====================
Validation with lxml
====================

Apart from the built-in DTD support in parsers, lxml currently supports three
schema languages: DTD_, `Relax NG`_ and `XML Schema`_.  All three provide
identical APIs in lxml, represented by validator classes with the obvious
names.

.. _DTD:          http://en.wikipedia.org/wiki/Document_Type_Definition
.. _`Relax NG`:   http://www.relaxng.org/
.. _`XML Schema`: http://www.w3.org/XML/Schema

There is also initial support for Schematron_.  However, it does not currently
support error reporting in the validation phase due to insufficiencies in the
implementation as of libxml2 2.6.29.

.. _Schematron:   http://www.ascc.net/xml/schematron

.. contents::
.. 
   1  DTD
   2  RelaxNG
   3  XMLSchema
   4  Schematron

The usual setup procedure::

  >>> from lxml import etree
  >>> from StringIO import StringIO


DTD
---

There are two places in lxml where DTDs are supported: parsers and the DTD
class.  If you pass a keyword option to a parser that requires DTD loading,
lxml will automatically include the DTD in the parsing process.  If you pass
the keyword for DTD validation, lxml (or rather libxml2) will use this DTD
right inside the parser and report failure or success when parsing terminates.

The parser support for DTDs depends on internal or external subsets of the XML
file.  This means that the XML file itself must either contain a DTD or must
reference a DTD to make this work.  If you want to validate an XML document
against a DTD that is not referenced by the document itself, you can use the
``DTD`` class.

To use the ``DTD`` class, you must first pass a filename or file-like object
into the constructor to parse a DTD::

  >>> f = StringIO("<!ELEMENT b EMPTY>")
  >>> dtd = etree.DTD(f)

Now you can use it to validate documents::

  >>> root = etree.XML("<b/>")
  >>> print dtd.validate(root)
  True

  >>> root = etree.XML("<b><a/></b>")
  >>> print dtd.validate(root)
  False

The reason for the validation failure can be found in the error log::

  >>> print dtd.error_log.filter_from_errors()[0]
  <string>:1:0:ERROR:VALID:DTD_NOT_EMPTY: Element b was declared EMPTY this one has content


RelaxNG
-------

The ``RelaxNG`` class takes an ElementTree object to construct a Relax NG
validator::

  >>> f = StringIO('''\
  ... <element name="a" xmlns="http://relaxng.org/ns/structure/1.0">
  ...  <zeroOrMore>
  ...     <element name="b">
  ...       <text />
  ...     </element>
  ...  </zeroOrMore>
  ... </element>
  ... ''')
  >>> relaxng_doc = etree.parse(f)
  >>> relaxng = etree.RelaxNG(relaxng_doc)

Alternatively, pass a filename to the ``file`` keyword argument to parse from
a file.  This also enables correct handling of include files from within the
RelaxNG parser.

You can then validate some ElementTree document against the schema. You'll get
back True if the document is valid against the Relax NG schema, and False if
not::

  >>> valid = StringIO('<a><b></b></a>')
  >>> doc = etree.parse(valid)
  >>> relaxng.validate(doc)
  True

  >>> invalid = StringIO('<a><c></c></a>')
  >>> doc2 = etree.parse(invalid)
  >>> relaxng.validate(doc2)
  False

Calling the schema object has the same effect as calling its validate
method. This is sometimes used in conditional statements::

  >>> invalid = StringIO('<a><c></c></a>')
  >>> doc2 = etree.parse(invalid)
  >>> if not relaxng(doc2):
  ...     print "invalid!"
  invalid!

If you prefer getting an exception when validating, you can use the
``assert_`` or ``assertValid`` methods::

  >>> relaxng.assertValid(doc2)
  Traceback (most recent call last):
    [...]
  DocumentInvalid: Did not expect element c there, line 1

  >>> relaxng.assert_(doc2)
  Traceback (most recent call last):
    [...]
  AssertionError: Did not expect element c there, line 1

If you want to find out why the validation failed in the second case, you can
look up the error log of the validation process and check it for relevant
messages::

  >>> log = relaxng.error_log
  >>> print log.last_error
  <string>:1:0:ERROR:RELAXNGV:ERR_LT_IN_ATTRIBUTE: Did not expect element c there

You can see that the error (ERROR) happened during RelaxNG validation
(RELAXNGV).  The message then tells you what went wrong.  Note that this error
log is local to the RelaxNG object.  It will only contain log entries that
appeared during the validation.  The DocumentInvalid exception raised by the
``assertValid`` method above provides access to the global error log (like all
other lxml exceptions).

Similar to XSLT, there's also a less efficient but easier shortcut method to
do one-shot RelaxNG validation::

  >>> doc.relaxng(relaxng_doc)
  True
  >>> doc2.relaxng(relaxng_doc)
  False


XMLSchema
---------

lxml.etree also has XML Schema (XSD) support, using the class
lxml.etree.XMLSchema.  The API is very similar to the Relax NG and DTD
classes.  Pass an ElementTree object to construct a XMLSchema validator::

  >>> f = StringIO('''\
  ... <xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema">
  ... <xsd:element name="a" type="AType"/>
  ... <xsd:complexType name="AType">
  ...   <xsd:sequence>
  ...     <xsd:element name="b" type="xsd:string" />
  ...   </xsd:sequence>
  ... </xsd:complexType>
  ... </xsd:schema>
  ... ''')
  >>> xmlschema_doc = etree.parse(f)
  >>> xmlschema = etree.XMLSchema(xmlschema_doc)

You can then validate some ElementTree document with this.  Like with RelaxNG,
you'll get back true if the document is valid against the XML schema, and
false if not::

  >>> valid = StringIO('<a><b></b></a>')
  >>> doc = etree.parse(valid)
  >>> xmlschema.validate(doc)
  True

  >>> invalid = StringIO('<a><c></c></a>')
  >>> doc2 = etree.parse(invalid)
  >>> xmlschema.validate(doc2)
  False

Calling the schema object has the same effect as calling its validate method.
This is sometimes used in conditional statements::

  >>> invalid = StringIO('<a><c></c></a>')
  >>> doc2 = etree.parse(invalid)
  >>> if not xmlschema(doc2):
  ...     print "invalid!"
  invalid!

If you prefer getting an exception when validating, you can use the
``assert_`` or ``assertValid`` methods::

  >>> xmlschema.assertValid(doc2)
  Traceback (most recent call last):
    [...]
  DocumentInvalid: Element 'c': This element is not expected. Expected is ( b )., line 1

  >>> xmlschema.assert_(doc2)
  Traceback (most recent call last):
    [...]
  AssertionError: Element 'c': This element is not expected. Expected is ( b )., line 1

Error reporting works as for the RelaxNG class::

  >>> log = xmlschema.error_log
  >>> error = log.last_error
  >>> print error.domain_name
  SCHEMASV
  >>> print error.type_name
  SCHEMAV_ELEMENT_CONTENT

If you were to print this log entry, you would get something like the
following.  Note that the error message depends on the libxml2 version in
use::

  <string>:1:ERROR::SCHEMAV_ELEMENT_CONTENT: Element 'c': This element is not expected. Expected is ( b ).

Similar to XSLT and RelaxNG, there's also a less efficient but easier shortcut
method to do XML Schema validation::

  >>> doc.xmlschema(xmlschema_doc)
  True
  >>> doc2.xmlschema(xmlschema_doc)
  False


Schematron
----------

Since version 2.0, lxml.etree features Schematron_ support, using the
class lxml.etree.Schematron.  It requires at least libxml2 2.6.21 to
work.  The API is the same as for the other validators.  Pass an
ElementTree object to construct a Schematron validator::

  >>> f = StringIO('''\
  ... <schema xmlns="http://www.ascc.net/xml/schematron" >
  ...   <pattern name="Sum equals 100%.">
  ...     <rule context="Total">
  ...       <assert test="sum(//Percent)=100">Sum is not 100%.</assert>
  ...     </rule>
  ...   </pattern>
  ... </schema>
  ... ''')

  >>> sct_doc = etree.parse(f)
  >>> schematron = etree.Schematron(sct_doc)

You can then validate some ElementTree document with this.  Like with RelaxNG,
you'll get back true if the document is valid against the schema, and false if
not::

  >>> valid = StringIO('''\
  ... <Total>
  ...   <Percent>20</Percent>
  ...   <Percent>30</Percent>
  ...   <Percent>50</Percent>
  ... </Total>
  ... ''')

  >>> doc = etree.parse(valid)
  >>> schematron.validate(doc)
  True

  >>> etree.SubElement(doc.getroot(), "Percent").text = "10"

  >>> schematron.validate(doc)
  False

Calling the schema object has the same effect as calling its validate method.
This is sometimes used in conditional statements::

  >>> is_valid = etree.Schematron(sct_doc)

  >>> if not is_valid(doc):
  ...     print "invalid!"
  invalid!

Note that libxml2 restricts error reporting to the parsing step (when creating
the Schematron instance).  There is not currently any support for error
reporting during validation.
