====================
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

lxml also provides support for ISO-`Schematron`_, based on the pure-XSLT
`skeleton implementation`_ of Schematron:

.. _Schematron: http://www.schematron.com
.. _`skeleton implementation`: http://www.schematron.com/implementation.html

There is also basic support for `pre-ISO-Schematron` through the libxml2
Schematron features. However, this does not currently support error reporting
in the validation phase due to insufficiencies in the implementation as of
libxml2 2.6.30.

.. _`pre-ISO-Schematron`:   http://www.ascc.net/xml/schematron

.. contents::
..
   1  Validation at parse time
   2  DTD
   3  RelaxNG
   4  XMLSchema
   5  Schematron
   6  (Pre-ISO-Schematron)

The usual setup procedure:

.. sourcecode:: pycon

  >>> from lxml import etree

..
  >>> try: from StringIO import StringIO
  ... except ImportError:
  ...    from io import BytesIO
  ...    def StringIO(s):
  ...        if isinstance(s, str): s = s.encode("UTF-8")
  ...        return BytesIO(s)


Validation at parse time
------------------------

The parser in lxml can do on-the-fly validation of a document against
a DTD or an XML schema.  The DTD is retrieved automatically based on
the DOCTYPE of the parsed document.  All you have to do is use a
parser that has DTD validation enabled:

.. sourcecode:: pycon

  >>> parser = etree.XMLParser(dtd_validation=True)

Obviously, a request for validation enables the DTD loading feature.
There are two other options that enable loading the DTD, but that do
not perform any validation.  The first is the ``load_dtd`` keyword
option, which simply loads the DTD into the parser and makes it
available to the document as external subset.  You can retrieve the
DTD from the parsed document using the ``docinfo`` property of the
result ElementTree object.  The internal subset is available as
``internalDTD``, the external subset is provided as ``externalDTD``.

The third way to activate DTD loading is with the
``attribute_defaults`` option, which loads the DTD and weaves
attribute default values into the document.  Again, no validation is
performed unless explicitly requested.

XML schema is supported in a similar way, but requires an explicit
schema to be provided:

.. sourcecode:: pycon

  >>> schema_root = etree.XML('''\
  ...   <xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema">
  ...     <xsd:element name="a" type="xsd:integer"/>
  ...   </xsd:schema>
  ... ''')
  >>> schema = etree.XMLSchema(schema_root)

  >>> parser = etree.XMLParser(schema = schema)
  >>> root = etree.fromstring("<a>5</a>", parser)

If the validation fails (be it for a DTD or an XML schema), the parser
will raise an exception:

.. sourcecode:: pycon

  >>> root = etree.fromstring("<a>no int</a>", parser)  # doctest: +ELLIPSIS
  Traceback (most recent call last):
  lxml.etree.XMLSyntaxError: Element 'a': 'no int' is not a valid value of the atomic type 'xs:integer'...

If you want the parser to succeed regardless of the outcome of the
validation, you should use a non validating parser and run the
validation separately after parsing the document.


DTD
---

As described above, 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:

.. sourcecode:: pycon

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

Now you can use it to validate documents:

.. sourcecode:: pycon

  >>> 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:

.. sourcecode:: pycon

  >>> 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

As an alternative to parsing from a file, you can use the
``external_id`` keyword argument to parse from a catalog.  The
following example reads the DocBook DTD in version 4.2, if available
in the system catalog:

.. sourcecode:: python

  dtd = etree.DTD(external_id = "-//OASIS//DTD DocBook XML V4.2//EN")

The DTD information is available as attributes on the DTD object. The method
``iterelements`` provides an iterator over the element declarations:

.. sourcecode:: pycon

  >>> dtd = etree.DTD(StringIO('<!ELEMENT a EMPTY><!ELEMENT b EMPTY>'))
  >>> for el in dtd.iterelements():
  ...     print(el.name)
  a
  b

The method ``elements`` returns the element declarations as a list:

.. sourcecode:: pycon

  >>> dtd = etree.DTD(StringIO('<!ELEMENT a EMPTY><!ELEMENT b EMPTY>'))
  >>> len(dtd.elements())
  2

An element declaration object provides the following attributes/methods:

  - ``name``: The name of the element;

  - ``type``: The element type, one of "undefined", "empty", "any", "mixed", or "element";

  - ``content``: Element content declaration (see below);

  - ``iterattributes()``: Return an iterator over attribute declarations (see below);

  - ``attributes()``: Return a list of attribute declarations.

The ``content`` attribute contains information about the content model of the element.
These element content declaration objects form a binary tree (via the ``left`` and ``right``
attributes), that makes it possible to reconstruct the content model expression. Here's a
list of all attributes:

  - ``name``: If this object represents an element in the content model expression,
    ``name`` is the name of the element, otherwise it is ``None``;

  - ``type``: The type of the node: one of "pcdata", "element", "seq", or "or";

  - ``occur``: How often this element (or this combination of elements) may occur:
    one of "once", "opt", "mult", or "plus"

  - ``left``: The left hand subexpression

  - ``right``: The right hand subexpression

For example, the element declaration ``<!ELEMENT a (a|b)+>`` results
in the following element content declaration objects:

.. sourcecode:: pycon

  >>> dtd = etree.DTD(StringIO('<!ELEMENT a (a|b)+>'))
  >>> content = dtd.elements()[0].content
  >>> content.type, content.occur, content.name
  ('or', 'plus', None)

  >>> left, right = content.left, content.right
  >>> left.type, left.occur, left.name
  ('element', 'once', 'a')
  >>> right.type, right.occur, right.name
  ('element', 'once', 'b')

Attributes declarations have the following attributes/methods:

  - ``name``: The name of the attribute;

  - ``elemname``: The name of the element the attribute belongs to;

  - ``type``: The attribute type, one of "cdata", "id", "idref", "idrefs", "entity",
    "entities", "nmtoken", "nmtokens", "enumeration", or "notation";

  - ``default``: The type of the default value, one of "none", "required", "implied",
    or "fixed";

  - ``defaultValue``: The default value;

  - ``itervalues()``: Return an iterator over the allowed attribute values (if the attribute
    is of type "enumeration");

  - ``values()``: Return a list of allowed attribute values.

Entity declarations are available via the ``iterentities`` and ``entities`` methods:

  >>> dtd = etree.DTD(StringIO('<!ENTITY hurz "&#x40;">'))
  >>> entity = dtd.entities()[0]
  >>> entity.name, entity.orig, entity.content
  ('hurz', '&#x40;', '@')


RelaxNG
-------

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

.. sourcecode:: pycon

  >>> 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:

.. sourcecode:: pycon

  >>> 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:

.. sourcecode:: pycon

  >>> 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:

.. sourcecode:: pycon

  >>> relaxng.assertValid(doc2)
  Traceback (most recent call last):
    ...
  lxml.etree.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:

.. sourcecode:: pycon

  >>> log = relaxng.error_log
  >>> print(log.last_error)
  <string>:1:0:ERROR:RELAXNGV:RELAXNG_ERR_ELEMWRONG: 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.  You can also
look at the error domain and its type directly:

.. sourcecode:: pycon

  >>> error = log.last_error
  >>> print(error.domain_name)
  RELAXNGV
  >>> print(error.type_name)
  RELAXNG_ERR_ELEMWRONG

Note that this error log is local to the RelaxNG object.  It will only
contain log entries that appeared during the validation.

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

.. sourcecode:: pycon

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

libxml2 does not currently support the `RelaxNG Compact Syntax`_.
However, if `rnc2rng`_ is installed, lxml 3.6 and later can use it
internally to parse the input schema.  It recognises the `.rnc` file
extension and also allows parsing an RNC schema from a string using
`RelaxNG.from_rnc_string()`.

Alternatively, the trang_ translator can convert the compact syntax
to the XML syntax, which can then be used with lxml.

.. _`rnc2rng`: https://pypi.python.org/pypi/rnc2rng
.. _`RelaxNG Compact Syntax`: http://relaxng.org/compact-tutorial.html
.. _trang: http://www.thaiopensource.com/relaxng/trang.html


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:

.. sourcecode:: pycon

  >>> 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:

.. sourcecode:: pycon

  >>> 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:

.. sourcecode:: pycon

  >>> 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:

.. sourcecode:: pycon

  >>> xmlschema.assertValid(doc2)
  Traceback (most recent call last):
    ...
  lxml.etree.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:

.. sourcecode:: pycon

  >>> 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:

.. sourcecode:: pycon

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

Schematron
----------

From version 2.3 on lxml features ISO-`Schematron`_ support built on the
de-facto reference implementation of Schematron, the pure-XSLT-1.0
`skeleton implementation`_. This is provided by the lxml.isoschematron package
that implements the Schematron class, with an API compatible to the other
validators'.  Pass an Element or ElementTree object to construct a Schematron
validator:

.. sourcecode:: pycon

  >>> from lxml import isoschematron
  >>> f = StringIO('''\
  ... <schema xmlns="http://purl.oclc.org/dsdl/schematron" >
  ...   <pattern id="sum_equals_100_percent">
  ...     <title>Sum equals 100%.</title>
  ...     <rule context="Total">
  ...       <assert test="sum(//Percent)=100">Sum is not 100%.</assert>
  ...     </rule>
  ...   </pattern>
  ... </schema>
  ... ''')

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

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

.. sourcecode:: pycon

  >>> 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 can be useful for conditional statements:

.. sourcecode:: pycon

  >>> is_valid = isoschematron.Schematron(sct_doc)

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

Built on a pure-xslt implementation, the actual validator is created as an
XSLT 1.0 stylesheet using these steps:

0. (Extract embedded Schematron from XML Schema or RelaxNG schema)
1. Process inclusions
2. Process abstract patterns
3. Compile the schematron schema to XSLT

To allow more control over the individual steps, isoschematron.Schematron
supports an extended API:

The ``include`` and ``expand`` keyword arguments can be used to switch off
steps 1) and 2).

To set parameters for steps 1), 2) and 3) dictionaries containing parameters
for XSLT can be provided using the keyword arguments ``include_params``,
``expand_params`` or ``compile_params``. Schematron automatically converts these
parameters to stylesheet parameters so you need not worry to set string
parameters using quotes or to use XSLT.strparam(). If you ever need to pass an
XPath as argument to the XSLT stylesheet you can pass in an etree.XPath object
(see XPath and XSLT with lxml: Stylesheet-parameters_ for background on this).

The ``phase`` parameter of the compile step is additionally exposed as a keyword
argument. If set, it overrides occurrence in ``compile_params``. Note that
isoschematron.Schematron might expose more common parameters as additional keyword
args in the future.

By setting ``store_schematron`` to True, the (included-and-expanded) schematron
document tree is stored and made available through the ``schematron`` property.

Similarly, setting ``store_xslt`` to True will result in the validation XSLT
document tree being kept; it can be retrieved through the ``validator_xslt``
property.

Finally, with ``store_report`` set to True (default: False), the resulting
validation report document gets stored and can be accessed as the
``validation_report`` property.

.. _Stylesheet-parameters: xpathxslt.html#stylesheet-parameters

Using the ``phase`` parameter of isoschematron.Schematron allows for selective
validation of predefined pattern groups:

.. sourcecode:: pycon

  >>> f = StringIO('''\
  ... <schema xmlns="http://purl.oclc.org/dsdl/schematron" >
  ...   <phase id="phase.sum_check">
  ...     <active pattern="sum_equals_100_percent"/>
  ...   </phase>
  ...   <phase id="phase.entries_check">
  ...     <active pattern="all_positive"/>
  ...   </phase>
  ...   <pattern id="sum_equals_100_percent">
  ...     <title>Sum equals 100%.</title>
  ...     <rule context="Total">
  ...       <assert test="sum(//Percent)=100">Sum is not 100%.</assert>
  ...     </rule>
  ...   </pattern>
  ...   <pattern id="all_positive">
  ...     <title>All entries must be positive.</title>
  ...     <rule context="Percent">
  ...       <assert test="number(.)>0">Number (<value-of select="."/>) not positive</assert>
  ...     </rule>
  ...   </pattern>
  ... </schema>
  ... ''')

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

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

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

  >>> invalid_positive = StringIO('''\
  ... <Total>
  ...   <Percent>0</Percent>
  ...   <Percent>50</Percent>
  ...   <Percent>50</Percent>
  ... </Total>
  ... ''')

  >>> doc = etree.parse(invalid_positive)

  >>> schematron.validate(doc)
  False

If the constraint of Percent entries being positive is not of interest in a
certain validation scenario, it can now be disabled:

.. sourcecode:: pycon

  >>> selective = isoschematron.Schematron(sct_doc, phase="phase.sum_check")
  >>> selective.validate(doc)
  True

The usage of validation phases is a unique feature of ISO-Schematron and can be
a very powerful tool e.g. for establishing validation stages or to provide
different validators for different "validation audiences".

Note: Some lxml distributions exclude the validation schema file due to licensing issues.
Since lxml 5.0, the validation of the user provided schema can be disabled with
``Schematron(..., validate_schema=False)``.
It is enabled by default if available and disabled otherwise.  Previous versions of
lxml always had it enabled and failed at import time if the file was not available.
Thus, some distributions chose to remove the entire ISO-Schematron support.
The module constant ``lxml.isoschematron.schematron_schema_valid_supported`` can be used
since lxml 5.0 to detect whether schema file validation is available.

(Pre-ISO-Schematron)
--------------------

In libxml2 versions that provide it,  lxml.etree features `pre-ISO-Schematron`_ support,
using the class lxml.etree.Schematron.  It requires at least libxml2 2.6.21 to
work but is no longer available in libxml2 2.15.  To test if lxml provides this,
use ``"schematron" in etree.LIBXML_FEATURES``.

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)
  assert schematron.validate(doc)

  etree.SubElement(doc.getroot(), "Percent").text = "10"
  assert not schematron.validate(doc)

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!")

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.
