import logging
import os
import sys
import yaml

from functools import partial
from lxml import etree
from collections import namedtuple
from pkg_resources import resource_listdir, resource_string, resource_isdir
from webob import Response

from xmodule.modulestore import Location
from xmodule.modulestore.exceptions import ItemNotFoundError, InsufficientSpecificationError, InvalidLocationError

from xblock.core import XBlock
from xblock.fields import Scope, Integer, Float, List, XBlockMixin, String
from xmodule.fields import RelativeTime
from xblock.fragment import Fragment
from xblock.runtime import Runtime
from xmodule.errortracker import exc_info_to_str
from xmodule.modulestore.locator import BlockUsageLocator

log = logging.getLogger(__name__)


def dummy_track(_event_type, _event):
    pass


class HTMLSnippet(object):
    """
    A base class defining an interface for an object that is able to present an
    html snippet, along with associated javascript and css
    """

    js = {}
    js_module_name = None

    css = {}

    @classmethod
    def get_javascript(cls):
        """
        Return a dictionary containing some of the following keys:

            coffee: A list of coffeescript fragments that should be compiled and
                    placed on the page

            js: A list of javascript fragments that should be included on the
            page

        All of these will be loaded onto the page in the CMS
        """
        # cdodge: We've moved the xmodule.coffee script from an outside directory into the xmodule area of common
        # this means we need to make sure that all xmodules include this dependency which had been previously implicitly
        # fulfilled in a different area of code
        coffee = cls.js.setdefault('coffee', [])
        fragment = resource_string(__name__, 'js/src/xmodule.coffee')

        if fragment not in coffee:
            coffee.insert(0, fragment)

        return cls.js

    @classmethod
    def get_css(cls):
        """
        Return a dictionary containing some of the following keys:

            css: A list of css fragments that should be applied to the html
                 contents of the snippet

            sass: A list of sass fragments that should be applied to the html
                  contents of the snippet

            scss: A list of scss fragments that should be applied to the html
                  contents of the snippet
        """
        return cls.css

    def get_html(self):
        """
        Return the html used to display this snippet
        """
        raise NotImplementedError(
            "get_html() must be provided by specific modules - not present in {0}"
            .format(self.__class__))


def shim_xmodule_js(fragment):
    """
    Set up the XBlock -> XModule shim on the supplied :class:`xblock.fragment.Fragment`
    """
    if not fragment.js_init_fn:
        fragment.initialize_js('XBlockToXModuleShim')


class XModuleMixin(XBlockMixin):
    """
    Fields and methods used by XModules internally.

    Adding this Mixin to an :class:`XBlock` allows it to cooperate with old-style :class:`XModules`
    """

    # Attributes for inspection of the descriptor

    # This indicates whether the xmodule is a problem-type.
    # It should respond to max_score() and grade(). It can be graded or ungraded
    # (like a practice problem).
    has_score = False

    # Class level variable

    # True if this descriptor always requires recalculation of grades, for
    # example if the score can change via an extrnal service, not just when the
    # student interacts with the module on the page.  A specific example is
    # FoldIt, which posts grade-changing updates through a separate API.
    always_recalculate_grades = False
    # The default implementation of get_icon_class returns the icon_class
    # attribute of the class
    #
    # This attribute can be overridden by subclasses, and
    # the function can also be overridden if the icon class depends on the data
    # in the module
    icon_class = 'other'

    display_name = String(
        display_name="Display Name",
        help="This name appears in the horizontal navigation at the top of the page.",
        scope=Scope.settings,
        # it'd be nice to have a useful default but it screws up other things; so,
        # use display_name_with_default for those
        default=None
    )

    @property
    def course_id(self):
        return self.runtime.course_id

    @property
    def id(self):
        return self.location.url()

    @property
    def category(self):
        return self.scope_ids.block_type

    @property
    def location(self):
        try:
            return Location(self.scope_ids.usage_id)
        except InvalidLocationError:
            if isinstance(self.scope_ids.usage_id, BlockUsageLocator):
                return self.scope_ids.usage_id
            else:
                return BlockUsageLocator(self.scope_ids.usage_id)

    @location.setter
    def location(self, value):
        self.scope_ids = self.scope_ids._replace(
            def_id=value,
            usage_id=value,
        )

    @property
    def url_name(self):
        if isinstance(self.location, Location):
            return self.location.name
        elif isinstance(self.location, BlockUsageLocator):
            return self.location.usage_id
        else:
            raise InsufficientSpecificationError()

    @property
    def display_name_with_default(self):
        """
        Return a display name for the module: use display_name if defined in
        metadata, otherwise convert the url name.
        """
        name = self.display_name
        if name is None:
            name = self.url_name.replace('_', ' ')
        return name

    def get_explicitly_set_fields_by_scope(self, scope=Scope.content):
        """
        Get a dictionary of the fields for the given scope which are set explicitly on this xblock. (Including
        any set to None.)
        """
        result = {}
        for field in self.fields.values():
            if (field.scope == scope and field.is_set_on(self)):
                result[field.name] = field.read_json(self)
        return result

    @property
    def xblock_kvs(self):
        """
        Use w/ caution. Really intended for use by the persistence layer.
        """
        # if caller wants kvs, caller's assuming it's up to date; so, decache it
        self.save()
        return self._field_data._kvs  # pylint: disable=protected-access

    def get_children(self):
        """Returns a list of XBlock instances for the children of
        this module"""

        if not self.has_children:
            return []

        if getattr(self, '_child_instances', None) is None:
            self._child_instances = []  # pylint: disable=attribute-defined-outside-init
            for child_loc in self.children:
                try:
                    child = self.runtime.get_block(child_loc)
                except ItemNotFoundError:
                    log.exception('Unable to load item {loc}, skipping'.format(loc=child_loc))
                    continue
                self._child_instances.append(child)

        return self._child_instances

    def get_required_module_descriptors(self):
        """Returns a list of XModuleDescriptor instances upon which this module depends, but are
        not children of this module"""
        return []

    def get_display_items(self):
        """
        Returns a list of descendent module instances that will display
        immediately inside this module.
        """
        items = []
        for child in self.get_children():
            items.extend(child.displayable_items())

        return items

    def displayable_items(self):
        """
        Returns list of displayable modules contained by this module. If this
        module is visible, should return [self].
        """
        return [self]

    def get_child_by(self, selector):
        """
        Return a child XBlock that matches the specified selector
        """
        for child in self.get_children():
            if selector(child):
                return child
        return None

    def get_icon_class(self):
        """
        Return a css class identifying this module in the context of an icon
        """
        return self.icon_class

    # Functions used in the LMS

    def get_score(self):
        """
        Score the student received on the problem, or None if there is no
        score.

        Returns:
          dictionary
             {'score': integer, from 0 to get_max_score(),
              'total': get_max_score()}

          NOTE (vshnayder): not sure if this was the intended return value, but
          that's what it's doing now.  I suspect that we really want it to just
          return a number.  Would need to change (at least) capa to match if we did that.
        """
        return None

    def max_score(self):
        """ Maximum score. Two notes:

            * This is generic; in abstract, a problem could be 3/5 points on one
              randomization, and 5/7 on another

            * In practice, this is a Very Bad Idea, and (a) will break some code
              in place (although that code should get fixed), and (b) break some
              analytics we plan to put in place.
        """
        return None

    def get_progress(self):
        """ Return a progress.Progress object that represents how far the
        student has gone in this module.  Must be implemented to get correct
        progress tracking behavior in nesting modules like sequence and
        vertical.

        If this module has no notion of progress, return None.
        """
        return None

    def bind_for_student(self, xmodule_runtime, field_data):
        """
        Set up this XBlock to act as an XModule instead of an XModuleDescriptor.

        :param xmodule_runtime: the runtime to use when accessing student facing methods
        :type xmodule_runtime: :class:`ModuleSystem`
        :param field_data: The :class:`FieldData` to use for all subsequent data access
        :type field_data: :class:`FieldData`
        """
        # pylint: disable=attribute-defined-outside-init
        self.xmodule_runtime = xmodule_runtime
        self._field_data = field_data


class ProxyAttribute(object):
    """
    A (python) descriptor that proxies attribute access.

    For example:

    class Foo(object):
        def __init__(self, value):
            self.foo_attr = value

    class Bar(object):
        foo = Foo('x')
        foo_attr = ProxyAttribute('foo', 'foo_attr')

    bar = Bar()

    assert bar.foo_attr == 'x'
    bar.foo_attr = 'y'
    assert bar.foo.foo_attr == 'y'
    del bar.foo_attr
    assert not hasattr(bar.foo, 'foo_attr')
    """
    def __init__(self, source, name):
        """
        :param source: The name of the attribute to proxy to
        :param name: The name of the attribute to proxy
        """
        self._source = source
        self._name = name

    def __get__(self, instance, owner):
        if instance is None:
            return self

        return getattr(getattr(instance, self._source), self._name)

    def __set__(self, instance, value):
        setattr(getattr(instance, self._source), self._name, value)

    def __delete__(self, instance):
        delattr(getattr(instance, self._source), self._name)


module_attr = partial(ProxyAttribute, '_xmodule')  # pylint: disable=invalid-name
descriptor_attr = partial(ProxyAttribute, 'descriptor')  # pylint: disable=invalid-name
module_runtime_attr = partial(ProxyAttribute, 'xmodule_runtime')  # pylint: disable=invalid-name


class XModule(XModuleMixin, HTMLSnippet, XBlock):  # pylint: disable=abstract-method
    """ Implements a generic learning module.

        Subclasses must at a minimum provide a definition for get_html in order
        to be displayed to users.

        See the HTML module for a simple example.
    """


    has_score = descriptor_attr('has_score')
    _field_data_cache = descriptor_attr('_field_data_cache')
    _field_data = descriptor_attr('_field_data')
    _dirty_fields = descriptor_attr('_dirty_fields')

    def __init__(self, descriptor, *args, **kwargs):
        """
        Construct a new xmodule

        runtime: An XBlock runtime allowing access to external resources

        descriptor: the XModuleDescriptor that this module is an instance of.

        field_data: A dictionary-like object that maps field names to values
            for those fields.
        """
        # Set the descriptor first so that we can proxy to it
        self.descriptor = descriptor
        super(XModule, self).__init__(*args, **kwargs)
        self._loaded_children = None
        self.system = self.runtime

    def __unicode__(self):
        return u'<x_module(id={0})>'.format(self.id)

    def handle_ajax(self, _dispatch, _data):
        """ dispatch is last part of the URL.
            data is a dictionary-like object with the content of the request"""
        return u""

    def xmodule_handler(self, request, suffix=None):
        """
        XBlock handler that wraps `handle_ajax`
        """
        response_data = self.handle_ajax(suffix, request.POST)
        return Response(response_data, content_type='application/json')

    def get_children(self):
        """
        Return module instances for all the children of this module.
        """
        if self._loaded_children is None:
            child_descriptors = self.get_child_descriptors()

            # This deliberately uses system.get_module, rather than runtime.get_block,
            # because we're looking at XModule children, rather than XModuleDescriptor children.
            # That means it can use the deprecated XModule apis, rather than future XBlock apis

            # TODO: Once we're in a system where this returns a mix of XModuleDescriptors
            # and XBlocks, we're likely to have to change this more
            children = [self.system.get_module(descriptor) for descriptor in child_descriptors]
            # get_module returns None if the current user doesn't have access
            # to the location.
            self._loaded_children = [c for c in children if c is not None]

        return self._loaded_children

    def get_child_descriptors(self):
        """
        Returns the descriptors of the child modules

        Overriding this changes the behavior of get_children and
        anything that uses get_children, such as get_display_items.

        This method will not instantiate the modules of the children
        unless absolutely necessary, so it is cheaper to call than get_children

        These children will be the same children returned by the
        descriptor unless descriptor.has_dynamic_children() is true.
        """
        return self.descriptor.get_children()

    def displayable_items(self):
        """
        Returns list of displayable modules contained by this module. If this
        module is visible, should return [self].
        """
        return [self.descriptor]

    # ~~~~~~~~~~~~~~~ XBlock API Wrappers ~~~~~~~~~~~~~~~~
    def student_view(self, context):
        """
        Return a fragment with the html from this XModule

        Doesn't yet add any of the javascript to the fragment, nor the css.
        Also doesn't expect any javascript binding, yet.

        Makes no use of the context parameter
        """
        return Fragment(self.get_html())


def policy_key(location):
    """
    Get the key for a location in a policy file.  (Since the policy file is
    specific to a course, it doesn't need the full location url).
    """
    return '{cat}/{name}'.format(cat=location.category, name=location.name)


Template = namedtuple("Template", "metadata data children")


class ResourceTemplates(object):
    """
    Gets the templates associated w/ a containing cls. The cls must have a 'template_dir_name' attribute.
    It finds the templates as directly in this directory under 'templates'.
    """
    @classmethod
    def templates(cls):
        """
        Returns a list of dictionary field: value objects that describe possible templates that can be used
        to seed a module of this type.

        Expects a class attribute template_dir_name that defines the directory
        inside the 'templates' resource directory to pull templates from
        """
        templates = []
        dirname = cls.get_template_dir()
        if dirname is not None:
            for template_file in resource_listdir(__name__, dirname):
                if not template_file.endswith('.yaml'):
                    log.warning("Skipping unknown template file %s", template_file)
                    continue
                template_content = resource_string(__name__, os.path.join(dirname, template_file))
                template = yaml.safe_load(template_content)
                template['template_id'] = template_file
                templates.append(template)

        return templates

    @classmethod
    def get_template_dir(cls):
        if getattr(cls, 'template_dir_name', None):
            dirname = os.path.join('templates', cls.template_dir_name)
            if not resource_isdir(__name__, dirname):
                log.warning("No resource directory {dir} found when loading {cls_name} templates".format(
                    dir=dirname,
                    cls_name=cls.__name__,
                ))
                return None
            else:
                return dirname
        else:
            return None

    @classmethod
    def get_template(cls, template_id):
        """
        Get a single template by the given id (which is the file name identifying it w/in the class's
        template_dir_name)

        """
        dirname = cls.get_template_dir()
        if dirname is not None:
            try:
                template_content = resource_string(__name__, os.path.join(dirname, template_id))
            except IOError:
                return None
            template = yaml.safe_load(template_content)
            template['template_id'] = template_id
            return template
        else:
            return None


class XModuleDescriptor(XModuleMixin, HTMLSnippet, ResourceTemplates, XBlock):
    """
    An XModuleDescriptor is a specification for an element of a course. This
    could be a problem, an organizational element (a group of content), or a
    segment of video, for example.

    XModuleDescriptors are independent and agnostic to the current student state
    on a problem. They handle the editing interface used by instructors to
    create a problem, and can generate XModules (which do know about student
    state).
    """
    entry_point = "xmodule.v1"
    module_class = XModule


    # VS[compat].  Backwards compatibility code that can go away after
    # importing 2012 courses.
    # A set of metadata key conversions that we want to make
    metadata_translations = {
        'slug': 'url_name',
        'name': 'display_name',
    }

    # ============================= STRUCTURAL MANIPULATION ===================
    def __init__(self, *args, **kwargs):
        """
        Construct a new XModuleDescriptor. The only required arguments are the
        system, used for interaction with external resources, and the
        definition, which specifies all the data needed to edit and display the
        problem (but none of the associated metadata that handles recordkeeping
        around the problem).

        This allows for maximal flexibility to add to the interface while
        preserving backwards compatibility.

        runtime: A DescriptorSystem for interacting with external resources

        field_data: A dictionary-like object that maps field names to values
            for those fields.

        XModuleDescriptor.__init__ takes the same arguments as xblock.core:XBlock.__init__
        """
        super(XModuleDescriptor, self).__init__(*args, **kwargs)
        self.system = self.runtime
        # update_version is the version which last updated this xblock v prev being the penultimate updater
        # leaving off original_version since it complicates creation w/o any obv value yet and is computable
        # by following previous until None
        # definition_locator is only used by mongostores which separate definitions from blocks
        self.edited_by = self.edited_on = self.previous_version = self.update_version = self.definition_locator = None
        self.xmodule_runtime = None

    def has_dynamic_children(self):
        """
        Returns True if this descriptor has dynamic children for a given
        student when the module is created.

        Returns False if the children of this descriptor are the same
        children that the module will return for any student.
        """
        return False

    @classmethod
    def _translate(cls, key):
        'VS[compat]'
        return cls.metadata_translations.get(key, key)

    # ================================= XML PARSING ============================
    @classmethod
    def parse_xml(cls, node, runtime, keys):
        """
        Interpret the parsed XML in `node`, creating an XModuleDescriptor.
        """
        xml = etree.tostring(node)
        # TODO: change from_xml to not take org and course, it can use self.system.
        block = cls.from_xml(xml, runtime, runtime.org, runtime.course)
        return block

    @classmethod
    def from_xml(cls, xml_data, system, org=None, course=None):
        """
        Creates an instance of this descriptor from the supplied xml_data.
        This may be overridden by subclasses.

        xml_data: A string of xml that will be translated into data and children
            for this module

        system is an XMLParsingSystem

        org and course are optional strings that will be used in the generated
            module's url identifiers
        """
        raise NotImplementedError('Modules must implement from_xml to be parsable from xml')

    def export_to_xml(self, resource_fs):
        """
        Returns an xml string representing this module, and all modules
        underneath it.  May also write required resources out to resource_fs.

        Assumes that modules have single parentage (that no module appears twice
        in the same course), and that it is thus safe to nest modules as xml
        children as appropriate.

        The returned XML should be able to be parsed back into an identical
        XModuleDescriptor using the from_xml method with the same system, org,
        and course
        """
        raise NotImplementedError('Modules must implement export_to_xml to enable xml export')

    # =============================== BUILTIN METHODS ==========================
    def __eq__(self, other):
        return (self.scope_ids == other.scope_ids and
                self.fields.keys() == other.fields.keys() and
                all(getattr(self, field.name) == getattr(other, field.name)
                    for field in self.fields.values()))

    def __repr__(self):
        return (
            "{0.__class__.__name__}("
            "{0.runtime!r}, "
            "{0._field_data!r}, "
            "{0.scope_ids!r}"
            ")".format(self)
        )

    @property
    def non_editable_metadata_fields(self):
        """
        Return the list of fields that should not be editable in Studio.

        When overriding, be sure to append to the superclasses' list.
        """
        # We are not allowing editing of xblock tag and name fields at this time (for any component).
        return [XBlock.tags, XBlock.name]


    @property
    def editable_metadata_fields(self):
        """
        Returns the metadata fields to be edited in Studio. These are fields with scope `Scope.settings`.

        Can be limited by extending `non_editable_metadata_fields`.
        """
        def jsonify_value(field, json_choice):
            if isinstance(json_choice, dict) and 'value' in json_choice:
                json_choice = dict(json_choice)  # make a copy so below doesn't change the original
                json_choice['value'] = field.to_json(json_choice['value'])
            else:
                json_choice = field.to_json(json_choice)
            return json_choice

        metadata_fields = {}

        # Only use the fields from this class, not mixins
        fields = getattr(self, 'unmixed_class', self.__class__).fields

        for field in fields.values():

            if field.scope != Scope.settings or field in self.non_editable_metadata_fields:
                continue

            # gets the 'default_value' and 'explicitly_set' attrs
            metadata_fields[field.name] = self.runtime.get_field_provenance(self, field)
            metadata_fields[field.name]['field_name'] = field.name
            metadata_fields[field.name]['display_name'] = field.display_name
            metadata_fields[field.name]['help'] = field.help
            metadata_fields[field.name]['value'] = field.read_json(self)

            # We support the following editors:
            # 1. A select editor for fields with a list of possible values (includes Booleans).
            # 2. Number editors for integers and floats.
            # 3. A generic string editor for anything else (editing JSON representation of the value).
            editor_type = "Generic"
            values = field.values
            if isinstance(values, (tuple, list)) and len(values) > 0:
                editor_type = "Select"
                values = [jsonify_value(field, json_choice) for json_choice in values]
            elif isinstance(field, Integer):
                editor_type = "Integer"
            elif isinstance(field, Float):
                editor_type = "Float"
            elif isinstance(field, List):
                editor_type = "List"
            elif isinstance(field, RelativeTime):
                editor_type = "RelativeTime"
            metadata_fields[field.name]['type'] = editor_type
            metadata_fields[field.name]['options'] = [] if values is None else values

        return metadata_fields

    # ~~~~~~~~~~~~~~~ XModule Indirection ~~~~~~~~~~~~~~~~
    @property
    def _xmodule(self):
        """
        Returns the XModule corresponding to this descriptor. Expects that the system
        already supports all of the attributes needed by xmodules
        """
        assert self.xmodule_runtime is not None
        assert self.xmodule_runtime.error_descriptor_class is not None
        if self.xmodule_runtime.xmodule_instance is None:
            try:
                self.xmodule_runtime.xmodule_instance = self.xmodule_runtime.construct_xblock_from_class(
                    self.module_class,
                    descriptor=self,
                    scope_ids=self.scope_ids,
                    field_data=self._field_data,
                )
                self.xmodule_runtime.xmodule_instance.save()
            except Exception:  # pylint: disable=broad-except
                if isinstance(self, self.xmodule_runtime.error_descriptor_class):
                    log.exception('Error creating an ErrorModule from an ErrorDescriptor')
                    raise

                log.exception('Error creating xmodule')
                descriptor = self.xmodule_runtime.error_descriptor_class.from_descriptor(
                    self,
                    error_msg=exc_info_to_str(sys.exc_info())
                )
                descriptor.xmodule_runtime = self.xmodule_runtime
                self.xmodule_runtime.xmodule_instance = descriptor._xmodule  # pylint: disable=protected-access
        return self.xmodule_runtime.xmodule_instance

    course_id = module_attr('course_id')
    displayable_items = module_attr('displayable_items')
    get_display_items = module_attr('get_display_items')
    get_icon_class = module_attr('get_icon_class')
    get_progress = module_attr('get_progress')
    get_score = module_attr('get_score')
    handle_ajax = module_attr('handle_ajax')
    max_score = module_attr('max_score')
    student_view = module_attr('student_view')
    get_child_descriptors = module_attr('get_child_descriptors')
    xmodule_handler = module_attr('xmodule_handler')

    # ~~~~~~~~~~~~~~~ XBlock API Wrappers ~~~~~~~~~~~~~~~~
    def studio_view(self, _context):
        """
        Return a fragment with the html from this XModuleDescriptor's editing view

        Doesn't yet add any of the javascript to the fragment, nor the css.
        Also doesn't expect any javascript binding, yet.

        Makes no use of the context parameter
        """
        return Fragment(self.get_html())


class ConfigurableFragmentWrapper(object):  # pylint: disable=abstract-method
    """
    Runtime mixin that allows for composition of many `wrap_child` wrappers
    """
    def __init__(self, wrappers=None, **kwargs):
        """
        :param wrappers: A list of wrappers, where each wrapper is:

            def wrapper(block, view, frag, context):
                ...
                return wrapped_frag
        """
        super(ConfigurableFragmentWrapper, self).__init__(**kwargs)
        if wrappers is not None:
            self.wrappers = wrappers
        else:
            self.wrappers = []

    def wrap_child(self, block, view, frag, context):
        """
        See :func:`Runtime.wrap_child`
        """
        for wrapper in self.wrappers:
            frag = wrapper(block, view, frag, context)

        return frag


class DescriptorSystem(ConfigurableFragmentWrapper, Runtime):  # pylint: disable=abstract-method
    """
    Base class for :class:`Runtime`s to be used with :class:`XModuleDescriptor`s
    """

    def __init__(self, load_item, resources_fs, error_tracker, **kwargs):
        """
        load_item: Takes a Location and returns an XModuleDescriptor

        resources_fs: A Filesystem object that contains all of the
            resources needed for the course

        error_tracker: A hook for tracking errors in loading the descriptor.
            Used for example to get a list of all non-fatal problems on course
            load, and display them to the user.

            A function of (error_msg). errortracker.py provides a
            handy make_error_tracker() function.

            Patterns for using the error handler:
               try:
                  x = access_some_resource()
                  check_some_format(x)
               except SomeProblem as err:
                  msg = 'Grommet {0} is broken: {1}'.format(x, str(err))
                  log.warning(msg)  # don't rely on tracker to log
                        # NOTE: we generally don't want content errors logged as errors
                  self.system.error_tracker(msg)
                  # work around
                  return 'Oops, couldn't load grommet'

               OR, if not in an exception context:

               if not check_something(thingy):
                  msg = "thingy {0} is broken".format(thingy)
                  log.critical(msg)
                  self.system.error_tracker(msg)

               NOTE: To avoid duplication, do not call the tracker on errors
               that you're about to re-raise---let the caller track them.
        """

        # Right now, usage_store is unused, and field_data is always supplanted
        # with an explicit field_data during construct_xblock, so None's suffice.
        super(DescriptorSystem, self).__init__(usage_store=None, field_data=None, **kwargs)

        self.load_item = load_item
        self.resources_fs = resources_fs
        self.error_tracker = error_tracker

    def get_block(self, block_id):
        """See documentation for `xblock.runtime:Runtime.get_block`"""
        return self.load_item(block_id)

    def get_field_provenance(self, xblock, field):
        """
        For the given xblock, return a dict for the field's current state:
        {
            'default_value': what json'd value will take effect if field is unset: either the field default or
            inherited value,
            'explicitly_set': boolean for whether the current value is set v default/inherited,
        }
        :param xblock:
        :param field:
        """
        # in runtime b/c runtime contains app-specific xblock behavior. Studio's the only app
        # which needs this level of introspection right now. runtime also is 'allowed' to know
        # about the kvs, dbmodel, etc.

        result = {}
        result['explicitly_set'] = xblock._field_data.has(xblock, field.name)
        try:
            block_inherited = xblock.xblock_kvs.inherited_settings
        except AttributeError:  # if inherited_settings doesn't exist on kvs
            block_inherited = {}
        if field.name in block_inherited:
            result['default_value'] = block_inherited[field.name]
        else:
            result['default_value'] = field.to_json(field.default)
        return result

    def render(self, block, view_name, context=None):
        if view_name == 'student_view':
            assert block.xmodule_runtime is not None
            if isinstance(block, (XModule, XModuleDescriptor)):
                to_render = block._xmodule
            else:
                to_render = block
            return block.xmodule_runtime.render(to_render, view_name, context)
        else:
            return super(DescriptorSystem, self).render(block, view_name, context)


class XMLParsingSystem(DescriptorSystem):
    def __init__(self, process_xml, policy, **kwargs):
        """
        policy: a policy dictionary for overriding xml metadata

        process_xml: Takes an xml string, and returns a XModuleDescriptor
            created from that xml
        """

        super(XMLParsingSystem, self).__init__(**kwargs)
        self.process_xml = process_xml
        self.policy = policy


class ModuleSystem(ConfigurableFragmentWrapper, Runtime):  # pylint: disable=abstract-method
    """
    This is an abstraction such that x_modules can function independent
    of the courseware (e.g. import into other types of courseware, LMS,
    or if we want to have a sandbox server for user-contributed content)

    ModuleSystem objects are passed to x_modules to provide access to system
    functionality.

    Note that these functions can be closures over e.g. a django request
    and user, or other environment-specific info.
    """
    def __init__(
            self, static_url, track_function, get_module, render_template,
            replace_urls, user=None, filestore=None,
            debug=False, hostname="", xqueue=None, publish=None, node_path="",
            anonymous_student_id='', course_id=None,
            open_ended_grading_interface=None, s3_interface=None,
            cache=None, can_execute_unsafe_code=None, replace_course_urls=None,
            replace_jump_to_id_urls=None, error_descriptor_class=None, **kwargs):
        """
        Create a closure around the system environment.

        static_url - the base URL to static assets

        track_function - function of (event_type, event), intended for logging
                         or otherwise tracking the event.
                         TODO: Not used, and has inconsistent args in different
                         files.  Update or remove.

        get_module - function that takes a descriptor and returns a corresponding
                         module instance object.  If the current user does not have
                         access to that location, returns None.

        render_template - a function that takes (template_file, context), and
                         returns rendered html.

        user - The user to base the random number generator seed off of for this
                         request

        filestore - A filestore ojbect.  Defaults to an instance of OSFS based
                         at settings.DATA_DIR.

        xqueue - Dict containing XqueueInterface object, as well as parameters
                    for the specific StudentModule:
                    xqueue = {'interface': XQueueInterface object,
                              'callback_url': Callback into the LMS,
                              'queue_name': Target queuename in Xqueue}

        replace_urls - TEMPORARY - A function like static_replace.replace_urls
                         that capa_module can use to fix up the static urls in
                         ajax results.

        anonymous_student_id - Used for tracking modules with student id

        course_id - the course_id containing this module

        publish(event) - A function that allows XModules to publish events (such as grade changes)

        cache - A cache object with two methods:
            .get(key) returns an object from the cache or None.
            .set(key, value, timeout_secs=None) stores a value in the cache with a timeout.

        can_execute_unsafe_code - A function returning a boolean, whether or
            not to allow the execution of unsafe, unsandboxed code.

        error_descriptor_class - The class to use to render XModules with errors

        """

        # Right now, usage_store is unused, and field_data is always supplanted
        # with an explicit field_data during construct_xblock, so None's suffice.
        super(ModuleSystem, self).__init__(usage_store=None, field_data=None, **kwargs)

        self.STATIC_URL = static_url
        self.xqueue = xqueue
        self.track_function = track_function
        self.filestore = filestore
        self.get_module = get_module
        self.render_template = render_template
        self.DEBUG = self.debug = debug
        self.HOSTNAME = self.hostname = hostname
        self.seed = user.id if user is not None else 0
        self.replace_urls = replace_urls
        self.node_path = node_path
        self.anonymous_student_id = anonymous_student_id
        self.course_id = course_id
        self.user_is_staff = user is not None and user.is_staff

        if publish is None:
            publish = lambda e: None

        self.publish = publish

        self.open_ended_grading_interface = open_ended_grading_interface
        self.s3_interface = s3_interface

        self.cache = cache or DoNothingCache()
        self.can_execute_unsafe_code = can_execute_unsafe_code or (lambda: False)
        self.replace_course_urls = replace_course_urls
        self.replace_jump_to_id_urls = replace_jump_to_id_urls
        self.error_descriptor_class = error_descriptor_class
        self.xmodule_instance = None

    def get(self, attr):
        """	provide uniform access to attributes (like etree)."""
        return self.__dict__.get(attr)

    def set(self, attr, val):
        """provide uniform access to attributes (like etree)"""
        self.__dict__[attr] = val

    def __repr__(self):
        return repr(self.__dict__)

    def __str__(self):
        return str(self.__dict__)

    @property
    def ajax_url(self):
        """
        The url prefix to be used by XModules to call into handle_ajax
        """
        return self.handler_url(self.xmodule_instance, 'xmodule_handler', '', '').rstrip('/?')


class DoNothingCache(object):
    """A duck-compatible object to use in ModuleSystem when there's no cache."""
    def get(self, _key):
        return None

    def set(self, key, value, timeout=None):
        pass
