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

class TranscriptsMixin(XBlock):
    """
    TranscriptsMixin class to encapsulate transcripts-related logic.
    """

    THREE_PLAY_MEDIA_API_DOMAIN = 'https://static.3playmedia.com/'

    threeplaymedia_streaming = Boolean(
        default=False,
        display_name=_('Direct 3PlayMedia'),
        scope=Scope.content,
        help=_("Direct <a href="http://www.3playmedia.com/">3PlayMedia</a> transcripts usage enabled.")
    )

    threeplaymedia_apikey = String(
        default='',
        display_name=_('3PlayMedia API Key'),
        help=_('You can generate a client token following official documentation of your video platform\'s API.'),
        scope=Scope.content,
        resettable_editor=False
    )

    threeplaymedia_file_id = String(
        default='',
        display_name=_('File Id'),
        help=_('3playmedia file id for download bind transcripts.'),
        scope=Scope.content,
        resettable_editor=False
    )

    @staticmethod

)

    action = String(display_name="Action",
                    help="Select the action to be performed "
                    "when the condition is met",
                    scope=Scope.content,
                    default="display_message",
                    values_provider=_actions_generator)

    condition = String(display_name="Flow control condition",
                       help="Select a conditon to evaluate",
                       scope=Scope.content,
                       default='single_problem',
                       values_provider=_conditions_generator)

    operator = String(display_name="Comparison type",
                      help="Select an operator for the condition",
                      scope=Scope.content,
                      default='eq',
                      values_provider=_operators_generator)

    ref_value = Integer(help="Enter the value to be used in "
                        "the comparison. (From 0 to 100)",
                        default=0,
                        scope=Scope.content,
                        display_name="Score percentage")

    tab_to = Integer(help="Number of unit tab to redirect to. (1, 2, 3...)",
                     default=1,
                     scope=Scope.content,
                     display_name="Tab to redirect to")

"the comparison. (From 0 to 100)",
                        default=0,
                        scope=Scope.content,
                        display_name="Score percentage")

    tab_to = Integer(help="Number of unit tab to redirect to. (1, 2, 3...)",
                     default=1,
                     scope=Scope.content,
                     display_name="Tab to redirect to")

    target_url = String(help="URL to redirect to, supports relative "
                        "or absolute urls",
                        scope=Scope.content,
                        display_name="URL to redirect to")

    target_id = String(help="Unit identifier to redirect to (Location id)",
                       scope=Scope.content,
                       display_name="Unit identifier to redirect to")

    message = String(help="Message for the learners to view "
                     "when the condition is met",
                     scope=Scope.content,
                     default='',
                     display_name="Message",
                     multiline_editor='html')

    problem_id = String(help="Problem id to use for the condition.  (Not the "
                        "complete problem locator. Only the 32 characters "
                        "alfanumeric id. "
                        "Example: 618c5933b8b544e4a4cc103d3e508378)",
                        scope=Scope.content,
                        display_name="Problem id")

class VideoXBlock(
        SettingsMixin, TranscriptsMixin, PlaybackStateMixin, LocationMixin,
        StudioEditableXBlockMixin, ContentStoreMixin, WorkbenchMixin, XBlock
):
    """
    Main VideoXBlock class, responsible for saving video settings and rendering it for students.

    VideoXBlock only provide a storage facilities for fields data, but not
    decide what fields to show to user. `BaseVideoPlayer` and it's subclassess
    declare what fields are required for proper configuration of a video.
    See `BaseVideoPlayer.basic_fields` and `BaseVideoPlayer.advanced_fields`.
    """

    icon_class = "video"

    display_name = String(
        default=_('Video'),
        display_name=_('Component Display Name'),
        help=_('The name students see. This name appears in the course ribbon and as a header for the video.'),
        scope=Scope.content,
    )

    href = String(
        default='',
        display_name=_('Video URL'),
        help=_('URL of the video page. E.g. https://example.wistia.com/medias/12345abcde'),
        scope=Scope.content
    )

    download_video_allowed = Boolean(
        default=False,
        scope=Scope.content,

    @XBlock.json_handler
    def on_select(self, data, suffix=''):
        self.position = data.get('position', 0)
        self.runtime.publish(self, {'position': self.position})
        return {'position': self.position}


@XBlock.needs('jinja')
class VerticalBlock(XBlock):
    """An XBlock which presents its children in a vertical list.

    See xmodule.vertical_module.VerticalModule in
    https://github.com/edx/edx-platform.
    """

    display_name = String(default=None, scope=Scope.content)

    has_children = True

    def __init__(self, *args, **kwargs):
        super(VerticalBlock, self).__init__(*args, **kwargs)
        self.templates_dirs = [os.path.join(os.path.dirname(__file__), 'templates')]
        self.get_template = self.runtime.service(self, 'jinja')

    def student_view(self, context=None):
        template = self.get_template('vertical.html', self.templates_dirs)
        child_frags = self.runtime.render_children(self, context=context)
        result = Fragment()
        result.add_frags_resources(child_frags)
        result.add_content(template.render(
            {'children': [frag.body_html() for frag in child_frags]}))
        return result

import pkg_resources

from xblock.core import XBlock
from xblock.fields import Scope, Integer, String
from xblock.fragment import Fragment


class OppiaXBlock(XBlock):
    """
    An XBlock providing an embedded Oppia exploration.
    """

    # Note: These fields are defined on the class, and can be accessed in the
    # code as self..

    oppiaid = String(
        help="ID of the Oppia exploration to embed",
        default=None,
        scope=Scope.content)
    src = String(
        help="Source URL of the site",
        default="https://www.oppia.org",
        scope=Scope.content)
    width = Integer(
        help="Width of the embedded exploration",
        default=700,
        scope=Scope.content)
    height = Integer(
        help="Height of the embedded exploration",
        default=500,
        scope=Scope.content)

)
    lesson_score = Float(
        scope=Scope.user_state,
        default=0
    )
    weight = Float(
        default=1,
        scope=Scope.settings
    )
    has_score = Boolean(
        display_name=_("Scored"),
        help=_("Select False if this component will not receive a numerical score from the Scorm"),
        default=True,
        scope=Scope.settings
    )
    icon_class = String(
        default="video",
        scope=Scope.settings,
    )
    width = Integer(
        display_name=_("Display Width (px)"),
        help=_('Width of iframe, if empty, the default 100%'),
        scope=Scope.settings
    )
    height = Integer(
        display_name=_("Display Height (px)"),
        help=_('Height of iframe'),
        default=450,
        scope=Scope.settings
    )

    has_author_view = True

from xblock.core import XBlock
from xblock.fields import Scope, Integer, String, Boolean
from xblock.fragment import Fragment

class videojsXBlock(XBlock):

    '''
    Icon of the XBlock. Values : [other (default), video, problem]
    '''
    icon_class = "video"

    '''
    Fields
    '''
    display_name = String(display_name="Display Name",
        default="Video JS",
        scope=Scope.settings,
        help="This name appears in the horizontal navigation at the top of the page.")

    url = String(display_name="Video URL",
        default="http://vjs.zencdn.net/v/oceans.mp4",
        scope=Scope.content,
        help="The URL for your video.")
    
    allow_download = Boolean(display_name="Video Download Allowed",
        default=True,
        scope=Scope.content,
        help="Allow students to download this video.")
    
    source_text = String(display_name="Source document button text",
        default="",

help=_("Display name for this module"),
        default="Scorm",
        scope=Scope.settings,
    )
    scorm_file = String(
        display_name=_("Upload scorm file"),
        scope=Scope.settings,
    )
    path_index_page = String(
        display_name=_("Path to the index page in scorm file"),
        scope=Scope.settings,
    )
    scorm_file_meta = Dict(
        scope=Scope.content
    )
    version_scorm = String(
        default="SCORM_12",
        scope=Scope.settings,
    )
    # save completion_status for SCORM_2004
    lesson_status = String(
        scope=Scope.user_state,
        default='not attempted'
    )
    success_status = String(
        scope=Scope.user_state,
        default='unknown'
    )
    data_scorm = Dict(
        scope=Scope.user_state,
        default={}
    )

source_text = String(display_name="Source document button text",
        default="",
        scope=Scope.content,
        help="Add a download link for the source file of your video. Use it for example to provide the PowerPoint or PDF file used for this video.")
    
    source_url = String(display_name="Source document URL",
        default="",
        scope=Scope.content,
        help="Add a download link for the source file of your video. Use it for example to provide the PowerPoint or PDF file used for this video.")
    
    start_time = String(display_name="Start time",
        default="",
        scope=Scope.content,
        help="The start and end time of your video. Equivalent to 'video.mp4#t=startTime,endTime' in the url.")
    
    end_time = String(display_name="End time",
        default="",
        scope=Scope.content,
        help="The start and end time of your video. Equivalent to 'video.mp4#t=startTime,endTime' in the url.")

    '''
    Util functions
    '''
    def load_resource(self, resource_path):
        """
        Gets the content of a resource
        """
        resource_content = pkg_resources.resource_string(__name__, resource_path)
        return unicode(resource_content)

    def render_template(self, template_path, context={}):
        """