How to use the commoncode.datautils.String function in commoncode

license_expression = String(
        label='license expression',
        help='The license expression for this package typically derived '
             'from its declared license or .')

    declared_license = String(
        label='declared license',
        help='The declared license mention, tag or text as found in a '
             'package manifest.')

    notice_text = String(
        label='notice text',
        help='A notice text for this package.')

    root_path = String(
        label='package root path',
        help='The path to the root of the package documented in this manifest '
             'if any, such as a Maven .pom or a npm package.json parent directory.')

    dependencies = List(
        item_type=DependentPackage,
        label='dependencies',
        help='A list of DependentPackage for this package. ')

    contains_source_code = TriBoolean(
        label='contains source code',
        help='Flag set to True if this package contains its own source code, None '
             'if this is unknown, False if not.')

    source_packages = List(
        item_type=String,

class ResourceNotInCache(Exception):
    pass


class UnknownResource(Exception):
    pass


@attr.s(slots=True)
class Header(object):
    """
    Represent a codebase header. Each tool that transforms the codebase
    should create a Header and append it to the codebase log_entries list.
    """
    tool_name = String(help='Name of the tool used such as scancode-toolkit.')
    tool_version = String(default='', help='Tool version used such as v1.2.3.')
    options = Mapping(help='Mapping of key/values describing the options used with this tool.')
    notice = String(default='', help='Notice text for this tool.')
    start_timestamp = String(help='Start timestamp for this header.')
    end_timestamp = String(help='End timestamp for this header.')
    message = String(help='Message text.')
    errors = List(help='List of error messages.')
    extra_data = Mapping(help='Mapping of extra key/values for this tool.')

    def to_dict(self):
        return attr.asdict(self, dict_factory=OrderedDict)

    @classmethod
    def from_dict(cls, **kwargs):
        """
        Return a Header object deserialized from a `kwargs` mapping of
        key/values. Unknown attributes are ignored.

release_date = Date(
        label='release date',
        help='Release date of the package')

    parties = List(
        item_type=Party,
        label='parties',
        help='A list of parties such as a person, project or organization.')

    keywords = List(
        item_type=str,
        label='keywords',
        help='A list of keywords.')

    homepage_url = String(
        label='homepage URL',
        help='URL to the homepage for this package.')

    download_url = String(
        label='Download URL',
        help='A direct download URL.')

    size = Integer(
        default=None,
        label='download size',
        help='size of the package download in bytes')

    sha1 = String(
        label='SHA1 checksum',
        help='SHA1 checksum for this download in hexadecimal')

label='MD5 checksum',
        help='MD5 checksum for this download in hexadecimal')

    sha256 = String(
        label='SHA256 checksum',
        help='SHA256 checksum for this download in hexadecimal')

    sha512 = String(
        label='SHA512 checksum',
        help='SHA512 checksum for this download in hexadecimal')

    bug_tracking_url = String(
        label='bug tracking URL',
        help='URL to the issue or bug tracker for this package')

    code_view_url = String(
        label='code view URL',
        help='a URL where the code can be browsed online')

    vcs_url = String(
        help='a URL to the VCS repository in the SPDX form of: '
             'https://github.com/nexb/scancode-toolkit.git@405aaa4b3 '
              'See SPDX specification "Package Download Location" '
              'at https://spdx.org/spdx-specification-21-web-version#h.49x2ik5 ')

    copyright = String(
        label='Copyright',
        help='Copyright statements for this package. Typically one per line.')

    license_expression = String(
        label='license expression',
        help='The license expression for this package typically derived '

)


@attr.s()
class Party(BaseModel):
    """
    A party is a person, project or organization related to a package.
    """

    type = String(
        validator=choices(PARTY_TYPES),
        label='party type',
        help='the type of this party: One of: '
            +', '.join(p for p in PARTY_TYPES if p))

    role = String(
        label='party role',
        help='A role for this party. Something such as author, '
             'maintainer, contributor, owner, packager, distributor, '
             'vendor, developer, owner, etc.')

    name = String(
        label='name',
        help='Name of this party.')

    email = String(
        label='email',
        help='Email for this party.')

    url = String(
        label='url',
        help='URL to a primary web page for this party.')

validator=choices(PARTY_TYPES),
        label='party type',
        help='the type of this party: One of: '
            +', '.join(p for p in PARTY_TYPES if p))

    role = String(
        label='party role',
        help='A role for this party. Something such as author, '
             'maintainer, contributor, owner, packager, distributor, '
             'vendor, developer, owner, etc.')

    name = String(
        label='name',
        help='Name of this party.')

    email = String(
        label='email',
        help='Email for this party.')

    url = String(
        label='url',
        help='URL to a primary web page for this party.')


@attr.s()
class BasePackage(BaseModel):
    """
    A base identifiable package object using discrete identifying attributes as
    specified here https://github.com/package-url/purl-spec.
    """

    # class-level attributes used to recognize a package

pass


@attr.s(slots=True)
class Header(object):
    """
    Represent a codebase header. Each tool that transforms the codebase
    should create a Header and append it to the codebase log_entries list.
    """
    tool_name = String(help='Name of the tool used such as scancode-toolkit.')
    tool_version = String(default='', help='Tool version used such as v1.2.3.')
    options = Mapping(help='Mapping of key/values describing the options used with this tool.')
    notice = String(default='', help='Notice text for this tool.')
    start_timestamp = String(help='Start timestamp for this header.')
    end_timestamp = String(help='End timestamp for this header.')
    message = String(help='Message text.')
    errors = List(help='List of error messages.')
    extra_data = Mapping(help='Mapping of extra key/values for this tool.')

    def to_dict(self):
        return attr.asdict(self, dict_factory=OrderedDict)

    @classmethod
    def from_dict(cls, **kwargs):
        """
        Return a Header object deserialized from a `kwargs` mapping of
        key/values. Unknown attributes are ignored.
        """
        known_attributes = set([
            'tool_name',
            'tool_version',
            'options',

class ResourceNotInCache(Exception):
    pass


class UnknownResource(Exception):
    pass


@attr.s(slots=True)
class Header(object):
    """
    Represent a codebase header. Each tool that transforms the codebase
    should create a Header and append it to the codebase log_entries list.
    """
    tool_name = String(help='Name of the tool used such as scancode-toolkit.')
    tool_version = String(default='', help='Tool version used such as v1.2.3.')
    options = Mapping(help='Mapping of key/values describing the options used with this tool.')
    notice = String(default='', help='Notice text for this tool.')
    start_timestamp = String(help='Start timestamp for this header.')
    end_timestamp = String(help='End timestamp for this header.')
    message = String(help='Message text.')
    errors = List(help='List of error messages.')
    extra_data = Mapping(help='Mapping of extra key/values for this tool.')

    def to_dict(self):
        return attr.asdict(self, dict_factory=OrderedDict)

    @classmethod
    def from_dict(cls, **kwargs):
        """
        Return a Header object deserialized from a `kwargs` mapping of

parties = List(
        item_type=Party,
        label='parties',
        help='A list of parties such as a person, project or organization.')

    keywords = List(
        item_type=str,
        label='keywords',
        help='A list of keywords.')

    homepage_url = String(
        label='homepage URL',
        help='URL to the homepage for this package.')

    download_url = String(
        label='Download URL',
        help='A direct download URL.')

    size = Integer(
        default=None,
        label='download size',
        help='size of the package download in bytes')

    sha1 = String(
        label='SHA1 checksum',
        help='SHA1 checksum for this download in hexadecimal')

    md5 = String(
        label='MD5 checksum',
        help='MD5 checksum for this download in hexadecimal')

help='A list of keywords.')

    homepage_url = String(
        label='homepage URL',
        help='URL to the homepage for this package.')

    download_url = String(
        label='Download URL',
        help='A direct download URL.')

    size = Integer(
        default=None,
        label='download size',
        help='size of the package download in bytes')

    sha1 = String(
        label='SHA1 checksum',
        help='SHA1 checksum for this download in hexadecimal')

    md5 = String(
        label='MD5 checksum',
        help='MD5 checksum for this download in hexadecimal')

    sha256 = String(
        label='SHA256 checksum',
        help='SHA256 checksum for this download in hexadecimal')

    sha512 = String(
        label='SHA512 checksum',
        help='SHA512 checksum for this download in hexadecimal')

    bug_tracking_url = String(