How to use the xblock.core.String function in XBlock

display_name=_("Verification Key"),
        help=_(
            "Enter the Base64 encoded Verification Key from your Azure Management Portal"
        ),
        default="",
        scope=Scope.settings
    )
    protection_type = String(
        display_name=_("Protection Type"),
        help=_(
            "This can be either blank (meaning unprotected), 'AES', or 'PlayReady'"
        ),
        default="",
        scope=Scope.settings
    )
    token_issuer = String(
        display_name=_("Token Issuer"),
        help=_(
            "This value must match what is in the 'Content Protection' area of the Azure Media Services portal"
        ),
        default="http://openedx.microsoft.com/",
        scope=Scope.settings
    )
    token_scope = String(
        display_name=_("Token Scope"),
        help=_(
            "This value must match what is in the 'Content Protection' area of the Azure Media Services portal"
        ),
        default="urn:xblock-azure-media-services",
        scope=Scope.settings
    )
    captions = List(

"""
    The xBlock to play videos from Azure Media Services.
    """

    RESOURCE = 'https://rest.media.azure.net'

    display_name = String(
        display_name=_("Display Name"),
        help=_(
            "Enter the name that students see for this component. "
            "Analytics reports may also use the display name to identify this component."
        ),
        scope=Scope.settings,
        default=_("Azure Media Services Video Player"),
    )
    video_url = String(
        display_name=_("Video Url"),
        help=_(
            "Enter the URL to your published video on Azure Media Services"
        ),
        default="",
        scope=Scope.settings
    )
    edx_video_id = String(
        default="",
        scope=Scope.settings
    )
    # Ultimately this should come via some secure means, but this is OK for a PoC
    verification_key = String(
        display_name=_("Verification Key"),
        help=_(
            "Enter the Base64 encoded Verification Key from your Azure Management Portal"

default=_("Azure Media Services Video Player"),
    )
    video_url = String(
        display_name=_("Video Url"),
        help=_(
            "Enter the URL to your published video on Azure Media Services"
        ),
        default="",
        scope=Scope.settings
    )
    edx_video_id = String(
        default="",
        scope=Scope.settings
    )
    # Ultimately this should come via some secure means, but this is OK for a PoC
    verification_key = String(
        display_name=_("Verification Key"),
        help=_(
            "Enter the Base64 encoded Verification Key from your Azure Management Portal"
        ),
        default="",
        scope=Scope.settings
    )
    protection_type = String(
        display_name=_("Protection Type"),
        help=_(
            "This can be either blank (meaning unprotected), 'AES', or 'PlayReady'"
        ),
        default="",
        scope=Scope.settings
    )
    token_issuer = String(

log = logging.getLogger(__name__)
loader = ResourceLoader(__name__)

# According to edx-platform vertical xblocks
CLASS_PRIORITY = ['video']


@XBlock.needs('i18n')
class AMSXBlock(StudioEditableXBlockMixin, XBlock):
    """
    The xBlock to play videos from Azure Media Services.
    """

    RESOURCE = 'https://rest.media.azure.net'

    display_name = String(
        display_name=_("Display Name"),
        help=_(
            "Enter the name that students see for this component. "
            "Analytics reports may also use the display name to identify this component."
        ),
        scope=Scope.settings,
        default=_("Azure Media Services Video Player"),
    )
    video_url = String(
        display_name=_("Video Url"),
        help=_(
            "Enter the URL to your published video on Azure Media Services"
        ),
        default="",
        scope=Scope.settings
    )

display_name=_("Protection Type"),
        help=_(
            "This can be either blank (meaning unprotected), 'AES', or 'PlayReady'"
        ),
        default="",
        scope=Scope.settings
    )
    token_issuer = String(
        display_name=_("Token Issuer"),
        help=_(
            "This value must match what is in the 'Content Protection' area of the Azure Media Services portal"
        ),
        default="http://openedx.microsoft.com/",
        scope=Scope.settings
    )
    token_scope = String(
        display_name=_("Token Scope"),
        help=_(
            "This value must match what is in the 'Content Protection' area of the Azure Media Services portal"
        ),
        default="urn:xblock-azure-media-services",
        scope=Scope.settings
    )
    captions = List(
        display_name=_("Captions"),
        help=_("A list of caption definitions"),
        scope=Scope.settings
    )
    caption_ids = List(
        default=[],
        scope=Scope.settings
    )

help=_(
            "Enter the name that students see for this component. "
            "Analytics reports may also use the display name to identify this component."
        ),
        scope=Scope.settings,
        default=_("Azure Media Services Video Player"),
    )
    video_url = String(
        display_name=_("Video Url"),
        help=_(
            "Enter the URL to your published video on Azure Media Services"
        ),
        default="",
        scope=Scope.settings
    )
    edx_video_id = String(
        default="",
        scope=Scope.settings
    )
    # Ultimately this should come via some secure means, but this is OK for a PoC
    verification_key = String(
        display_name=_("Verification Key"),
        help=_(
            "Enter the Base64 encoded Verification Key from your Azure Management Portal"
        ),
        default="",
        scope=Scope.settings
    )
    protection_type = String(
        display_name=_("Protection Type"),
        help=_(
            "This can be either blank (meaning unprotected), 'AES', or 'PlayReady'"

captions = List(
        display_name=_("Captions"),
        help=_("A list of caption definitions"),
        scope=Scope.settings
    )
    caption_ids = List(
        default=[],
        scope=Scope.settings
    )
    transcripts_enabled = Boolean(
        display_name=_("Transcripts enabled"),
        help=_("Transcripts switch"),
        default=True,
        scope=Scope.settings
    )
    download_url = String(
        display_name=_("Video Download URL"),
        help=_("A download URL"),
        scope=Scope.settings
    )

    # These are what become visible in the Mixin editor
    editable_fields = (
        'display_name', 'video_url', 'verification_key', 'protection_type',
        'token_issuer', 'token_scope', 'captions', 'transcripts_enabled', 'download_url',
        'edx_video_id', 'caption_ids'
    )

    def studio_view(self, context):
        """
        Render a form for editing this XBlock.
        """

scope=Scope.settings
    )
    edx_video_id = String(
        default="",
        scope=Scope.settings
    )
    # Ultimately this should come via some secure means, but this is OK for a PoC
    verification_key = String(
        display_name=_("Verification Key"),
        help=_(
            "Enter the Base64 encoded Verification Key from your Azure Management Portal"
        ),
        default="",
        scope=Scope.settings
    )
    protection_type = String(
        display_name=_("Protection Type"),
        help=_(
            "This can be either blank (meaning unprotected), 'AES', or 'PlayReady'"
        ),
        default="",
        scope=Scope.settings
    )
    token_issuer = String(
        display_name=_("Token Issuer"),
        help=_(
            "This value must match what is in the 'Content Protection' area of the Azure Media Services portal"
        ),
        default="http://openedx.microsoft.com/",
        scope=Scope.settings
    )
    token_scope = String(

class HelloWorldBlock(XBlock):
    """A simple block: just show some fixed content."""
    def fallback_view(self, view_name, context=None):  # pylint: disable=W0613
        """Provide a fallback view handler"""
        return Fragment(u"Hello, world!")


class HtmlBlock(XBlock):
    """Render content as HTML.

    The content can have $PLACEHOLDERS, which will be substituted with values
    from the context.

    """

    content = String(help="The HTML to display", scope=Scope.content, default=u"<b>DEFAULT</b>")

    def fallback_view(self, view_name, context=None):
        """Provide a fallback view handler"""
        context = context or {}
        return Fragment(Template(self.content).substitute(**context))

    @classmethod
    def parse_xml(cls, node, runtime, keys, id_generator):
        """
        Parse the XML for an HTML block.

        The entire subtree under `node` is re-serialized, and set as the
        content of the XBlock.

        """
        block = runtime.construct_xblock_from_class(cls, keys)