XML Schema: Advanced Types

<xs:simpleType name="Sku">
  <xs:restriction base="xs:string">
    <xs:pattern value="[A-Z]{3}-[0-9]{2}"/>       <!-- regular expression -->
  </xs:restriction>
</xs:simpleType>

<xs:simpleType name="Currency">
  <xs:restriction base="xs:string">
    <xs:enumeration value="EUR"/>
    <xs:enumeration value="GBP"/>
    <xs:enumeration value="USD"/>
  </xs:restriction>
</xs:simpleType>

<xs:simpleType name="Price">
  <xs:restriction base="xs:decimal">
    <xs:minInclusive value="0"/>
    <xs:fractionDigits value="2"/>
  </xs:restriction>
</xs:simpleType>

Other facets: minLength/maxLength, length, maxInclusive, minExclusive, totalDigits, whiteSpace.

Named types, extension and keys

Define a xs:complexType name="Address" once and reuse it for billing and delivery addresses.
Extension derives a type that adds elements or attributes; restriction narrows one.
xs:key / xs:keyref / xs:unique enforce identity constraints with XPath, like primary and foreign keys.
targetNamespace puts the schema's elements in a namespace; with elementFormDefault="qualified" local elements are namespaced too (almost always what you want).

from lxml import etree schema = etree.XMLSchema(etree.fromstring(b""" <xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" targetNamespace="urn:example:shop" xmlns="urn:example:shop" elementFormDefault="qualified"> <xs:simpleType name="Sku"> <xs:restriction base="xs:string"><xs:pattern value="[A-Z]{3}-[0-9]{2}"/></xs:restriction> </xs:simpleType> <xs:simpleType name="Currency"> <xs:restriction base="xs:string"> <xs:enumeration value="EUR"/><xs:enumeration value="GBP"/><xs:enumeration value="USD"/> </xs:restriction> </xs:simpleType> <xs:element name="catalog"> <xs:complexType> <xs:sequence> <xs:element name="product" maxOccurs="unbounded"> <xs:complexType> <xs:attribute name="sku" type="Sku" use="required"/> <xs:attribute name="currency" type="Currency" use="required"/> </xs:complexType> </xs:element> </xs:sequence> </xs:complexType> <xs:unique name="uniqueSku"> <xs:selector xpath="s:product" xmlns:s="urn:example:shop"/> <xs:field xpath="@sku"/> </xs:unique> </xs:element> </xs:schema>""")) tests = { "ok": '<product sku="TEA-01" currency="EUR"/><product sku="MUG-07" currency="GBP"/>', "bad pattern": '<product sku="tea-1" currency="EUR"/>', "bad enum": '<product sku="TEA-01" currency="BTC"/>', "duplicate sku": '<product sku="TEA-01" currency="EUR"/><product sku="TEA-01" currency="USD"/>', } for name, body in tests.items(): doc = etree.fromstring(f'<catalog xmlns="urn:example:shop">{body}</catalog>'.encode()) ok = schema.validate(doc) print(f"{name}: {'valid' if ok else 'invalid - ' + schema.error_log[0].message}")

ok: valid bad pattern: invalid - Element '{urn:example:shop}product', attribute 'sku': [facet 'pattern'] The value 'tea-1' is not accepted by the pattern '[A-Z]{3}-[0-9]{2}'. bad enum: invalid - Element '{urn:example:shop}product', attribute 'currency': [facet 'enumeration'] The value 'BTC' is not an element of the set {'EUR', 'GBP', 'USD'}. duplicate sku: invalid - Element '{urn:example:shop}product': Duplicate key-sequence ['TEA-01'] in unique identity-constraint '{urn:example:shop}uniqueSku'.

XSD 1.0 vs 1.1

XSD 1.1 (2012) adds assertions (xs:assert test="@end ge @start"), conditional type assignment and xs:all with repeats. libxml2 (lxml, PHP) and the JDK's built-in validator support 1.0 only; for 1.1 use Saxon-EE or Xerces-J's 1.1 build, or express cross-field rules in Schematron.

Restricting simple types with facets

Named types, extension and keys

XSD 1.0 vs 1.1

Test yourself