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:

>>> 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:

>>> 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:

>>> 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:

>>> 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

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:

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:

>>> 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:

>>> 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:

>>> 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:

>>> 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):
  ...
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:

>>> 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:

>>> 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:

>>> 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.

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):
  ...
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:

>>> 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

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:

>>> 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:

>>> 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:

>>> 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.

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

>>> 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:

>>> 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)

Since version 2.0, lxml.etree features pre-ISO-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.