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