How to use the xblock.fields.Sentinel function in XBlock

NONE: Identifies data agnostic to the user of the :class:`.XBlock`.  The
        data is related to no particular user.  All users see the same data.
        For instance, the definition of a problem.

    ONE: Identifies data particular to a single user of the :class:`.XBlock`.
        For instance, a student's answer to a problem.

    ALL: Identifies data aggregated while the block is used by many users.
        The data is related to all the users.  For instance, a count of how
        many students have answered a question, or a histogram of the answers
        submitted by all students.

    """
    NONE = Sentinel('UserScope.NONE')
    ONE = Sentinel('UserScope.ONE')
    ALL = Sentinel('UserScope.ALL')

    @classmethod
    def scopes(cls):
        """
        Return a list of valid/understood class scopes.
        Why do we need this? I believe it is not used anywhere.
        """
        return [cls.NONE, cls.ONE, cls.ALL]


UNSET = Sentinel("fields.UNSET")


ScopeBase = namedtuple('ScopeBase', 'user block name')

cls(user, block)
            for user in UserScope.scopes()
            for block in BlockScope.scopes()
            if cls(user, block) not in named_scopes
        ]

    def __new__(cls, user, block, name=None):
        """Create a new Scope, with an optional name."""

        if name is None:
            name = u'{}_{}'.format(user, block)

        return ScopeBase.__new__(cls, user, block, name)

    children = Sentinel('Scope.children')
    parent = Sentinel('Scope.parent')

    def __unicode__(self):
        return self.name

    def __eq__(self, other):
        return isinstance(other, Scope) and self.user == other.user and self.block == other.block


class ScopeIds(namedtuple('ScopeIds', 'user_id block_type def_id usage_id')):
    """
    A simple wrapper to collect all of the ids needed to correctly identify an XBlock
    (or other classes deriving from ScopedStorageMixin) to a FieldData.
    These identifiers match up with BlockScope and UserScope attributes, so that,
    for instance, the `def_id` identifies scopes that use BlockScope.DEFINITION.
    """
    __slots__ = ()

return named_scopes + [
            cls(user, block)
            for user in UserScope.scopes()
            for block in BlockScope.scopes()
            if cls(user, block) not in named_scopes
        ]

    def __new__(cls, user, block, name=None):
        """Create a new Scope, with an optional name."""

        if name is None:
            name = u'{}_{}'.format(user, block)

        return ScopeBase.__new__(cls, user, block, name)

    children = Sentinel('Scope.children')
    parent = Sentinel('Scope.parent')

    def __unicode__(self):
        return self.name

    def __eq__(self, other):
        return isinstance(other, Scope) and self.user == other.user and self.block == other.block


class ScopeIds(namedtuple('ScopeIds', 'user_id block_type def_id usage_id')):
    """
    A simple wrapper to collect all of the ids needed to correctly identify an XBlock
    (or other classes deriving from ScopedStorageMixin) to a FieldData.
    These identifiers match up with BlockScope and UserScope attributes, so that,
    for instance, the `def_id` identifies scopes that use BlockScope.DEFINITION.
    """

A simple wrapper to collect all of the ids needed to correctly identify an XBlock
    (or other classes deriving from ScopedStorageMixin) to a FieldData.
    These identifiers match up with BlockScope and UserScope attributes, so that,
    for instance, the `def_id` identifies scopes that use BlockScope.DEFINITION.
    """
    __slots__ = ()


# Define special reference that can be used as a field's default in field
# definition to signal that the field should default to a unique string value
# calculated at runtime.
UNIQUE_ID = Sentinel("fields.UNIQUE_ID")

# define a placeholder ('nil') value to indicate when nothing has been stored
# in the cache ("None" may be a valid value in the cache, so we cannot use it).
NO_CACHE_VALUE = Sentinel("fields.NO_CACHE_VALUE")

# define a placeholder value that indicates that a value is explicitly dirty,
# because it was explicitly set
EXPLICITLY_SET = Sentinel("fields.EXPLICITLY_SET")

# Fields that cannot have runtime-generated defaults. These are special,
# because they define the structure of XBlock trees.
NO_GENERATED_DEFAULTS = ('parent', 'children')


class Field(Nameable):
    """
    A field class that can be used as a class attribute to define what data the
    class will want to refer to.

    When the class is instantiated, it will be available as an instance

NONE: Identifies data agnostic to the user of the :class:`.XBlock`.  The
        data is related to no particular user.  All users see the same data.
        For instance, the definition of a problem.

    ONE: Identifies data particular to a single user of the :class:`.XBlock`.
        For instance, a student's answer to a problem.

    ALL: Identifies data aggregated while the block is used by many users.
        The data is related to all the users.  For instance, a count of how
        many students have answered a question, or a histogram of the answers
        submitted by all students.

    """
    NONE = Sentinel('UserScope.NONE')
    ONE = Sentinel('UserScope.ONE')
    ALL = Sentinel('UserScope.ALL')

    @classmethod
    def scopes(cls):
        """
        Return a list of valid/understood class scopes.
        Why do we need this? I believe it is not used anywhere.
        """
        return [cls.NONE, cls.ONE, cls.ALL]


UNSET = Sentinel("fields.UNSET")


ScopeBase = namedtuple('ScopeBase', 'user block name')

"""
    __slots__ = ()


# Define special reference that can be used as a field's default in field
# definition to signal that the field should default to a unique string value
# calculated at runtime.
UNIQUE_ID = Sentinel("fields.UNIQUE_ID")

# define a placeholder ('nil') value to indicate when nothing has been stored
# in the cache ("None" may be a valid value in the cache, so we cannot use it).
NO_CACHE_VALUE = Sentinel("fields.NO_CACHE_VALUE")

# define a placeholder value that indicates that a value is explicitly dirty,
# because it was explicitly set
EXPLICITLY_SET = Sentinel("fields.EXPLICITLY_SET")

# Fields that cannot have runtime-generated defaults. These are special,
# because they define the structure of XBlock trees.
NO_GENERATED_DEFAULTS = ('parent', 'children')


class Field(Nameable):
    """
    A field class that can be used as a class attribute to define what data the
    class will want to refer to.

    When the class is instantiated, it will be available as an instance
    attribute of the same name, by proxying through to the field-data service on
    the containing object.

    Parameters:

:class:`.Scope` for a field.

    USAGE: The data is related to a particular use of a block in a course.

    DEFINITION: The data is related to the definition of the block.  Although
        unusual, one block definition can be used in more than one place in a
        course.

    TYPE: The data is related to all instances of this type of XBlock.

    ALL: The data is common to all blocks.  This can be useful for storing
        information that is purely about the student.

    """
    USAGE = Sentinel('BlockScope.USAGE')
    DEFINITION = Sentinel('BlockScope.DEFINITION')
    TYPE = Sentinel('BlockScope.TYPE')
    ALL = Sentinel('BlockScope.ALL')

    @classmethod
    def scopes(cls):
        """
        Return a list of valid/understood class scopes.
        """
        # Why do we need this? This should either
        # * Be bubbled to the places where it is used (AcidXBlock).
        # * Be automatic. Look for all members of a type.
        return [cls.USAGE, cls.DEFINITION, cls.TYPE, cls.ALL]


class UserScope(object):
    """

def __eq__(self, other):
        """ Equality is based on being of the same class, and having same name
        """
        return isinstance(other, Sentinel) and self.name == other.name

class ScopeIds(namedtuple('ScopeIds', 'user_id block_type def_id usage_id')):
    """
    A simple wrapper to collect all of the ids needed to correctly identify an XBlock
    (or other classes deriving from ScopedStorageMixin) to a FieldData.
    These identifiers match up with BlockScope and UserScope attributes, so that,
    for instance, the `def_id` identifies scopes that use BlockScope.DEFINITION.
    """
    __slots__ = ()


# Define special reference that can be used as a field's default in field
# definition to signal that the field should default to a unique string value
# calculated at runtime.
UNIQUE_ID = Sentinel("fields.UNIQUE_ID")

# define a placeholder ('nil') value to indicate when nothing has been stored
# in the cache ("None" may be a valid value in the cache, so we cannot use it).
NO_CACHE_VALUE = Sentinel("fields.NO_CACHE_VALUE")

# define a placeholder value that indicates that a value is explicitly dirty,
# because it was explicitly set
EXPLICITLY_SET = Sentinel("fields.EXPLICITLY_SET")

# Fields that cannot have runtime-generated defaults. These are special,
# because they define the structure of XBlock trees.
NO_GENERATED_DEFAULTS = ('parent', 'children')


class Field(Nameable):
    """

:class:`.Scope` for a field.

    NONE: Identifies data agnostic to the user of the :class:`.XBlock`.  The
        data is related to no particular user.  All users see the same data.
        For instance, the definition of a problem.

    ONE: Identifies data particular to a single user of the :class:`.XBlock`.
        For instance, a student's answer to a problem.

    ALL: Identifies data aggregated while the block is used by many users.
        The data is related to all the users.  For instance, a count of how
        many students have answered a question, or a histogram of the answers
        submitted by all students.

    """
    NONE = Sentinel('UserScope.NONE')
    ONE = Sentinel('UserScope.ONE')
    ALL = Sentinel('UserScope.ALL')

    @classmethod
    def scopes(cls):
        """
        Return a list of valid/understood class scopes.
        Why do we need this? I believe it is not used anywhere.
        """
        return [cls.NONE, cls.ONE, cls.ALL]


UNSET = Sentinel("fields.UNSET")


ScopeBase = namedtuple('ScopeBase', 'user block name')