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+# $Id: frontend.py 9540 2024-02-17 10:36:59Z milde $
+# Author: David Goodger <goodger@python.org>
+# Copyright: This module has been placed in the public domain.
+
+"""
+Command-line and common processing for Docutils front-end tools.
+
+This module is provisional.
+Major changes will happen with the switch from the deprecated
+"optparse" module to "arparse".
+
+Applications should use the high-level API provided by `docutils.core`.
+See https://docutils.sourceforge.io/docs/api/runtime-settings.html.
+
+Exports the following classes:
+
+* `OptionParser`: Standard Docutils command-line processing.
+  Deprecated. Will be replaced by an ArgumentParser.
+* `Option`: Customized version of `optparse.Option`; validation support.
+  Deprecated. Will be removed.
+* `Values`: Runtime settings; objects are simple structs
+  (``object.attribute``).  Supports cumulative list settings (attributes).
+  Deprecated. Will be removed.
+* `ConfigParser`: Standard Docutils config file processing.
+  Provisional. Details will change.
+
+Also exports the following functions:
+
+Interface function:
+  `get_default_settings()`.  New in 0.19.
+
+Option callbacks:
+  `store_multiple()`, `read_config_file()`. Deprecated.
+
+Setting validators:
+  `validate_encoding()`, `validate_encoding_error_handler()`,
+  `validate_encoding_and_error_handler()`,
+  `validate_boolean()`, `validate_ternary()`,
+  `validate_nonnegative_int()`, `validate_threshold()`,
+  `validate_colon_separated_string_list()`,
+  `validate_comma_separated_list()`,
+  `validate_url_trailing_slash()`,
+  `validate_dependency_file()`,
+  `validate_strip_class()`
+  `validate_smartquotes_locales()`.
+
+  Provisional.
+
+Misc:
+  `make_paths_absolute()`, `filter_settings_spec()`. Provisional.
+"""
+
+__docformat__ = 'reStructuredText'
+
+
+import codecs
+import configparser
+import optparse
+from optparse import SUPPRESS_HELP
+import os
+import os.path
+from pathlib import Path
+import sys
+import warnings
+
+import docutils
+from docutils import io, utils
+
+
+def store_multiple(option, opt, value, parser, *args, **kwargs):
+    """
+    Store multiple values in `parser.values`.  (Option callback.)
+
+    Store `None` for each attribute named in `args`, and store the value for
+    each key (attribute name) in `kwargs`.
+    """
+    for attribute in args:
+        setattr(parser.values, attribute, None)
+    for key, value in kwargs.items():
+        setattr(parser.values, key, value)
+
+
+def read_config_file(option, opt, value, parser):
+    """
+    Read a configuration file during option processing.  (Option callback.)
+    """
+    try:
+        new_settings = parser.get_config_file_settings(value)
+    except ValueError as err:
+        parser.error(err)
+    parser.values.update(new_settings, parser)
+
+
+def validate_encoding(setting, value=None, option_parser=None,
+                      config_parser=None, config_section=None):
+    # All arguments except `value` are ignored
+    # (kept for compatibility with "optparse" module).
+    # If there is only one positional argument, it is interpreted as `value`.
+    if value is None:
+        value = setting
+    if value == '':
+        return None  # allow overwriting a config file value
+    try:
+        codecs.lookup(value)
+    except LookupError:
+        raise LookupError('setting "%s": unknown encoding: "%s"'
+                          % (setting, value))
+    return value
+
+
+def validate_encoding_error_handler(setting, value=None, option_parser=None,
+                                    config_parser=None, config_section=None):
+    # All arguments except `value` are ignored
+    # (kept for compatibility with "optparse" module).
+    # If there is only one positional argument, it is interpreted as `value`.
+    if value is None:
+        value = setting
+    try:
+        codecs.lookup_error(value)
+    except LookupError:
+        raise LookupError(
+            'unknown encoding error handler: "%s" (choices: '
+            '"strict", "ignore", "replace", "backslashreplace", '
+            '"xmlcharrefreplace", and possibly others; see documentation for '
+            'the Python ``codecs`` module)' % value)
+    return value
+
+
+def validate_encoding_and_error_handler(
+    setting, value, option_parser, config_parser=None, config_section=None):
+    """
+    Side-effect: if an error handler is included in the value, it is inserted
+    into the appropriate place as if it were a separate setting/option.
+    """
+    if ':' in value:
+        encoding, handler = value.split(':')
+        validate_encoding_error_handler(handler)
+        if config_parser:
+            config_parser.set(config_section, setting + '_error_handler',
+                              handler)
+        else:
+            setattr(option_parser.values, setting + '_error_handler', handler)
+    else:
+        encoding = value
+    return validate_encoding(encoding)
+
+
+def validate_boolean(setting, value=None, option_parser=None,
+                     config_parser=None, config_section=None):
+    """Check/normalize boolean settings:
+         True:  '1', 'on', 'yes', 'true'
+         False: '0', 'off', 'no','false', ''
+
+    All arguments except `value` are ignored
+    (kept for compatibility with "optparse" module).
+    If there is only one positional argument, it is interpreted as `value`.
+    """
+    if value is None:
+        value = setting
+    if isinstance(value, bool):
+        return value
+    try:
+        return OptionParser.booleans[value.strip().lower()]
+    except KeyError:
+        raise LookupError('unknown boolean value: "%s"' % value)
+
+
+def validate_ternary(setting, value=None, option_parser=None,
+                     config_parser=None, config_section=None):
+    """Check/normalize three-value settings:
+         True:  '1', 'on', 'yes', 'true'
+         False: '0', 'off', 'no','false', ''
+         any other value: returned as-is.
+
+    All arguments except `value` are ignored
+    (kept for compatibility with "optparse" module).
+    If there is only one positional argument, it is interpreted as `value`.
+    """
+    if value is None:
+        value = setting
+    if isinstance(value, bool) or value is None:
+        return value
+    try:
+        return OptionParser.booleans[value.strip().lower()]
+    except KeyError:
+        return value
+
+
+def validate_nonnegative_int(setting, value=None, option_parser=None,
+                             config_parser=None, config_section=None):
+    # All arguments except `value` are ignored
+    # (kept for compatibility with "optparse" module).
+    # If there is only one positional argument, it is interpreted as `value`.
+    if value is None:
+        value = setting
+    value = int(value)
+    if value < 0:
+        raise ValueError('negative value; must be positive or zero')
+    return value
+
+
+def validate_threshold(setting, value=None, option_parser=None,
+                       config_parser=None, config_section=None):
+    # All arguments except `value` are ignored
+    # (kept for compatibility with "optparse" module).
+    # If there is only one positional argument, it is interpreted as `value`.
+    if value is None:
+        value = setting
+    try:
+        return int(value)
+    except ValueError:
+        try:
+            return OptionParser.thresholds[value.lower()]
+        except (KeyError, AttributeError):
+            raise LookupError('unknown threshold: %r.' % value)
+
+
+def validate_colon_separated_string_list(
+        setting, value=None, option_parser=None,
+        config_parser=None, config_section=None):
+    # All arguments except `value` are ignored
+    # (kept for compatibility with "optparse" module).
+    # If there is only one positional argument, it is interpreted as `value`.
+    if value is None:
+        value = setting
+    if not isinstance(value, list):
+        value = value.split(':')
+    else:
+        last = value.pop()
+        value.extend(last.split(':'))
+    return value
+
+
+def validate_comma_separated_list(setting, value=None, option_parser=None,
+                                  config_parser=None, config_section=None):
+    """Check/normalize list arguments (split at "," and strip whitespace).
+
+    All arguments except `value` are ignored
+    (kept for compatibility with "optparse" module).
+    If there is only one positional argument, it is interpreted as `value`.
+    """
+    if value is None:
+        value = setting
+    # `value` may be ``bytes``, ``str``, or a ``list`` (when  given as
+    # command line option and "action" is "append").
+    if not isinstance(value, list):
+        value = [value]
+    # this function is called for every option added to `value`
+    # -> split the last item and append the result:
+    last = value.pop()
+    items = [i.strip(' \t\n') for i in last.split(',') if i.strip(' \t\n')]
+    value.extend(items)
+    return value
+
+
+def validate_math_output(setting, value=None, option_parser=None,
+                         config_parser=None, config_section=None):
+    """Check "math-output" setting, return list with "format" and "options".
+
+    See also https://docutils.sourceforge.io/docs/user/config.html#math-output
+
+    Argument list for compatibility with "optparse" module.
+    All arguments except `value` are ignored.
+    If there is only one positional argument, it is interpreted as `value`.
+    """
+    if value is None:
+        value = setting
+
+    formats = ('html', 'latex', 'mathml', 'mathjax')
+    tex2mathml_converters = ('', 'latexml', 'ttm', 'blahtexml', 'pandoc')
+
+    if not value:
+        return []
+    values = value.split(maxsplit=1)
+    format = values[0].lower()
+    try:
+        options = values[1]
+    except IndexError:
+        options = ''
+    if format not in formats:
+        raise LookupError(f'Unknown math output format: "{value}",\n'
+                          f'    choose from {formats}.')
+    if format == 'mathml':
+        converter = options.lower()
+        if converter not in tex2mathml_converters:
+            raise LookupError(f'MathML converter "{options}" not supported,\n'
+                              f'    choose from {tex2mathml_converters}.')
+        options = converter
+    return [format, options]
+
+
+def validate_url_trailing_slash(setting, value=None, option_parser=None,
+                                config_parser=None, config_section=None):
+    # All arguments except `value` are ignored
+    # (kept for compatibility with "optparse" module).
+    # If there is only one positional argument, it is interpreted as `value`.
+    if value is None:
+        value = setting
+    if not value:
+        return './'
+    elif value.endswith('/'):
+        return value
+    else:
+        return value + '/'
+
+
+def validate_dependency_file(setting, value=None, option_parser=None,
+                             config_parser=None, config_section=None):
+    # All arguments except `value` are ignored
+    # (kept for compatibility with "optparse" module).
+    # If there is only one positional argument, it is interpreted as `value`.
+    if value is None:
+        value = setting
+    try:
+        return utils.DependencyList(value)
+    except OSError:
+        # TODO: warn/info?
+        return utils.DependencyList(None)
+
+
+def validate_strip_class(setting, value=None, option_parser=None,
+                         config_parser=None, config_section=None):
+    # All arguments except `value` are ignored
+    # (kept for compatibility with "optparse" module).
+    # If there is only one positional argument, it is interpreted as `value`.
+    if value is None:
+        value = setting
+    # value is a comma separated string list:
+    value = validate_comma_separated_list(value)
+    # validate list elements:
+    for cls in value:
+        normalized = docutils.nodes.make_id(cls)
+        if cls != normalized:
+            raise ValueError('Invalid class value %r (perhaps %r?)'
+                             % (cls, normalized))
+    return value
+
+
+def validate_smartquotes_locales(setting, value=None, option_parser=None,
+                                 config_parser=None, config_section=None):
+    """Check/normalize a comma separated list of smart quote definitions.
+
+    Return a list of (language-tag, quotes) string tuples.
+
+    All arguments except `value` are ignored
+    (kept for compatibility with "optparse" module).
+    If there is only one positional argument, it is interpreted as `value`.
+    """
+    if value is None:
+        value = setting
+    # value is a comma separated string list:
+    value = validate_comma_separated_list(value)
+    # validate list elements
+    lc_quotes = []
+    for item in value:
+        try:
+            lang, quotes = item.split(':', 1)
+        except AttributeError:
+            # this function is called for every option added to `value`
+            # -> ignore if already a tuple:
+            lc_quotes.append(item)
+            continue
+        except ValueError:
+            raise ValueError('Invalid value "%s".'
+                             ' Format is "<language>:<quotes>".'
+                             % item.encode('ascii', 'backslashreplace'))
+        # parse colon separated string list:
+        quotes = quotes.strip()
+        multichar_quotes = quotes.split(':')
+        if len(multichar_quotes) == 4:
+            quotes = multichar_quotes
+        elif len(quotes) != 4:
+            raise ValueError('Invalid value "%s". Please specify 4 quotes\n'
+                             '    (primary open/close; secondary open/close).'
+                             % item.encode('ascii', 'backslashreplace'))
+        lc_quotes.append((lang, quotes))
+    return lc_quotes
+
+
+def make_paths_absolute(pathdict, keys, base_path=None):
+    """
+    Interpret filesystem path settings relative to the `base_path` given.
+
+    Paths are values in `pathdict` whose keys are in `keys`.  Get `keys` from
+    `OptionParser.relative_path_settings`.
+    """
+    if base_path is None:
+        base_path = Path.cwd()
+    else:
+        base_path = Path(base_path)
+    for key in keys:
+        if key in pathdict:
+            value = pathdict[key]
+            if isinstance(value, list):
+                value = [str((base_path/path).resolve()) for path in value]
+            elif value:
+                value = str((base_path/value).resolve())
+            pathdict[key] = value
+
+
+def make_one_path_absolute(base_path, path):
+    # deprecated, will be removed
+    warnings.warn('frontend.make_one_path_absolute() will be removed '
+                  'in Docutils 0.23.', DeprecationWarning, stacklevel=2)
+    return os.path.abspath(os.path.join(base_path, path))
+
+
+def filter_settings_spec(settings_spec, *exclude, **replace):
+    """Return a copy of `settings_spec` excluding/replacing some settings.
+
+    `settings_spec` is a tuple of configuration settings
+    (cf. `docutils.SettingsSpec.settings_spec`).
+
+    Optional positional arguments are names of to-be-excluded settings.
+    Keyword arguments are option specification replacements.
+    (See the html4strict writer for an example.)
+    """
+    settings = list(settings_spec)
+    # every third item is a sequence of option tuples
+    for i in range(2, len(settings), 3):
+        newopts = []
+        for opt_spec in settings[i]:
+            # opt_spec is ("<help>", [<option strings>], {<keyword args>})
+            opt_name = [opt_string[2:].replace('-', '_')
+                        for opt_string in opt_spec[1]
+                        if opt_string.startswith('--')][0]
+            if opt_name in exclude:
+                continue
+            if opt_name in replace.keys():
+                newopts.append(replace[opt_name])
+            else:
+                newopts.append(opt_spec)
+        settings[i] = tuple(newopts)
+    return tuple(settings)
+
+
+class Values(optparse.Values):
+    """Storage for option values.
+
+    Updates list attributes by extension rather than by replacement.
+    Works in conjunction with the `OptionParser.lists` instance attribute.
+
+    Deprecated. Will be removed.
+    """
+
+    def __init__(self, *args, **kwargs):
+        warnings.warn('frontend.Values class will be removed '
+                      'in Docutils 0.21 or later.',
+                      DeprecationWarning, stacklevel=2)
+        super().__init__(*args, **kwargs)
+        if getattr(self, 'record_dependencies', None) is None:
+            # Set up dummy dependency list.
+            self.record_dependencies = utils.DependencyList()
+
+    def update(self, other_dict, option_parser):
+        if isinstance(other_dict, Values):
+            other_dict = other_dict.__dict__
+        other_dict = dict(other_dict)  # also works with ConfigParser sections
+        for setting in option_parser.lists.keys():
+            if hasattr(self, setting) and setting in other_dict:
+                value = getattr(self, setting)
+                if value:
+                    value += other_dict[setting]
+                    del other_dict[setting]
+        self._update_loose(other_dict)
+
+    def copy(self):
+        """Return a shallow copy of `self`."""
+        with warnings.catch_warnings():
+            warnings.filterwarnings('ignore', category=DeprecationWarning)
+            return self.__class__(defaults=self.__dict__)
+
+    def setdefault(self, name, default):
+        """Return ``self.name`` or ``default``.
+
+        If ``self.name`` is unset, set ``self.name = default``.
+        """
+        if getattr(self, name, None) is None:
+            setattr(self, name, default)
+        return getattr(self, name)
+
+
+class Option(optparse.Option):
+    """Add validation and override support to `optparse.Option`.
+
+    Deprecated. Will be removed.
+    """
+
+    ATTRS = optparse.Option.ATTRS + ['validator', 'overrides']
+
+    def __init__(self, *args, **kwargs):
+        warnings.warn('The frontend.Option class will be removed '
+                      'in Docutils 0.21 or later.',
+                      DeprecationWarning, stacklevel=2)
+        super().__init__(*args, **kwargs)
+
+    def process(self, opt, value, values, parser):
+        """
+        Call the validator function on applicable settings and
+        evaluate the 'overrides' option.
+        Extends `optparse.Option.process`.
+        """
+        result = super().process(opt, value, values, parser)
+        setting = self.dest
+        if setting:
+            if self.validator:
+                value = getattr(values, setting)
+                try:
+                    new_value = self.validator(setting, value, parser)
+                except Exception as err:
+                    raise optparse.OptionValueError(
+                        'Error in option "%s":\n    %s'
+                        % (opt, io.error_string(err)))
+                setattr(values, setting, new_value)
+            if self.overrides:
+                setattr(values, self.overrides, None)
+        return result
+
+
+class OptionParser(optparse.OptionParser, docutils.SettingsSpec):
+    """
+    Settings parser for command-line and library use.
+
+    The `settings_spec` specification here and in other Docutils components
+    are merged to build the set of command-line options and runtime settings
+    for this process.
+
+    Common settings (defined below) and component-specific settings must not
+    conflict.  Short options are reserved for common settings, and components
+    are restricted to using long options.
+
+    Deprecated.
+    Will be replaced by a subclass of `argparse.ArgumentParser`.
+    """
+
+    standard_config_files = [
+        '/etc/docutils.conf',           # system-wide
+        './docutils.conf',              # project-specific
+        '~/.docutils']                  # user-specific
+    """Docutils configuration files, using ConfigParser syntax.
+
+    Filenames will be tilde-expanded later. Later files override earlier ones.
+    """
+
+    threshold_choices = 'info 1 warning 2 error 3 severe 4 none 5'.split()
+    """Possible inputs for for --report and --halt threshold values."""
+
+    thresholds = {'info': 1, 'warning': 2, 'error': 3, 'severe': 4, 'none': 5}
+    """Lookup table for --report and --halt threshold values."""
+
+    booleans = {'1': True, 'on': True, 'yes': True, 'true': True, '0': False,
+                'off': False, 'no': False, 'false': False, '': False}
+    """Lookup table for boolean configuration file settings."""
+
+    default_error_encoding = (getattr(sys.stderr, 'encoding', None)
+                              or io._locale_encoding  # noqa
+                              or 'ascii')
+
+    default_error_encoding_error_handler = 'backslashreplace'
+
+    settings_spec = (
+        'General Docutils Options',
+        None,
+        (('Output destination name. Obsoletes the <destination> '
+          'positional argument. Default: None (stdout).',
+          ['--output'], {'metavar': '<destination>'}),
+         ('Specify the document title as metadata.',
+          ['--title'], {'metavar': '<title>'}),
+         ('Include a "Generated by Docutils" credit and link.',
+          ['--generator', '-g'], {'action': 'store_true',
+                                  'validator': validate_boolean}),
+         ('Do not include a generator credit.',
+          ['--no-generator'], {'action': 'store_false', 'dest': 'generator'}),
+         ('Include the date at the end of the document (UTC).',
+          ['--date', '-d'], {'action': 'store_const', 'const': '%Y-%m-%d',
+                             'dest': 'datestamp'}),
+         ('Include the time & date (UTC).',
+          ['--time', '-t'], {'action': 'store_const',
+                             'const': '%Y-%m-%d %H:%M UTC',
+                             'dest': 'datestamp'}),
+         ('Do not include a datestamp of any kind.',
+          ['--no-datestamp'], {'action': 'store_const', 'const': None,
+                               'dest': 'datestamp'}),
+         ('Base directory for absolute paths when reading '
+          'from the local filesystem. Default "/".',
+          ['--root-prefix'],
+          {'default': '/', 'metavar': '<path>'}),
+         ('Include a "View document source" link.',
+          ['--source-link', '-s'], {'action': 'store_true',
+                                    'validator': validate_boolean}),
+         ('Use <URL> for a source link; implies --source-link.',
+          ['--source-url'], {'metavar': '<URL>'}),
+         ('Do not include a "View document source" link.',
+          ['--no-source-link'],
+          {'action': 'callback', 'callback': store_multiple,
+           'callback_args': ('source_link', 'source_url')}),
+         ('Link from section headers to TOC entries.  (default)',
+          ['--toc-entry-backlinks'],
+          {'dest': 'toc_backlinks', 'action': 'store_const', 'const': 'entry',
+           'default': 'entry'}),
+         ('Link from section headers to the top of the TOC.',
+          ['--toc-top-backlinks'],
+          {'dest': 'toc_backlinks', 'action': 'store_const', 'const': 'top'}),
+         ('Disable backlinks to the table of contents.',
+          ['--no-toc-backlinks'],
+          {'dest': 'toc_backlinks', 'action': 'store_false'}),
+         ('Link from footnotes/citations to references. (default)',
+          ['--footnote-backlinks'],
+          {'action': 'store_true', 'default': True,
+           'validator': validate_boolean}),
+         ('Disable backlinks from footnotes and citations.',
+          ['--no-footnote-backlinks'],
+          {'dest': 'footnote_backlinks', 'action': 'store_false'}),
+         ('Enable section numbering by Docutils.  (default)',
+          ['--section-numbering'],
+          {'action': 'store_true', 'dest': 'sectnum_xform',
+           'default': True, 'validator': validate_boolean}),
+         ('Disable section numbering by Docutils.',
+          ['--no-section-numbering'],
+          {'action': 'store_false', 'dest': 'sectnum_xform'}),
+         ('Remove comment elements from the document tree.',
+          ['--strip-comments'],
+          {'action': 'store_true', 'validator': validate_boolean}),
+         ('Leave comment elements in the document tree. (default)',
+          ['--leave-comments'],
+          {'action': 'store_false', 'dest': 'strip_comments'}),
+         ('Remove all elements with classes="<class>" from the document tree. '
+          'Warning: potentially dangerous; use with caution. '
+          '(Multiple-use option.)',
+          ['--strip-elements-with-class'],
+          {'action': 'append', 'dest': 'strip_elements_with_classes',
+           'metavar': '<class>', 'validator': validate_strip_class}),
+         ('Remove all classes="<class>" attributes from elements in the '
+          'document tree. Warning: potentially dangerous; use with caution. '
+          '(Multiple-use option.)',
+          ['--strip-class'],
+          {'action': 'append', 'dest': 'strip_classes',
+           'metavar': '<class>', 'validator': validate_strip_class}),
+         ('Report system messages at or higher than <level>: "info" or "1", '
+          '"warning"/"2" (default), "error"/"3", "severe"/"4", "none"/"5"',
+          ['--report', '-r'], {'choices': threshold_choices, 'default': 2,
+                               'dest': 'report_level', 'metavar': '<level>',
+                               'validator': validate_threshold}),
+         ('Report all system messages.  (Same as "--report=1".)',
+          ['--verbose', '-v'], {'action': 'store_const', 'const': 1,
+                                'dest': 'report_level'}),
+         ('Report no system messages.  (Same as "--report=5".)',
+          ['--quiet', '-q'], {'action': 'store_const', 'const': 5,
+                              'dest': 'report_level'}),
+         ('Halt execution at system messages at or above <level>.  '
+          'Levels as in --report.  Default: 4 (severe).',
+          ['--halt'], {'choices': threshold_choices, 'dest': 'halt_level',
+                       'default': 4, 'metavar': '<level>',
+                       'validator': validate_threshold}),
+         ('Halt at the slightest problem.  Same as "--halt=info".',
+          ['--strict'], {'action': 'store_const', 'const': 1,
+                         'dest': 'halt_level'}),
+         ('Enable a non-zero exit status for non-halting system messages at '
+          'or above <level>.  Default: 5 (disabled).',
+          ['--exit-status'], {'choices': threshold_choices,
+                              'dest': 'exit_status_level',
+                              'default': 5, 'metavar': '<level>',
+                              'validator': validate_threshold}),
+         ('Enable debug-level system messages and diagnostics.',
+          ['--debug'], {'action': 'store_true',
+                        'validator': validate_boolean}),
+         ('Disable debug output.  (default)',
+          ['--no-debug'], {'action': 'store_false', 'dest': 'debug'}),
+         ('Send the output of system messages to <file>.',
+          ['--warnings'], {'dest': 'warning_stream', 'metavar': '<file>'}),
+         ('Enable Python tracebacks when Docutils is halted.',
+          ['--traceback'], {'action': 'store_true', 'default': None,
+                            'validator': validate_boolean}),
+         ('Disable Python tracebacks.  (default)',
+          ['--no-traceback'], {'dest': 'traceback', 'action': 'store_false'}),
+         ('Specify the encoding and optionally the '
+          'error handler of input text.  Default: <auto-detect>:strict.',
+          ['--input-encoding', '-i'],
+          {'metavar': '<name[:handler]>',
+           'validator': validate_encoding_and_error_handler}),
+         ('Specify the error handler for undecodable characters.  '
+          'Choices: "strict" (default), "ignore", and "replace".',
+          ['--input-encoding-error-handler'],
+          {'default': 'strict', 'validator': validate_encoding_error_handler}),
+         ('Specify the text encoding and optionally the error handler for '
+          'output.  Default: utf-8:strict.',
+          ['--output-encoding', '-o'],
+          {'metavar': '<name[:handler]>', 'default': 'utf-8',
+           'validator': validate_encoding_and_error_handler}),
+         ('Specify error handler for unencodable output characters; '
+          '"strict" (default), "ignore", "replace", '
+          '"xmlcharrefreplace", "backslashreplace".',
+          ['--output-encoding-error-handler'],
+          {'default': 'strict', 'validator': validate_encoding_error_handler}),
+         ('Specify text encoding and optionally error handler '
+          'for error output.  Default: %s:%s.'
+          % (default_error_encoding, default_error_encoding_error_handler),
+          ['--error-encoding', '-e'],
+          {'metavar': '<name[:handler]>', 'default': default_error_encoding,
+           'validator': validate_encoding_and_error_handler}),
+         ('Specify the error handler for unencodable characters in '
+          'error output.  Default: %s.'
+          % default_error_encoding_error_handler,
+          ['--error-encoding-error-handler'],
+          {'default': default_error_encoding_error_handler,
+           'validator': validate_encoding_error_handler}),
+         ('Specify the language (as BCP 47 language tag).  Default: en.',
+          ['--language', '-l'], {'dest': 'language_code', 'default': 'en',
+                                 'metavar': '<name>'}),
+         ('Write output file dependencies to <file>.',
+          ['--record-dependencies'],
+          {'metavar': '<file>', 'validator': validate_dependency_file,
+           'default': None}),           # default set in Values class
+         ('Read configuration settings from <file>, if it exists.',
+          ['--config'], {'metavar': '<file>', 'type': 'string',
+                         'action': 'callback', 'callback': read_config_file}),
+         ("Show this program's version number and exit.",
+          ['--version', '-V'], {'action': 'version'}),
+         ('Show this help message and exit.',
+          ['--help', '-h'], {'action': 'help'}),
+         # Typically not useful for non-programmatical use:
+         (SUPPRESS_HELP, ['--id-prefix'], {'default': ''}),
+         (SUPPRESS_HELP, ['--auto-id-prefix'], {'default': '%'}),
+         # Hidden options, for development use only:
+         (SUPPRESS_HELP, ['--dump-settings'], {'action': 'store_true'}),
+         (SUPPRESS_HELP, ['--dump-internals'], {'action': 'store_true'}),
+         (SUPPRESS_HELP, ['--dump-transforms'], {'action': 'store_true'}),
+         (SUPPRESS_HELP, ['--dump-pseudo-xml'], {'action': 'store_true'}),
+         (SUPPRESS_HELP, ['--expose-internal-attribute'],
+          {'action': 'append', 'dest': 'expose_internals',
+           'validator': validate_colon_separated_string_list}),
+         (SUPPRESS_HELP, ['--strict-visitor'], {'action': 'store_true'}),
+         ))
+    """Runtime settings and command-line options common to all Docutils front
+    ends.  Setting specs specific to individual Docutils components are also
+    used (see `populate_from_components()`)."""
+
+    settings_defaults = {'_disable_config': None,
+                         '_source': None,
+                         '_destination': None,
+                         '_config_files': None}
+    """Defaults for settings without command-line option equivalents.
+
+    See https://docutils.sourceforge.io/docs/user/config.html#internal-settings
+    """
+
+    config_section = 'general'
+
+    version_template = ('%%prog (Docutils %s%s, Python %s, on %s)'
+                        % (docutils.__version__,
+                           docutils.__version_details__
+                           and ' [%s]'%docutils.__version_details__ or '',
+                           sys.version.split()[0], sys.platform))
+    """Default version message."""
+
+    def __init__(self, components=(), defaults=None, read_config_files=False,
+                 *args, **kwargs):
+        """Set up OptionParser instance.
+
+        `components` is a list of Docutils components each containing a
+        ``.settings_spec`` attribute.
+        `defaults` is a mapping of setting default overrides.
+        """
+
+        self.lists = {}
+        """Set of list-type settings."""
+
+        self.config_files = []
+        """List of paths of applied configuration files."""
+
+        self.relative_path_settings = ['warning_stream']  # will be modified
+
+        warnings.warn('The frontend.OptionParser class will be replaced '
+                      'by a subclass of argparse.ArgumentParser '
+                      'in Docutils 0.21 or later.',
+                      DeprecationWarning, stacklevel=2)
+        super().__init__(option_class=Option, add_help_option=None,
+                         formatter=optparse.TitledHelpFormatter(width=78),
+                         *args, **kwargs)
+        if not self.version:
+            self.version = self.version_template
+        self.components = (self, *components)
+        self.populate_from_components(self.components)
+        self.defaults.update(defaults or {})
+        if read_config_files and not self.defaults['_disable_config']:
+            try:
+                config_settings = self.get_standard_config_settings()
+            except ValueError as err:
+                self.error(err)
+            self.defaults.update(config_settings.__dict__)
+
+    def populate_from_components(self, components):
+        """Collect settings specification from components.
+
+        For each component, populate from the `SettingsSpec.settings_spec`
+        structure, then from the `SettingsSpec.settings_defaults` dictionary.
+        After all components have been processed, check for and populate from
+        each component's `SettingsSpec.settings_default_overrides` dictionary.
+        """
+        for component in components:
+            if component is None:
+                continue
+            settings_spec = component.settings_spec
+            self.relative_path_settings.extend(
+                component.relative_path_settings)
+            for i in range(0, len(settings_spec), 3):
+                title, description, option_spec = settings_spec[i:i+3]
+                if title:
+                    group = optparse.OptionGroup(self, title, description)
+                    self.add_option_group(group)
+                else:
+                    group = self        # single options
+                for (help_text, option_strings, kwargs) in option_spec:
+                    option = group.add_option(help=help_text, *option_strings,
+                                              **kwargs)
+                    if kwargs.get('action') == 'append':
+                        self.lists[option.dest] = True
+                if component.settings_defaults:
+                    self.defaults.update(component.settings_defaults)
+        for component in components:
+            if component and component.settings_default_overrides:
+                self.defaults.update(component.settings_default_overrides)
+
+    @classmethod
+    def get_standard_config_files(cls):
+        """Return list of config files, from environment or standard."""
+        if 'DOCUTILSCONFIG' in os.environ:
+            config_files = os.environ['DOCUTILSCONFIG'].split(os.pathsep)
+        else:
+            config_files = cls.standard_config_files
+        return [os.path.expanduser(f) for f in config_files if f.strip()]
+
+    def get_standard_config_settings(self):
+        with warnings.catch_warnings():
+            warnings.filterwarnings('ignore', category=DeprecationWarning)
+            settings = Values()
+        for filename in self.get_standard_config_files():
+            settings.update(self.get_config_file_settings(filename), self)
+        return settings
+
+    def get_config_file_settings(self, config_file):
+        """Returns a dictionary containing appropriate config file settings."""
+        config_parser = ConfigParser()
+        # parse config file, add filename if found and successfully read.
+        applied = set()
+        with warnings.catch_warnings():
+            warnings.filterwarnings('ignore', category=DeprecationWarning)
+            self.config_files += config_parser.read(config_file, self)
+            settings = Values()
+        for component in self.components:
+            if not component:
+                continue
+            for section in (tuple(component.config_section_dependencies or ())
+                            + (component.config_section,)):
+                if section in applied:
+                    continue
+                applied.add(section)
+                if config_parser.has_section(section):
+                    settings.update(config_parser[section], self)
+        make_paths_absolute(settings.__dict__,
+                            self.relative_path_settings,
+                            os.path.dirname(config_file))
+        return settings.__dict__
+
+    def check_values(self, values, args):
+        """Store positional arguments as runtime settings."""
+        values._source, values._destination = self.check_args(args)
+        make_paths_absolute(values.__dict__, self.relative_path_settings)
+        values._config_files = self.config_files
+        return values
+
+    def check_args(self, args):
+        source = destination = None
+        if args:
+            source = args.pop(0)
+            if source == '-':           # means stdin
+                source = None
+        if args:
+            destination = args.pop(0)
+            if destination == '-':      # means stdout
+                destination = None
+        if args:
+            self.error('Maximum 2 arguments allowed.')
+        if source and source == destination:
+            self.error('Do not specify the same file for both source and '
+                       'destination.  It will clobber the source file.')
+        return source, destination
+
+    def set_defaults_from_dict(self, defaults):
+        # deprecated, will be removed
+        warnings.warn('OptionParser.set_defaults_from_dict() will be removed '
+                      'in Docutils 0.22 or with the switch to ArgumentParser.',
+                      DeprecationWarning, stacklevel=2)
+        self.defaults.update(defaults)
+
+    def get_default_values(self):
+        """Needed to get custom `Values` instances."""
+        with warnings.catch_warnings():
+            warnings.filterwarnings('ignore', category=DeprecationWarning)
+            defaults = Values(self.defaults)
+        defaults._config_files = self.config_files
+        return defaults
+
+    def get_option_by_dest(self, dest):
+        """
+        Get an option by its dest.
+
+        If you're supplying a dest which is shared by several options,
+        it is undefined which option of those is returned.
+
+        A KeyError is raised if there is no option with the supplied
+        dest.
+        """
+        for group in self.option_groups + [self]:
+            for option in group.option_list:
+                if option.dest == dest:
+                    return option
+        raise KeyError('No option with dest == %r.' % dest)
+
+
+class ConfigParser(configparser.RawConfigParser):
+    """Parser for Docutils configuration files.
+
+    See https://docutils.sourceforge.io/docs/user/config.html.
+
+    Option key normalization includes conversion of '-' to '_'.
+
+    Config file encoding is "utf-8". Encoding errors are reported
+    and the affected file(s) skipped.
+
+    This class is provisional and will change in future versions.
+    """
+
+    old_settings = {
+        'pep_stylesheet': ('pep_html writer', 'stylesheet'),
+        'pep_stylesheet_path': ('pep_html writer', 'stylesheet_path'),
+        'pep_template': ('pep_html writer', 'template')}
+    """{old setting: (new section, new setting)} mapping, used by
+    `handle_old_config`, to convert settings from the old [options] section.
+    """
+
+    old_warning = (
+        'The "[option]" section is deprecated.\n'
+        'Support for old-format configuration files will be removed in '
+        'Docutils 2.0.  Please revise your configuration files.  '
+        'See <https://docutils.sourceforge.io/docs/user/config.html>, '
+        'section "Old-Format Configuration Files".')
+
+    not_utf8_error = """\
+Unable to read configuration file "%s": content not encoded as UTF-8.
+Skipping "%s" configuration file.
+"""
+
+    def read(self, filenames, option_parser=None):
+        # Currently, if a `docutils.frontend.OptionParser` instance is
+        # supplied, setting values are validated.
+        if option_parser is not None:
+            warnings.warn('frontend.ConfigParser.read(): parameter '
+                          '"option_parser" will be removed '
+                          'in Docutils 0.21 or later.',
+                          DeprecationWarning, stacklevel=2)
+        read_ok = []
+        if isinstance(filenames, str):
+            filenames = [filenames]
+        for filename in filenames:
+            # Config files are UTF-8-encoded:
+            try:
+                read_ok += super().read(filename, encoding='utf-8')
+            except UnicodeDecodeError:
+                sys.stderr.write(self.not_utf8_error % (filename, filename))
+                continue
+            if 'options' in self:
+                self.handle_old_config(filename)
+            if option_parser is not None:
+                self.validate_settings(filename, option_parser)
+        return read_ok
+
+    def handle_old_config(self, filename):
+        warnings.warn_explicit(self.old_warning, ConfigDeprecationWarning,
+                               filename, 0)
+        options = self.get_section('options')
+        if not self.has_section('general'):
+            self.add_section('general')
+        for key, value in options.items():
+            if key in self.old_settings:
+                section, setting = self.old_settings[key]
+                if not self.has_section(section):
+                    self.add_section(section)
+            else:
+                section = 'general'
+                setting = key
+            if not self.has_option(section, setting):
+                self.set(section, setting, value)
+        self.remove_section('options')
+
+    def validate_settings(self, filename, option_parser):
+        """
+        Call the validator function and implement overrides on all applicable
+        settings.
+        """
+        for section in self.sections():
+            for setting in self.options(section):
+                try:
+                    option = option_parser.get_option_by_dest(setting)
+                except KeyError:
+                    continue
+                if option.validator:
+                    value = self.get(section, setting)
+                    try:
+                        new_value = option.validator(
+                            setting, value, option_parser,
+                            config_parser=self, config_section=section)
+                    except Exception as err:
+                        raise ValueError(f'Error in config file "{filename}", '
+                                         f'section "[{section}]":\n'
+                                         f'    {io.error_string(err)}\n'
+                                         f'        {setting} = {value}')
+                    self.set(section, setting, new_value)
+                if option.overrides:
+                    self.set(section, option.overrides, None)
+
+    def optionxform(self, optionstr):
+        """
+        Lowercase and transform '-' to '_'.
+
+        So the cmdline form of option names can be used in config files.
+        """
+        return optionstr.lower().replace('-', '_')
+
+    def get_section(self, section):
+        """
+        Return a given section as a dictionary.
+
+        Return empty dictionary if the section doesn't exist.
+
+        Deprecated. Use the configparser "Mapping Protocol Access" and
+        catch KeyError.
+        """
+        warnings.warn('frontend.OptionParser.get_section() '
+                      'will be removed in Docutils 0.21 or later.',
+                      DeprecationWarning, stacklevel=2)
+        try:
+            return dict(self[section])
+        except KeyError:
+            return {}
+
+
+class ConfigDeprecationWarning(FutureWarning):
+    """Warning for deprecated configuration file features."""
+
+
+def get_default_settings(*components):
+    """Return default runtime settings for `components`.
+
+    Return a `frontend.Values` instance with defaults for generic Docutils
+    settings and settings from the `components` (`SettingsSpec` instances).
+
+    This corresponds to steps 1 and 2 in the `runtime settings priority`__.
+
+    __ https://docutils.sourceforge.io/docs/api/runtime-settings.html
+       #settings-priority
+    """
+    with warnings.catch_warnings():
+        warnings.filterwarnings('ignore', category=DeprecationWarning)
+        return OptionParser(components).get_default_values()