#
# Copyright (c), 2016-2020, SISSA (International School for Advanced Studies).
# All rights reserved.
# This file is distributed under the terms of the MIT License.
# See the file 'LICENSE' in the root directory of the present
# distribution, or http://opensource.org/licenses/MIT.
#
# @author Davide Brunato <[email protected]>
#
"""
This module contains base functions and classes XML Schema components.
"""
import re
from typing import Optional

from ..exceptions import XMLSchemaValueError, XMLSchemaTypeError
from ..names import XSD_ANNOTATION, XSD_APPINFO, XSD_DOCUMENTATION, XML_LANG, \
    XSD_ANY_TYPE, XSD_ANY_SIMPLE_TYPE, XSD_ANY_ATOMIC_TYPE, XSD_ID, XSD_QNAME, \
    XSD_OVERRIDE, XSD_NOTATION_TYPE, XSD_DECIMAL
from ..etree import is_etree_element, etree_tostring
from ..helpers import get_qname, local_name, get_prefixed_qname
from .exceptions import XMLSchemaParseError, XMLSchemaValidationError

XSD_TYPE_DERIVATIONS = {'extension', 'restriction'}
XSD_ELEMENT_DERIVATIONS = {'extension', 'restriction', 'substitution'}

XSD_VALIDATION_MODES = {'strict', 'lax', 'skip'}
"""
XML Schema validation modes
Ref.: https://www.w3.org/TR/xmlschema11-1/#key-va
"""


def check_validation_mode(validation):
    if validation not in XSD_VALIDATION_MODES:
        raise XMLSchemaValueError("validation mode can be 'strict', "
                                  "'lax' or 'skip': %r" % validation)


class XsdValidator:
    """
    Common base class for XML Schema validator, that represents a PSVI (Post Schema Validation
    Infoset) information item. A concrete XSD validator have to report its validity collecting
    building errors and implementing the properties.

    :param validation: defines the XSD validation mode to use for build the validator, \
    its value can be 'strict', 'lax' or 'skip'. Strict mode is the default.
    :type validation: str

    :ivar validation: XSD validation mode.
    :vartype validation: str
    :ivar errors: XSD validator building errors.
    :vartype errors: list
    """
    xsd_version = elem = None

    def __init__(self, validation='strict'):
        self.validation = validation
        self.errors = []

    @property
    def built(self):
        """
        Property that is ``True`` if XSD validator has been fully parsed and built,
        ``False`` otherwise. For schemas the property is checked on all global
        components. For XSD components check only the building of local subcomponents.
        """
        raise NotImplementedError()

    @property
    def validation_attempted(self):
        """
        Property that returns the *validation status* of the XSD validator.
        It can be 'full', 'partial' or 'none'.

        | https://www.w3.org/TR/xmlschema-1/#e-validation_attempted
        | https://www.w3.org/TR/2012/REC-xmlschema11-1-20120405/#e-validation_attempted
        """
        raise NotImplementedError()

    @property
    def validity(self):
        """
        Property that returns the XSD validator's validity.
        It can be ‘valid’, ‘invalid’ or ‘notKnown’.

        | https://www.w3.org/TR/xmlschema-1/#e-validity
        | https://www.w3.org/TR/2012/REC-xmlschema11-1-20120405/#e-validity
        """
        if self.validation == 'skip':
            return 'notKnown'
        elif self.errors or any(comp.errors for comp in self.iter_components()):
            return 'invalid'
        elif self.built:
            return 'valid'
        else:
            return 'notKnown'

    def iter_components(self, xsd_classes=None):
        """
        Creates an iterator for traversing all XSD components of the validator.

        :param xsd_classes: returns only a specific class/classes of components, \
        otherwise returns all components.
        """
        raise NotImplementedError()

    @property
    def all_errors(self):
        """
        A list with all the building errors of the XSD validator and its components.
        """
        errors = []
        for comp in self.iter_components():
            if comp.errors:
                errors.extend(comp.errors)
        return errors

    def copy(self):
        validator = object.__new__(self.__class__)
        validator.__dict__.update(self.__dict__)
        validator.errors = self.errors[:]
        return validator

    __copy__ = copy

    def parse_error(self, error, elem=None, validation=None):
        """
        Helper method for registering parse errors. Does nothing if validation mode is 'skip'.
        Il validation mode is 'lax' collects the error, otherwise raise the error.

        :param error: can be a parse error or an error message.
        :param elem: the Element instance related to the error, for default uses the 'elem' \
        attribute of the validator, if it's present.
        :param validation: overrides the default validation mode of the validator.
        """
        if validation:
            check_validation_mode(validation)
        else:
            validation = self.validation

        if validation == 'skip':
            return
        elif elem is None:
            elem = self.elem
        elif not is_etree_element(elem):
            msg = "the argument 'elem' must be an Element instance, not {!r}."
            raise XMLSchemaTypeError(msg.format(elem))

        if isinstance(error, XMLSchemaParseError):
            error.validator = self
            error.namespaces = getattr(self, 'namespaces', None)
            error.elem = elem
            error.source = getattr(self, 'source', None)
        elif isinstance(error, Exception):
            message = str(error).strip()
            if message[0] in '\'"' and message[0] == message[-1]:
                message = message.strip('\'"')
            error = XMLSchemaParseError(self, message, elem)
        elif isinstance(error, str):
            error = XMLSchemaParseError(self, error, elem)
        else:
            msg = "'error' argument must be an exception or a string, not {!r}."
            raise XMLSchemaTypeError(msg.format(error))

        if validation == 'lax':
            self.errors.append(error)
        else:
            raise error

    def _parse_xpath_default_namespace(self, elem):
        """
        Parse XSD 1.1 xpathDefaultNamespace attribute for schema, alternative, assert, assertion
        and selector declarations, checking if the value is conforming to the specification. In
        case the attribute is missing or for wrong attribute values defaults to ''.
        """
        try:
            value = elem.attrib['xpathDefaultNamespace']
        except KeyError:
            return ''

        value = value.strip()
        if value == '##local':
            return ''
        elif value == '##defaultNamespace':
            return getattr(self, 'default_namespace')
        elif value == '##targetNamespace':
            return getattr(self, 'target_namespace')
        elif len(value.split()) == 1:
            return value
        else:
            admitted_values = ('##defaultNamespace', '##targetNamespace', '##local')
            msg = "wrong value %r for 'xpathDefaultNamespace' attribute, can be (anyURI | %s)."
            self.parse_error(msg % (value, ' | '.join(admitted_values)), elem)
            return ''


class XsdComponent(XsdValidator):
    """
    Class for XSD components. See: https://www.w3.org/TR/xmlschema-ref/

    :param elem: ElementTree's node containing the definition.
    :param schema: the XMLSchema object that owns the definition.
    :param parent: the XSD parent, `None` means that is a global component that \
    has the schema as parent.
    :param name: name of the component, maybe overwritten by the parse of the `elem` argument.

    :cvar qualified: for name matching, unqualified matching may be admitted only \
    for elements and attributes.
    :vartype qualified: bool
    """
    _REGEX_SPACE = re.compile(r'\s')
    _REGEX_SPACES = re.compile(r'\s+')
    _ADMITTED_TAGS = ()

    parent = None
    name = None
    ref = None
    annotation = None
    qualified = True
    redefine = None

    def __init__(self, elem, schema, parent=None, name: Optional[str] = None):
        super(XsdComponent, self).__init__(schema.validation)
        if name:
            self.name = name
        if parent is not None:
            self.parent = parent
        self.schema = schema
        self.maps = schema.maps
        self.elem = elem

    def __setattr__(self, name, value):
        super(XsdComponent, self).__setattr__(name, value)
        if name == 'elem':
            if value.tag not in self._ADMITTED_TAGS:
                msg = "wrong XSD element {!r} for {!r}, must be one of {!r}"
                raise XMLSchemaValueError(
                    msg.format(value.tag, self.__class__, self._ADMITTED_TAGS)
                )
            self._parse()

    @property
    def xsd_version(self):
        return self.schema.XSD_VERSION

    def is_global(self):
        """Returns `True` if the instance is a global component, `False` if it's local."""
        return self.parent is None

    def is_override(self):
        """Returns `True` if the instance is an override of a global component."""
        if self.parent is not None:
            return False
        return any(self.elem in x for x in self.schema.root if x.tag == XSD_OVERRIDE)

    @property
    def schema_elem(self):
        """The reference element of the schema for the component instance."""
        return self.elem

    @property
    def source(self):
        """Property that references to schema source."""
        return self.schema.source

    @property
    def target_namespace(self):
        """Property that references to schema's targetNamespace."""
        return self.schema.target_namespace if self.ref is None else self.ref.target_namespace

    @property
    def default_namespace(self):
        """Property that references to schema's default namespaces."""
        return self.schema.namespaces.get('')

    @property
    def namespaces(self):
        """Property that references to schema's namespace mapping."""
        return self.schema.namespaces

    @property
    def any_type(self):
        """Property that references to the xs:anyType instance of the global maps."""
        return self.maps.types[XSD_ANY_TYPE]

    @property
    def any_simple_type(self):
        """Property that references to the xs:anySimpleType instance of the global maps."""
        return self.maps.types[XSD_ANY_SIMPLE_TYPE]

    @property
    def any_atomic_type(self):
        """Property that references to the xs:anyAtomicType instance of the global maps."""
        return self.maps.types[XSD_ANY_ATOMIC_TYPE]

    def __repr__(self):
        if self.ref is not None:
            return '%s(ref=%r)' % (self.__class__.__name__, self.prefixed_name)
        else:
            return '%s(name=%r)' % (self.__class__.__name__, self.prefixed_name)

    def _parse(self):
        del self.errors[:]
        for child in self.elem:
            if child.tag == XSD_ANNOTATION:
                self.annotation = XsdAnnotation(child, self.schema, self)
                break
            elif not callable(child.tag):
                break

    def _parse_reference(self):
        """
        Helper method for referable components. Returns `True` if a valid reference QName
        is found without any error, otherwise returns `None`. Sets an id-related name for
        the component ('nameless_<id of the instance>') if both the attributes 'ref' and
        'name' are missing.
        """
        ref = self.elem.get('ref')
        if ref is None:
            if 'name' in self.elem.attrib:
                return
            elif self.parent is None:
                self.parse_error("missing attribute 'name' in a global %r" % type(self))
            else:
                self.parse_error(
                    "missing both attributes 'name' and 'ref' in local %r" % type(self)
                )
        elif 'name' in self.elem.attrib:
            self.parse_error("attributes 'name' and 'ref' are mutually exclusive")
        elif self.parent is None:
            self.parse_error("attribute 'ref' not allowed in a global %r" % type(self))
        else:
            try:
                self.name = self.schema.resolve_qname(ref)
            except (KeyError, ValueError, RuntimeError) as err:
                self.parse_error(err)
            else:
                if self._parse_child_component(self.elem, strict=False) is not None:
                    self.parse_error("a reference component cannot have "
                                     "child definitions/declarations")
                return True

    def _parse_child_component(self, elem, strict=True):
        child = None
        for e in elem:
            if e.tag == XSD_ANNOTATION or callable(e.tag):
                continue
            elif not strict:
                return e
            elif child is not None:
                msg = "too many XSD components, unexpected {!r} found at position {}"
                self.parse_error(msg.format(child, elem[:].index(e)), elem)
                break
            else:
                child = e
        return child

    def _parse_target_namespace(self):
        """
        XSD 1.1 targetNamespace attribute in elements and attributes declarations.
        """
        if 'targetNamespace' not in self.elem.attrib:
            return

        self._target_namespace = self.elem.attrib['targetNamespace'].strip()
        if 'name' not in self.elem.attrib:
            self.parse_error("attribute 'name' must be present when "
                             "'targetNamespace' attribute is provided")
        if 'form' in self.elem.attrib:
            self.parse_error("attribute 'form' must be absent when "
                             "'targetNamespace' attribute is provided")
        if self._target_namespace != self.schema.target_namespace:
            if self.parent is None:
                self.parse_error("a global %s must have the same namespace as "
                                 "its parent schema" % self.__class__.__name__)

            xsd_type = self.get_parent_type()
            if not xsd_type or xsd_type.parent is not None:
                pass
            elif xsd_type.derivation != 'restriction' or xsd_type.base_type.name == XSD_ANY_TYPE:
                self.parse_error("a declaration contained in a global complexType "
                                 "must have the same namespace as its parent schema")

        if not self._target_namespace:
            self.name = local_name(self.name)
        else:
            self.name = '{%s}%s' % (self._target_namespace, local_name(self.name))

    @property
    def local_name(self):
        """The local part of the name of the component, or `None` if the name is `None`."""
        return local_name(self.name)

    @property
    def qualified_name(self):
        """The name of the component in extended format, or `None` if the name is `None`."""
        return get_qname(self.target_namespace, self.name)

    @property
    def prefixed_name(self):
        """The name of the component in prefixed format, or `None` if the name is `None`."""
        return get_prefixed_qname(self.name, self.namespaces)

    @property
    def id(self):
        """The ``'id'`` attribute of the component tag, ``None`` if missing."""
        return self.elem.get('id')

    @property
    def validation_attempted(self):
        return 'full' if self.built else 'partial'

    @property
    def built(self):
        raise NotImplementedError()

    def is_matching(self, name, default_namespace=None, **kwargs):
        """
        Returns `True` if the component name is matching the name provided as argument,
        `False` otherwise. For XSD elements the matching is extended to substitutes.

        :param name: a local or fully-qualified name.
        :param default_namespace: used if it's not None and not empty for completing \
        the name argument in case it's a local name.
        :param kwargs: additional options that can be used by certain components.
        """
        if not name:
            return self.name == name
        elif name[0] == '{':
            return self.qualified_name == name
        elif not default_namespace:
            return self.name == name or not self.qualified and self.local_name == name
        else:
            qname = '{%s}%s' % (default_namespace, name)
            return self.qualified_name == qname or not self.qualified and self.local_name == name

    def match(self, name, default_namespace=None, **kwargs):
        """
        Returns the component if its name is matching the name provided as argument,
        `None` otherwise.
        """
        return self if self.is_matching(name, default_namespace, **kwargs) else None

    def get_global(self):
        """Returns the global XSD component that contains the component instance."""
        if self.parent is None:
            return self
        component = self.parent
        while component is not self:  # pragma: no cover
            if component.parent is None:
                return component
            component = component.parent

    def get_parent_type(self):
        """
        Returns the nearest XSD type that contains the component instance,
        or `None` if the component doesn't have an XSD type parent.
        """
        component = self.parent
        while component is not self and component is not None:
            if isinstance(component, XsdType):
                return component
            component = component.parent

    def iter_components(self, xsd_classes=None):
        """
        Creates an iterator for XSD subcomponents.

        :param xsd_classes: provide a class or a tuple of classes to iterates over only a \
        specific classes of components.
        """
        if xsd_classes is None or isinstance(self, xsd_classes):
            yield self

    def iter_ancestors(self, xsd_classes=None):
        """
        Creates an iterator for XSD ancestor components, schema excluded. Stops when the component
        is global or if the ancestor is not an instance of the specified class/classes.

        :param xsd_classes: provide a class or a tuple of classes to iterates over only a \
        specific classes of components.
        """
        ancestor = self
        while True:
            ancestor = ancestor.parent
            if ancestor is None or xsd_classes and not isinstance(ancestor, xsd_classes):
                break
            yield ancestor

    def tostring(self, indent='', max_lines=None, spaces_for_tab=4):
        """Serializes the XML elements that declare or define the component to a string."""
        return etree_tostring(self.schema_elem, self.namespaces, indent, max_lines, spaces_for_tab)


class XsdAnnotation(XsdComponent):
    """
    Class for XSD *annotation* definitions.

    :ivar appinfo: a list containing the xs:appinfo children.
    :ivar documentation: a list containing the xs:documentation children.

    ..  <annotation
          id = ID
          {any attributes with non-schema namespace . . .}>
          Content: (appinfo | documentation)*
        </annotation>

    ..  <appinfo
          source = anyURI
          {any attributes with non-schema namespace . . .}>
          Content: ({any})*
        </appinfo>

    ..  <documentation
          source = anyURI
          xml:lang = language
          {any attributes with non-schema namespace . . .}>
          Content: ({any})*
        </documentation>
    """
    _ADMITTED_TAGS = {XSD_ANNOTATION}

    @property
    def built(self):
        return True

    def _parse(self):
        del self.errors[:]
        self.appinfo = []
        self.documentation = []
        for child in self.elem:
            if child.tag == XSD_APPINFO:
                for key in child.attrib:
                    if key != 'source':
                        self.parse_error("wrong attribute %r for appinfo declaration." % key)
                self.appinfo.append(child)
            elif child.tag == XSD_DOCUMENTATION:
                for key in child.attrib:
                    if key not in ['source', XML_LANG]:
                        self.parse_error("wrong attribute %r for documentation declaration." % key)
                self.documentation.append(child)


class XsdType(XsdComponent):
    """Common base class for XSD types."""

    abstract = False
    block = None
    base_type = None
    derivation = None
    _final = None

    @property
    def final(self):
        return self.schema.final_default if self._final is None else self._final

    @property
    def built(self):
        raise NotImplementedError()

    @property
    def content_type_label(self):
        """The content type classification."""
        raise NotImplementedError()

    @property
    def sequence_type(self):
        """The XPath sequence type associated with the content."""
        raise NotImplementedError()

    @property
    def root_type(self):
        """
        The root type of the type definition hierarchy. For an atomic type
        is the primitive type. For a list is the primitive type of the item.
        For a union is the base union type. For a complex type is xs:anyType.
        """
        if self.is_complex() and self.attributes:
            return self.maps.types[XSD_ANY_TYPE]
        elif self.base_type is None:
            return self if self.is_simple() else self.maps.types[XSD_ANY_TYPE]

        try:
            if self.base_type.is_simple():
                return self.base_type.primitive_type
            else:
                return self.base_type.content.primitive_type
        except AttributeError:
            # The type has complex or XsdList content
            return self.base_type.root_type

    @property
    def simple_type(self):
        """
        Property that is the instance itself for a simpleType. For a
        complexType is the instance's *content* if this is a simpleType
        or `None` if the instance's *content* is a model group.
        """
        raise NotImplementedError()

    @property
    def model_group(self):
        """
        Property that is `None` for a simpleType. For a complexType is
        the instance's *content* if this is a model group or `None` if
        the instance's *content* is a simpleType.
        """
        raise NotImplementedError()

    @staticmethod
    def is_simple():
        """Returns `True` if the instance is a simpleType, `False` otherwise."""
        raise NotImplementedError()

    @staticmethod
    def is_complex():
        """Returns `True` if the instance is a complexType, `False` otherwise."""
        raise NotImplementedError()

    @staticmethod
    def is_atomic():
        """Returns `True` if the instance is an atomic simpleType, `False` otherwise."""
        return False

    @staticmethod
    def is_list():
        """Returns `True` if the instance is a list simpleType, `False` otherwise."""
        return False

    @staticmethod
    def is_union():
        """Returns `True` if the instance is a union simpleType, `False` otherwise."""
        return False

    @staticmethod
    def is_datetime():
        """
        Returns `True` if the instance is a datetime/duration XSD builtin-type, `False` otherwise.
        """
        return False

    def is_empty(self):
        """Returns `True` if the instance has an empty content, `False` otherwise."""
        raise NotImplementedError()

    def is_emptiable(self):
        """Returns `True` if the instance has an emptiable value or content, `False` otherwise."""
        raise NotImplementedError()

    def has_simple_content(self):
        """
        Returns `True` if the instance has a simple content, `False` otherwise.
        """
        raise NotImplementedError()

    def has_complex_content(self):
        """
        Returns `True` if the instance is a complexType with mixed or element-only
        content, `False` otherwise.
        """
        raise NotImplementedError()

    def has_mixed_content(self):
        """
        Returns `True` if the instance is a complexType with mixed content, `False` otherwise.
        """
        raise NotImplementedError()

    def is_element_only(self):
        """
        Returns `True` if the instance is a complexType with element-only content,
        `False` otherwise.
        """
        raise NotImplementedError()

    def is_derived(self, other, derivation=None):
        raise NotImplementedError()

    def is_extension(self):
        return self.derivation == 'extension'

    def is_restriction(self):
        return self.derivation == 'restriction'

    def is_blocked(self, xsd_element):
        """
        Returns `True` if the base type derivation is blocked, `False` otherwise.
        """
        xsd_type = xsd_element.type
        if self is xsd_type:
            return False

        block = ('%s %s' % (xsd_element.block, xsd_type.block)).strip()
        if not block:
            return False
        block = {x for x in block.split() if x in ('extension', 'restriction')}

        return any(self.is_derived(xsd_type, derivation) for derivation in block)

    def is_dynamic_consistent(self, other):
        return other.name == XSD_ANY_TYPE or self.is_derived(other) or \
            hasattr(other, 'member_types') and \
            any(self.is_derived(mt) for mt in other.member_types)  # pragma: no cover

    def is_key(self):
        return self.name == XSD_ID or self.is_derived(self.maps.types[XSD_ID])

    def is_qname(self):
        return self.name == XSD_QNAME or self.is_derived(self.maps.types[XSD_QNAME])

    def is_notation(self):
        return self.name == XSD_NOTATION_TYPE or self.is_derived(self.maps.types[XSD_NOTATION_TYPE])

    def is_decimal(self):
        return self.name == XSD_DECIMAL or self.is_derived(self.maps.types[XSD_DECIMAL])

    def text_decode(self, text):
        raise NotImplementedError()


class ValidationMixin:
    """
    Mixin for implementing XML data validators/decoders. A derived class must implement the
    methods `iter_decode` and `iter_encode`.
    """
    def validate(self, source, use_defaults=True, namespaces=None):
        """
        Validates an XML data against the XSD schema/component instance.

        :param source: the source of XML data. For a schema can be a path \
        to a file or an URI of a resource or an opened file-like object or an Element Tree \
        instance or a string containing XML data. For other XSD components can be a string \
        for an attribute or a simple type validators, or an ElementTree's Element otherwise.
        :param use_defaults: indicates whether to use default values for filling missing data.
        :param namespaces: is an optional mapping from namespace prefix to URI.
        :raises: :exc:`XMLSchemaValidationError` if XML *data* instance is not a valid.
        """
        for error in self.iter_errors(source, use_defaults=use_defaults, namespaces=namespaces):
            raise error

    def is_valid(self, source, use_defaults=True, namespaces=None):
        """
        Like :meth:`validate` except that do not raises an exception but returns ``True`` if
        the XML document is valid, ``False`` if it's invalid.

        :param source: the source of XML data. For a schema can be a path \
        to a file or an URI of a resource or an opened file-like object or an Element Tree \
        instance or a string containing XML data. For other XSD components can be a string \
        for an attribute or a simple type validators, or an ElementTree's Element otherwise.
        :param use_defaults: indicates whether to use default values for filling missing data.
        :param namespaces: is an optional mapping from namespace prefix to URI.
        """
        return next(
            self.iter_errors(source, use_defaults=use_defaults, namespaces=namespaces), None
        ) is None

    def iter_errors(self, source, use_defaults=True, namespaces=None):
        """
        Creates an iterator for the errors generated by the validation of an XML data
        against the XSD schema/component instance.

        :param source: the source of XML data. For a schema can be a path \
        to a file or an URI of a resource or an opened file-like object or an Element Tree \
        instance or a string containing XML data. For other XSD components can be a string \
        for an attribute or a simple type validators, or an ElementTree's Element otherwise.
        :param use_defaults: Use schema's default values for filling missing data.
        :param namespaces: is an optional mapping from namespace prefix to URI.
        """
        for result in self.iter_decode(source, use_defaults=use_defaults, namespaces=namespaces):
            if isinstance(result, XMLSchemaValidationError):
                yield result
            else:
                del result

    def decode(self, source, validation='strict', **kwargs):
        """
        Decodes XML data.

        :param source: the XML data. Can be a string for an attribute or for a simple \
        type components or a dictionary for an attribute group or an ElementTree's \
        Element for other components.
        :param validation: the validation mode. Can be 'lax', 'strict' or 'skip.
        :param kwargs: optional keyword arguments for the method :func:`iter_decode`.
        :return: a dictionary like object if the XSD component is an element, a \
        group or a complex type; a list if the XSD component is an attribute group; \
        a simple data type object otherwise. If *validation* argument is 'lax' a 2-items \
        tuple is returned, where the first item is the decoded object and the second item \
        is a list containing the errors.
        :raises: :exc:`XMLSchemaValidationError` if the object is not decodable by \
        the XSD component, or also if it's invalid when ``validation='strict'`` is provided.
        """
        check_validation_mode(validation)

        result, errors = None, []
        for result in self.iter_decode(source, validation, **kwargs):  # pragma: no cover
            if not isinstance(result, XMLSchemaValidationError):
                break
            elif validation == 'strict':
                raise result
            else:
                errors.append(result)

        return (result, errors) if validation == 'lax' else result

    def encode(self, obj, validation='strict', **kwargs):
        """
        Encodes data to XML.

        :param obj: the data to be encoded to XML.
        :param validation: the validation mode. Can be 'lax', 'strict' or 'skip.
        :param kwargs: optional keyword arguments for the method :func:`iter_encode`.
        :return: An element tree's Element if the original data is a structured data or \
        a string if it's simple type datum. If *validation* argument is 'lax' a 2-items \
        tuple is returned, where the first item is the encoded object and the second item \
        is a list containing the errors.
        :raises: :exc:`XMLSchemaValidationError` if the object is not encodable by the XSD \
        component, or also if it's invalid when ``validation='strict'`` is provided.
        """
        check_validation_mode(validation)

        result, errors = None, []
        for result in self.iter_encode(obj, validation=validation, **kwargs):  # pragma: no cover
            if not isinstance(result, XMLSchemaValidationError):
                break
            elif validation == 'strict':
                raise result
            else:
                errors.append(result)

        return (result, errors) if validation == 'lax' else result

    def iter_decode(self, source, validation='lax', **kwargs):
        """
        Creates an iterator for decoding an XML source to a Python object.

        :param source: the XML data source.
        :param validation: the validation mode. Can be 'lax', 'strict' or 'skip.
        :param kwargs: keyword arguments for the decoder API.
        :return: Yields a decoded object, eventually preceded by a sequence of \
        validation or decoding errors.
        """
        raise NotImplementedError()

    def iter_encode(self, obj, validation='lax', **kwargs):
        """
        Creates an iterator for Encode data to an Element.

        :param obj: The data that has to be encoded.
        :param validation: The validation mode. Can be 'lax', 'strict' or 'skip'.
        :param kwargs: keyword arguments for the encoder API.
        :return: Yields an Element, eventually preceded by a sequence of validation \
        or encoding errors.
        """
        raise NotImplementedError()

    def validation_error(self, validation, error, obj=None,
                         source=None, namespaces=None, **_kwargs):
        """
        Helper method for generating and updating validation errors. If validation
        mode is 'lax' or 'skip' returns the error, otherwise raises the error.

        :param validation: an error-compatible validation mode: can be 'lax' or 'strict'.
        :param error: an error instance or the detailed reason of failed validation.
        :param obj: the instance related to the error.
        :param source: the XML resource related to the validation process.
        :param namespaces: is an optional mapping from namespace prefix to URI.
        :param _kwargs: keyword arguments of the validation process that are not used.
        """
        check_validation_mode(validation)
        if isinstance(error, XMLSchemaValidationError):
            if error.namespaces is None and namespaces is not None:
                error.namespaces = namespaces
            if error.source is None and source is not None:
                error.source = source
            if error.obj is None and obj is not None:
                error.obj = obj
            if error.elem is None and is_etree_element(obj):
                error.elem = obj
        elif isinstance(error, Exception):
            error = XMLSchemaValidationError(self, obj, str(error), source, namespaces)
        else:
            error = XMLSchemaValidationError(self, obj, error, source, namespaces)

        if validation == 'strict' and error.elem is not None:
            raise error
        return error