : returns the node name
+* `type` : returns a string indicating the node type
+* `content` : returns the content of the node, it is based on xmlNodeGetContent() and hence is recursive.
+* `parent` , `children`, `last`, `next`, `prev`, `doc`, `properties`: pointing to the associated element in 
the tree, those may return None in case no such link exists.
+
+Also note the need to explicitly deallocate documents with freeDoc() . Reference counting for libxml2 trees 
would need quite a lot of work to function properly, and rather than risk memory leaks if not implemented 
correctly it sounds safer to have an explicit function to free a tree. The wrapper python objects like doc, 
root or child are them automatically garbage collected.
+
+### validate.py
+
+This test check the validation interfaces and redirection of error messages:
+
+```plaintext
+import libxml2
+
+#deactivate error messages from the validation
+def noerr(ctx, str):
+    pass
+
+libxml2.registerErrorHandler(noerr, None)
+
+ctxt = libxml2.createFileParserCtxt("invalid.xml")
+ctxt.validate(1)
+ctxt.parseDocument()
+doc = ctxt.doc()
+valid = ctxt.isValid()
+doc.freeDoc()
+if valid != 0:
+    print "validity check failed"
+```
+
+The first thing to notice is the call to registerErrorHandler(), it defines a new error handler global to 
the library. It is used to avoid seeing the error messages when trying to validate the invalid document.
+
+The main interest of that test is the creation of a parser context with createFileParserCtxt() and how the 
behaviour can be changed before calling parseDocument() . Similarly the information resulting from the 
parsing phase is also available using context methods.
+
+Contexts like nodes are defined as class and the libxml2 wrappers maps the C function interfaces in terms of 
objects method as much as possible. The best to get a complete view of what methods are supported is to look 
at the [libxml2.py](http://libxml2.py) module containing all the wrappers.
+
+### push.py
+
+This test show how to activate the push parser interface:
+
+```plaintext
+import libxml2
+
+ctxt = libxml2.createPushParser(None, "<foo", 4, "test.xml")
+ctxt.parseChunk("/>", 2, 1)
+doc = ctxt.doc()
+
+doc.freeDoc()
+```
+
+The context is created with a special call based on the xmlCreatePushParser() from the C library. The first 
argument is an optional SAX callback object, then the initial set of data, the length and the name of the 
resource in case URI-References need to be computed by the parser.
+
+Then the data are pushed using the parseChunk() method, the last call setting the third argument terminate 
to 1.
+
+### pushSAX.py
+
+this test show the use of the event based parsing interfaces. In this case the parser does not build a 
document, but provides callback information as the parser makes progresses analyzing the data being provided:
+
+```plaintext
+import libxml2
+log = ""
+
+class callback:
+    def startDocument(self):
+        global log
+        log = log + "startDocument:"
+
+    def endDocument(self):
+        global log
+        log = log + "endDocument:"
+
+    def startElement(self, tag, attrs):
+        global log
+        log = log + "startElement %s %s:" % (tag, attrs)
+
+    def endElement(self, tag):
+        global log
+        log = log + "endElement %s:" % (tag)
+
+    def characters(self, data):
+        global log
+        log = log + "characters: %s:" % (data)
+
+    def warning(self, msg):
+        global log
+        log = log + "warning: %s:" % (msg)
+
+    def error(self, msg):
+        global log
+        log = log + "error: %s:" % (msg)
+
+    def fatalError(self, msg):
+        global log
+        log = log + "fatalError: %s:" % (msg)
+
+handler = callback()
+
+ctxt = libxml2.createPushParser(handler, "<foo", 4, "test.xml")
+chunk = " url='tst'>b"
+ctxt.parseChunk(chunk, len(chunk), 0)
+chunk = "ar</foo>"
+ctxt.parseChunk(chunk, len(chunk), 1)
+
+reference = "startDocument:startElement foo {'url': 'tst'}:" + \ 
+            "characters: bar:endElement foo:endDocument:"
+if log != reference:
+    print "Error got: %s" % log
+    print "Expected: %s" % reference
+```
+
+The key object in that test is the handler, it provides a number of entry points which can be called by the 
parser as it makes progresses to indicate the information set obtained. The full set of callback is larger 
than what the callback class in that specific example implements (see the SAX definition for a complete 
list). The wrapper will only call those supplied by the object when activated. The startElement receives the 
names of the element and a dictionary containing the attributes carried by this element.
+
+Also note that the reference string generated from the callback shows a single character call even though 
the string "bar" is passed to the parser from 2 different call to parseChunk()
+
+### xpath.py
+
+This is a basic test of XPath wrappers support
+
+```plaintext
+import libxml2
+
+doc = libxml2.parseFile("tst.xml")
+ctxt = doc.xpathNewContext()
+res = ctxt.xpathEval("//*")
+if len(res) != 2:
+    print "xpath query: wrong node set size"
+    sys.exit(1)
+if res[0].name != "doc" or res[1].name != "foo":
+    print "xpath query: wrong node set value"
+    sys.exit(1)
+doc.freeDoc()
+ctxt.xpathFreeContext()
+```
+
+This test parses a file, then create an XPath context to evaluate XPath expression on it. The xpathEval() 
method execute an XPath query and returns the result mapped in a Python way. String and numbers are natively 
converted, and node sets are returned as a tuple of libxml2 Python nodes wrappers. Like the document, the 
XPath context need to be freed explicitly, also not that the result of the XPath query may point back to the 
document tree and hence the document must be freed after the result of the query is used.
+
+### xpathext.py
+
+This test shows how to extend the XPath engine with functions written in python:
+
+```plaintext
+import libxml2
+
+def foo(ctx, x):
+    return x + 1
+
+doc = libxml2.parseFile("tst.xml")
+ctxt = doc.xpathNewContext()
+libxml2.registerXPathFunction(ctxt._o, "foo", None, foo)
+res = ctxt.xpathEval("foo(1)")
+if res != 2:
+    print "xpath extension failure"
+doc.freeDoc()
+ctxt.xpathFreeContext()
+```
+
+Note how the extension function is registered with the context (but that part is not yet finalized, this may 
change slightly in the future).
+
+### tstxpath.py
+
+This test is similar to the previous one but shows how the extension function can access the XPath 
evaluation context:
+
+```plaintext
+def foo(ctx, x):
+    global called
+
+    #
+    # test that access to the XPath evaluation contexts
+    #
+    pctxt = libxml2.xpathParserContext(_obj=ctx)
+    ctxt = pctxt.context()
+    called = ctxt.function()
+    return x + 1
+```
+
+All the interfaces around the XPath parser(or rather evaluation) context are not finalized, but it should be 
sufficient to do contextual work at the evaluation point.
+
+### Memory debugging
+
+last but not least, all tests starts with the following prologue:
+
+```plaintext
+#memory debug specific
+libxml2.debugMemory(1)
+```
+
+and ends with the following epilogue:
+
+```plaintext
+#memory debug specific
+libxml2.cleanupParser()
+if libxml2.debugMemory(1) == 0:
+    print "OK"
+else:
+    print "Memory leak %d bytes" % (libxml2.debugMemory(1))
+    libxml2.dumpMemory()
+```
+
+Those activate the memory debugging interface of libxml2 where all allocated block in the library are 
tracked. The prologue then cleans up the library state and checks that all allocated memory has been freed. 
If not it calls dumpMemory() which saves that list in a `.memdump` file.
+
+Daniel Veillard
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