import errno
import os
import sys
from contextlib import contextmanager
from itertools import repeat
from functools import update_wrapper

from .types import convert_type, IntRange, BOOL
from .utils import make_str, make_default_short_help, echo, get_os_args
from .exceptions import ClickException, UsageError, BadParameter, Abort, \
     MissingParameter
from .termui import prompt, confirm
from .formatting import HelpFormatter, join_options
from .parser import OptionParser, split_opt
from .globals import push_context, pop_context

from ._compat import PY2, isidentifier, iteritems
from ._unicodefun import _check_for_unicode_literals, _verify_python3_env


_missing = object()


SUBCOMMAND_METAVAR = 'COMMAND [ARGS]...'
SUBCOMMANDS_METAVAR = 'COMMAND1 [ARGS]... [COMMAND2 [ARGS]...]...'


def _bashcomplete(cmd, prog_name, complete_var=None):
    """Internal handler for the bash completion support."""
    if complete_var is None:
        complete_var = '_%s_COMPLETE' % (prog_name.replace('-', '_')).upper()
    complete_instr = os.environ.get(complete_var)
    if not complete_instr:
        return

    from ._bashcomplete import bashcomplete
    if bashcomplete(cmd, prog_name, complete_var, complete_instr):
        sys.exit(1)


def _check_multicommand(base_command, cmd_name, cmd, register=False):
    if not base_command.chain or not isinstance(cmd, MultiCommand):
        return
    if register:
        hint = 'It is not possible to add multi commands as children to ' \
               'another multi command that is in chain mode'
    else:
        hint = 'Found a multi command as subcommand to a multi command ' \
               'that is in chain mode.  This is not supported'
    raise RuntimeError('%s.  Command "%s" is set to chain and "%s" was '
                       'added as subcommand but it in itself is a '
                       'multi command.  ("%s" is a %s within a chained '
                       '%s named "%s").  This restriction was supposed to '
                       'be lifted in 6.0 but the fix was flawed.  This '
                       'will be fixed in Click 7.0' % (
                           hint, base_command.name, cmd_name,
                           cmd_name, cmd.__class__.__name__,
                           base_command.__class__.__name__,
                           base_command.name))


def batch(iterable, batch_size):
    return list(zip(*repeat(iter(iterable), batch_size)))


def invoke_param_callback(callback, ctx, param, value):
    code = getattr(callback, '__code__', None)
    args = getattr(code, 'co_argcount', 3)

    if args < 3:
        # This will become a warning in Click 3.0:
        from warnings import warn
        warn(Warning('Invoked legacy parameter callback "%s".  The new '
                     'signature for such callbacks starting with '
                     'click 2.0 is (ctx, param, value).'
                     % callback), stacklevel=3)
        return callback(ctx, value)
    return callback(ctx, param, value)


@contextmanager
def augment_usage_errors(ctx, param=None):
    """Context manager that attaches extra information to exceptions that
    fly.
    """
    try:
        yield
    except BadParameter as e:
        if e.ctx is None:
            e.ctx = ctx
        if param is not None and e.param is None:
            e.param = param
        raise
    except UsageError as e:
        if e.ctx is None:
            e.ctx = ctx
        raise


def iter_params_for_processing(invocation_order, declaration_order):
    """Given a sequence of parameters in the order as should be considered
    for processing and an iterable of parameters that exist, this returns
    a list in the correct order as they should be processed.
    """
    def sort_key(item):
        try:
            idx = invocation_order.index(item)
        except ValueError:
            idx = float('inf')
        return (not item.is_eager, idx)

    return sorted(declaration_order, key=sort_key)


class Context(object):
    """The context is a special internal object that holds state relevant
    for the script execution at every single level.  It's normally invisible
    to commands unless they opt-in to getting access to it.

    The context is useful as it can pass internal objects around and can
    control special execution features such as reading data from
    environment variables.

    A context can be used as context manager in which case it will call
    :meth:`close` on teardown.

    .. versionadded:: 2.0
       Added the `resilient_parsing`, `help_option_names`,
       `token_normalize_func` parameters.

    .. versionadded:: 3.0
       Added the `allow_extra_args` and `allow_interspersed_args`
       parameters.

    .. versionadded:: 4.0
       Added the `color`, `ignore_unknown_options`, and
       `max_content_width` parameters.

    :param command: the command class for this context.
    :param parent: the parent context.
    :param info_name: the info name for this invocation.  Generally this
                      is the most descriptive name for the script or
                      command.  For the toplevel script it is usually
                      the name of the script, for commands below it it's
                      the name of the script.
    :param obj: an arbitrary object of user data.
    :param auto_envvar_prefix: the prefix to use for automatic environment
                               variables.  If this is `None` then reading
                               from environment variables is disabled.  This
                               does not affect manually set environment
                               variables which are always read.
    :param default_map: a dictionary (like object) with default values
                        for parameters.
    :param terminal_width: the width of the terminal.  The default is
                           inherit from parent context.  If no context
                           defines the terminal width then auto
                           detection will be applied.
    :param max_content_width: the maximum width for content rendered by
                              Click (this currently only affects help
                              pages).  This defaults to 80 characters if
                              not overridden.  In other words: even if the
                              terminal is larger than that, Click will not
                              format things wider than 80 characters by
                              default.  In addition to that, formatters might
                              add some safety mapping on the right.
    :param resilient_parsing: if this flag is enabled then Click will
                              parse without any interactivity or callback
                              invocation.  This is useful for implementing
                              things such as completion support.
    :param allow_extra_args: if this is set to `True` then extra arguments
                             at the end will not raise an error and will be
                             kept on the context.  The default is to inherit
                             from the command.
    :param allow_interspersed_args: if this is set to `False` then options
                                    and arguments cannot be mixed.  The
                                    default is to inherit from the command.
    :param ignore_unknown_options: instructs click to ignore options it does
                                   not know and keeps them for later
                                   processing.
    :param help_option_names: optionally a list of strings that define how
                              the default help parameter is named.  The
                              default is ``['--help']``.
    :param token_normalize_func: an optional function that is used to
                                 normalize tokens (options, choices,
                                 etc.).  This for instance can be used to
                                 implement case insensitive behavior.
    :param color: controls if the terminal supports ANSI colors or not.  The
                  default is autodetection.  This is only needed if ANSI
                  codes are used in texts that Click prints which is by
                  default not the case.  This for instance would affect
                  help output.
    """

    def __init__(self, command, parent=None, info_name=None, obj=None,
                 auto_envvar_prefix=None, default_map=None,
                 terminal_width=None, max_content_width=None,
                 resilient_parsing=False, allow_extra_args=None,
                 allow_interspersed_args=None,
                 ignore_unknown_options=None, help_option_names=None,
                 token_normalize_func=None, color=None):
        #: the parent context or `None` if none exists.
        self.parent = parent
        #: the :class:`Command` for this context.
        self.command = command
        #: the descriptive information name
        self.info_name = info_name
        #: the parsed parameters except if the value is hidden in which
        #: case it's not remembered.
        self.params = {}
        #: the leftover arguments.
        self.args = []
        #: protected arguments.  These are arguments that are prepended
        #: to `args` when certain parsing scenarios are encountered but
        #: must be never propagated to another arguments.  This is used
        #: to implement nested parsing.
        self.protected_args = []
        if obj is None and parent is not None:
            obj = parent.obj
        #: the user object stored.
        self.obj = obj
        self._meta = getattr(parent, 'meta', {})

        #: A dictionary (-like object) with defaults for parameters.
        if default_map is None \
           and parent is not None \
           and parent.default_map is not None:
            default_map = parent.default_map.get(info_name)
        self.default_map = default_map

        #: This flag indicates if a subcommand is going to be executed. A
        #: group callback can use this information to figure out if it's
        #: being executed directly or because the execution flow passes
        #: onwards to a subcommand. By default it's None, but it can be
        #: the name of the subcommand to execute.
        #:
        #: If chaining is enabled this will be set to ``'*'`` in case
        #: any commands are executed.  It is however not possible to
        #: figure out which ones.  If you require this knowledge you
        #: should use a :func:`resultcallback`.
        self.invoked_subcommand = None

        if terminal_width is None and parent is not None:
            terminal_width = parent.terminal_width
        #: The width of the terminal (None is autodetection).
        self.terminal_width = terminal_width

        if max_content_width is None and parent is not None:
            max_content_width = parent.max_content_width
        #: The maximum width of formatted content (None implies a sensible
        #: default which is 80 for most things).
        self.max_content_width = max_content_width

        if allow_extra_args is None:
            allow_extra_args = command.allow_extra_args
        #: Indicates if the context allows extra args or if it should
        #: fail on parsing.
        #:
        #: .. versionadded:: 3.0
        self.allow_extra_args = allow_extra_args

        if allow_interspersed_args is None:
            allow_interspersed_args = command.allow_interspersed_args
        #: Indicates if the context allows mixing of arguments and
        #: options or not.
        #:
        #: .. versionadded:: 3.0
        self.allow_interspersed_args = allow_interspersed_args

        if ignore_unknown_options is None:
            ignore_unknown_options = command.ignore_unknown_options
        #: Instructs click to ignore options that a command does not
        #: understand and will store it on the context for later
        #: processing.  This is primarily useful for situations where you
        #: want to call into external programs.  Generally this pattern is
        #: strongly discouraged because it's not possibly to losslessly
        #: forward all arguments.
        #:
        #: .. versionadded:: 4.0
        self.ignore_unknown_options = ignore_unknown_options

        if help_option_names is None:
            if parent is not None:
                help_option_names = parent.help_option_names
            else:
                help_option_names = ['--help']

        #: The names for the help options.
        self.help_option_names = help_option_names

        if token_normalize_func is None and parent is not None:
            token_normalize_func = parent.token_normalize_func

        #: An optional normalization function for tokens.  This is
        #: options, choices, commands etc.
        self.token_normalize_func = token_normalize_func

        #: Indicates if resilient parsing is enabled.  In that case Click
        #: will do its best to not cause any failures.
        self.resilient_parsing = resilient_parsing

        # If there is no envvar prefix yet, but the parent has one and
        # the command on this level has a name, we can expand the envvar
        # prefix automatically.
        if auto_envvar_prefix is None:
            if parent is not None \
               and parent.auto_envvar_prefix is not None and \
               self.info_name is not None:
                auto_envvar_prefix = '%s_%s' % (parent.auto_envvar_prefix,
                                           self.info_name.upper())
        else:
            self.auto_envvar_prefix = auto_envvar_prefix.upper()
        self.auto_envvar_prefix = auto_envvar_prefix

        if color is None and parent is not None:
            color = parent.color

        #: Controls if styling output is wanted or not.
        self.color = color

        self._close_callbacks = []
        self._depth = 0

    def __enter__(self):
        self._depth += 1
        push_context(self)
        return self

    def __exit__(self, exc_type, exc_value, tb):
        self._depth -= 1
        if self._depth == 0:
            self.close()
        pop_context()

    @contextmanager
    def scope(self, cleanup=True):
        """This helper method can be used with the context object to promote
        it to the current thread local (see :func:`get_current_context`).
        The default behavior of this is to invoke the cleanup functions which
        can be disabled by setting `cleanup` to `False`.  The cleanup
        functions are typically used for things such as closing file handles.

        If the cleanup is intended the context object can also be directly
        used as a context manager.

        Example usage::

            with ctx.scope():
                assert get_current_context() is ctx

        This is equivalent::

            with ctx:
                assert get_current_context() is ctx

        .. versionadded:: 5.0

        :param cleanup: controls if the cleanup functions should be run or
                        not.  The default is to run these functions.  In
                        some situations the context only wants to be
                        temporarily pushed in which case this can be disabled.
                        Nested pushes automatically defer the cleanup.
        """
        if not cleanup:
            self._depth += 1
        try:
            with self as rv:
                yield rv
        finally:
            if not cleanup:
                self._depth -= 1

    @property
    def meta(self):
        """This is a dictionary which is shared with all the contexts
        that are nested.  It exists so that click utiltiies can store some
        state here if they need to.  It is however the responsibility of
        that code to manage this dictionary well.

        The keys are supposed to be unique dotted strings.  For instance
        module paths are a good choice for it.  What is stored in there is
        irrelevant for the operation of click.  However what is important is
        that code that places data here adheres to the general semantics of
        the system.

        Example usage::

            LANG_KEY = __name__ + '.lang'

            def set_language(value):
                ctx = get_current_context()
                ctx.meta[LANG_KEY] = value

            def get_language():
                return get_current_context().meta.get(LANG_KEY, 'en_US')

        .. versionadded:: 5.0
        """
        return self._meta

    def make_formatter(self):
        """Creates the formatter for the help and usage output."""
        return HelpFormatter(width=self.terminal_width,
                             max_width=self.max_content_width)

    def call_on_close(self, f):
        """This decorator remembers a function as callback that should be
        executed when the context tears down.  This is most useful to bind
        resource handling to the script execution.  For instance, file objects
        opened by the :class:`File` type will register their close callbacks
        here.

        :param f: the function to execute on teardown.
        """
        self._close_callbacks.append(f)
        return f

    def close(self):
        """Invokes all close callbacks."""
        for cb in self._close_callbacks:
            cb()
        self._close_callbacks = []

    @property
    def command_path(self):
        """The computed command path.  This is used for the ``usage``
        information on the help page.  It's automatically created by
        combining the info names of the chain of contexts to the root.
        """
        rv = ''
        if self.info_name is not None:
            rv = self.info_name
        if self.parent is not None:
            rv = self.parent.command_path + ' ' + rv
        return rv.lstrip()

    def find_root(self):
        """Finds the outermost context."""
        node = self
        while node.parent is not None:
            node = node.parent
        return node

    def find_object(self, object_type):
        """Finds the closest object of a given type."""
        node = self
        while node is not None:
            if isinstance(node.obj, object_type):
                return node.obj
            node = node.parent

    def ensure_object(self, object_type):
        """Like :meth:`find_object` but sets the innermost object to a
        new instance of `object_type` if it does not exist.
        """
        rv = self.find_object(object_type)
        if rv is None:
            self.obj = rv = object_type()
        return rv

    def lookup_default(self, name):
        """Looks up the default for a parameter name.  This by default
        looks into the :attr:`default_map` if available.
        """
        if self.default_map is not None:
            rv = self.default_map.get(name)
            if callable(rv):
                rv = rv()
            return rv

    def fail(self, message):
        """Aborts the execution of the program with a specific error
        message.

        :param message: the error message to fail with.
        """
        raise UsageError(message, self)

    def abort(self):
        """Aborts the script."""
        raise Abort()

    def exit(self, code=0):
        """Exits the application with a given exit code."""
        sys.exit(code)

    def get_usage(self):
        """Helper method to get formatted usage string for the current
        context and command.
        """
        return self.command.get_usage(self)

    def get_help(self):
        """Helper method to get formatted help page for the current
        context and command.
        """
        return self.command.get_help(self)

    def invoke(*args, **kwargs):
        """Invokes a command callback in exactly the way it expects.  There
        are two ways to invoke this method:

        1.  the first argument can be a callback and all other arguments and
            keyword arguments are forwarded directly to the function.
        2.  the first argument is a click command object.  In that case all
            arguments are forwarded as well but proper click parameters
            (options and click arguments) must be keyword arguments and Click
            will fill in defaults.

        Note that before Click 3.2 keyword arguments were not properly filled
        in against the intention of this code and no context was created.  For
        more information about this change and why it was done in a bugfix
        release see :ref:`upgrade-to-3.2`.
        """
        self, callback = args[:2]
        ctx = self

        # It's also possible to invoke another command which might or
        # might not have a callback.  In that case we also fill
        # in defaults and make a new context for this command.
        if isinstance(callback, Command):
            other_cmd = callback
            callback = other_cmd.callback
            ctx = Context(other_cmd, info_name=other_cmd.name, parent=self)
            if callback is None:
                raise TypeError('The given command does not have a '
                                'callback that can be invoked.')

            for param in other_cmd.params:
                if param.name not in kwargs and param.expose_value:
                    kwargs[param.name] = param.get_default(ctx)

        args = args[2:]
        with augment_usage_errors(self):
            with ctx:
                return callback(*args, **kwargs)

    def forward(*args, **kwargs):
        """Similar to :meth:`invoke` but fills in default keyword
        arguments from the current context if the other command expects
        it.  This cannot invoke callbacks directly, only other commands.
        """
        self, cmd = args[:2]

        # It's also possible to invoke another command which might or
        # might not have a callback.
        if not isinstance(cmd, Command):
            raise TypeError('Callback is not a command.')

        for param in self.params:
            if param not in kwargs:
                kwargs[param] = self.params[param]

        return self.invoke(cmd, **kwargs)


class BaseCommand(object):
    """The base command implements the minimal API contract of commands.
    Most code will never use this as it does not implement a lot of useful
    functionality but it can act as the direct subclass of alternative
    parsing methods that do not depend on the Click parser.

    For instance, this can be used to bridge Click and other systems like
    argparse or docopt.

    Because base commands do not implement a lot of the API that other
    parts of Click take for granted, they are not supported for all
    operations.  For instance, they cannot be used with the decorators
    usually and they have no built-in callback system.

    .. versionchanged:: 2.0
       Added the `context_settings` parameter.

    :param name: the name of the command to use unless a group overrides it.
    :param context_settings: an optional dictionary with defaults that are
                             passed to the context object.
    """
    #: the default for the :attr:`Context.allow_extra_args` flag.
    allow_extra_args = False
    #: the default for the :attr:`Context.allow_interspersed_args` flag.
    allow_interspersed_args = True
    #: the default for the :attr:`Context.ignore_unknown_options` flag.
    ignore_unknown_options = False

    def __init__(self, name, context_settings=None):
        #: the name the command thinks it has.  Upon registering a command
        #: on a :class:`Group` the group will default the command name
        #: with this information.  You should instead use the
        #: :class:`Context`\'s :attr:`~Context.info_name` attribute.
        self.name = name
        if context_settings is None:
            context_settings = {}
        #: an optional dictionary with defaults passed to the context.
        self.context_settings = context_settings

    def get_usage(self, ctx):
        raise NotImplementedError('Base commands cannot get usage')

    def get_help(self, ctx):
        raise NotImplementedError('Base commands cannot get help')

    def make_context(self, info_name, args, parent=None, **extra):
        """This function when given an info name and arguments will kick
        off the parsing and create a new :class:`Context`.  It does not
        invoke the actual command callback though.

        :param info_name: the info name for this invokation.  Generally this
                          is the most descriptive name for the script or
                          command.  For the toplevel script it's usually
                          the name of the script, for commands below it it's
                          the name of the script.
        :param args: the arguments to parse as list of strings.
        :param parent: the parent context if available.
        :param extra: extra keyword arguments forwarded to the context
                      constructor.
        """
        for key, value in iteritems(self.context_settings):
            if key not in extra:
                extra[key] = value
        ctx = Context(self, info_name=info_name, parent=parent, **extra)
        with ctx.scope(cleanup=False):
            self.parse_args(ctx, args)
        return ctx

    def parse_args(self, ctx, args):
        """Given a context and a list of arguments this creates the parser
        and parses the arguments, then modifies the context as necessary.
        This is automatically invoked by :meth:`make_context`.
        """
        raise NotImplementedError('Base commands do not know how to parse '
                                  'arguments.')

    def invoke(self, ctx):
        """Given a context, this invokes the command.  The default
        implementation is raising a not implemented error.
        """
        raise NotImplementedError('Base commands are not invokable by default')

    def main(self, args=None, prog_name=None, complete_var=None,
             standalone_mode=True, **extra):
        """This is the way to invoke a script with all the bells and
        whistles as a command line application.  This will always terminate
        the application after a call.  If this is not wanted, ``SystemExit``
        needs to be caught.

        This method is also available by directly calling the instance of
        a :class:`Command`.

        .. versionadded:: 3.0
           Added the `standalone_mode` flag to control the standalone mode.

        :param args: the arguments that should be used for parsing.  If not
                     provided, ``sys.argv[1:]`` is used.
        :param prog_name: the program name that should be used.  By default
                          the program name is constructed by taking the file
                          name from ``sys.argv[0]``.
        :param complete_var: the environment variable that controls the
                             bash completion support.  The default is
                             ``"_<prog_name>_COMPLETE"`` with prog name in
                             uppercase.
        :param standalone_mode: the default behavior is to invoke the script
                                in standalone mode.  Click will then
                                handle exceptions and convert them into
                                error messages and the function will never
                                return but shut down the interpreter.  If
                                this is set to `False` they will be
                                propagated to the caller and the return
                                value of this function is the return value
                                of :meth:`invoke`.
        :param extra: extra keyword arguments are forwarded to the context
                      constructor.  See :class:`Context` for more information.
        """
        # If we are in Python 3, we will verify that the environment is
        # sane at this point of reject further execution to avoid a
        # broken script.
        if not PY2:
            _verify_python3_env()
        else:
            _check_for_unicode_literals()

        if args is None:
            args = get_os_args()
        else:
            args = list(args)

        if prog_name is None:
            prog_name = make_str(os.path.basename(
                sys.argv and sys.argv[0] or __file__))

        # Hook for the Bash completion.  This only activates if the Bash
        # completion is actually enabled, otherwise this is quite a fast
        # noop.
        _bashcomplete(self, prog_name, complete_var)

        try:
            try:
                with self.make_context(prog_name, args, **extra) as ctx:
                    rv = self.invoke(ctx)
                    if not standalone_mode:
                        return rv
                    ctx.exit()
            except (EOFError, KeyboardInterrupt):
                echo(file=sys.stderr)
                raise Abort()
            except ClickException as e:
                if not standalone_mode:
                    raise
                e.show()
                sys.exit(e.exit_code)
            except IOError as e:
                if e.errno == errno.EPIPE:
                    sys.exit(1)
                else:
                    raise
        except Abort:
            if not standalone_mode:
                raise
            echo('Aborted!', file=sys.stderr)
            sys.exit(1)

    def __call__(self, *args, **kwargs):
        """Alias for :meth:`main`."""
        return self.main(*args, **kwargs)


class Command(BaseCommand):
    """Commands are the basic building block of command line interfaces in
    Click.  A basic command handles command line parsing and might dispatch
    more parsing to commands nested below it.

    .. versionchanged:: 2.0
       Added the `context_settings` parameter.

    :param name: the name of the command to use unless a group overrides it.
    :param context_settings: an optional dictionary with defaults that are
                             passed to the context object.
    :param callback: the callback to invoke.  This is optional.
    :param params: the parameters to register with this command.  This can
                   be either :class:`Option` or :class:`Argument` objects.
    :param help: the help string to use for this command.
    :param epilog: like the help string but it's printed at the end of the
                   help page after everything else.
    :param short_help: the short help to use for this command.  This is
                       shown on the command listing of the parent command.
    :param add_help_option: by default each command registers a ``--help``
                            option.  This can be disabled by this parameter.
    """

    def __init__(self, name, context_settings=None, callback=None,
                 params=None, help=None, epilog=None, short_help=None,
                 options_metavar='[OPTIONS]', add_help_option=True):
        BaseCommand.__init__(self, name, context_settings)
        #: the callback to execute when the command fires.  This might be
        #: `None` in which case nothing happens.
        self.callback = callback
        #: the list of parameters for this command in the order they
        #: should show up in the help page and execute.  Eager parameters
        #: will automatically be handled before non eager ones.
        self.params = params or []
        self.help = help
        self.epilog = epilog
        self.options_metavar = options_metavar
        if short_help is None and help:
            short_help = make_default_short_help(help)
        self.short_help = short_help
        self.add_help_option = add_help_option

    def get_usage(self, ctx):
        formatter = ctx.make_formatter()
        self.format_usage(ctx, formatter)
        return formatter.getvalue().rstrip('\n')

    def get_params(self, ctx):
        rv = self.params
        help_option = self.get_help_option(ctx)
        if help_option is not None:
            rv = rv + [help_option]
        return rv

    def format_usage(self, ctx, formatter):
        """Writes the usage line into the formatter."""
        pieces = self.collect_usage_pieces(ctx)
        formatter.write_usage(ctx.command_path, ' '.join(pieces))

    def collect_usage_pieces(self, ctx):
        """Returns all the pieces that go into the usage line and returns
        it as a list of strings.
        """
        rv = [self.options_metavar]
        for param in self.get_params(ctx):
            rv.extend(param.get_usage_pieces(ctx))
        return rv

    def get_help_option_names(self, ctx):
        """Returns the names for the help option."""
        all_names = set(ctx.help_option_names)
        for param in self.params:
            all_names.difference_update(param.opts)
            all_names.difference_update(param.secondary_opts)
        return all_names

    def get_help_option(self, ctx):
        """Returns the help option object."""
        help_options = self.get_help_option_names(ctx)
        if not help_options or not self.add_help_option:
            return

        def show_help(ctx, param, value):
            if value and not ctx.resilient_parsing:
                echo(ctx.get_help(), color=ctx.color)
                ctx.exit()
        return Option(help_options, is_flag=True,
                      is_eager=True, expose_value=False,
                      callback=show_help,
                      help='Show this message and exit.')

    def make_parser(self, ctx):
        """Creates the underlying option parser for this command."""
        parser = OptionParser(ctx)
        parser.allow_interspersed_args = ctx.allow_interspersed_args
        parser.ignore_unknown_options = ctx.ignore_unknown_options
        for param in self.get_params(ctx):
            param.add_to_parser(parser, ctx)
        return parser

    def get_help(self, ctx):
        """Formats the help into a string and returns it.  This creates a
        formatter and will call into the following formatting methods:
        """
        formatter = ctx.make_formatter()
        self.format_help(ctx, formatter)
        return formatter.getvalue().rstrip('\n')

    def format_help(self, ctx, formatter):
        """Writes the help into the formatter if it exists.

        This calls into the following methods:

        -   :meth:`format_usage`
        -   :meth:`format_help_text`
        -   :meth:`format_options`
        -   :meth:`format_epilog`
        """
        self.format_usage(ctx, formatter)
        self.format_help_text(ctx, formatter)
        self.format_options(ctx, formatter)
        self.format_epilog(ctx, formatter)

    def format_help_text(self, ctx, formatter):
        """Writes the help text to the formatter if it exists."""
        if self.help:
            formatter.write_paragraph()
            with formatter.indentation():
                formatter.write_text(self.help)

    def format_options(self, ctx, formatter):
        """Writes all the options into the formatter if they exist."""
        opts = []
        for param in self.get_params(ctx):
            rv = param.get_help_record(ctx)
            if rv is not None:
                opts.append(rv)

        if opts:
            with formatter.section('Options'):
                formatter.write_dl(opts)

    def format_epilog(self, ctx, formatter):
        """Writes the epilog into the formatter if it exists."""
        if self.epilog:
            formatter.write_paragraph()
            with formatter.indentation():
                formatter.write_text(self.epilog)

    def parse_args(self, ctx, args):
        parser = self.make_parser(ctx)
        opts, args, param_order = parser.parse_args(args=args)

        for param in iter_params_for_processing(
                param_order, self.get_params(ctx)):
            value, args = param.handle_parse_result(ctx, opts, args)

        if args and not ctx.allow_extra_args and not ctx.resilient_parsing:
            ctx.fail('Got unexpected extra argument%s (%s)'
                     % (len(args) != 1 and 's' or '',
                        ' '.join(map(make_str, args))))

        ctx.args = args
        return args

    def invoke(self, ctx):
        """Given a context, this invokes the attached callback (if it exists)
        in the right way.
        """
        if self.callback is not None:
            return ctx.invoke(self.callback, **ctx.params)


class MultiCommand(Command):
    """A multi command is the basic implementation of a command that
    dispatches to subcommands.  The most common version is the
    :class:`Group`.

    :param invoke_without_command: this controls how the multi command itself
                                   is invoked.  By default it's only invoked
                                   if a subcommand is provided.
    :param no_args_is_help: this controls what happens if no arguments are
                            provided.  This option is enabled by default if
                            `invoke_without_command` is disabled or disabled
                            if it's enabled.  If enabled this will add
                            ``--help`` as argument if no arguments are
                            passed.
    :param subcommand_metavar: the string that is used in the documentation
                               to indicate the subcommand place.
    :param chain: if this is set to `True` chaining of multiple subcommands
                  is enabled.  This restricts the form of commands in that
                  they cannot have optional arguments but it allows
                  multiple commands to be chained together.
    :param result_callback: the result callback to attach to this multi
                            command.
    """
    allow_extra_args = True
    allow_interspersed_args = False

    def __init__(self, name=None, invoke_without_command=False,
                 no_args_is_help=None, subcommand_metavar=None,
                 chain=False, result_callback=None, **attrs):
        Command.__init__(self, name, **attrs)
        if no_args_is_help is None:
            no_args_is_help = not invoke_without_command
        self.no_args_is_help = no_args_is_help
        self.invoke_without_command = invoke_without_command
        if subcommand_metavar is None:
            if chain:
                subcommand_metavar = SUBCOMMANDS_METAVAR
            else:
                subcommand_metavar = SUBCOMMAND_METAVAR
        self.subcommand_metavar = subcommand_metavar
        self.chain = chain
        #: The result callback that is stored.  This can be set or
        #: overridden with the :func:`resultcallback` decorator.
        self.result_callback = result_callback

        if self.chain:
            for param in self.params:
                if isinstance(param, Argument) and not param.required:
                    raise RuntimeError('Multi commands in chain mode cannot '
                                       'have optional arguments.')

    def collect_usage_pieces(self, ctx):
        rv = Command.collect_usage_pieces(self, ctx)
        rv.append(self.subcommand_metavar)
        return rv

    def format_options(self, ctx, formatter):
        Command.format_options(self, ctx, formatter)
        self.format_commands(ctx, formatter)

    def resultcallback(self, replace=False):
        """Adds a result callback to the chain command.  By default if a
        result callback is already registered this will chain them but
        this can be disabled with the `replace` parameter.  The result
        callback is invoked with the return value of the subcommand
        (or the list of return values from all subcommands if chaining
        is enabled) as well as the parameters as they would be passed
        to the main callback.

        Example::

            @click.group()
            @click.option('-i', '--input', default=23)
            def cli(input):
                return 42

            @cli.resultcallback()
            def process_result(result, input):
                return result + input

        .. versionadded:: 3.0

        :param replace: if set to `True` an already existing result
                        callback will be removed.
        """
        def decorator(f):
            old_callback = self.result_callback
            if old_callback is None or replace:
                self.result_callback = f
                return f
            def function(__value, *args, **kwargs):
                return f(old_callback(__value, *args, **kwargs),
                         *args, **kwargs)
            self.result_callback = rv = update_wrapper(function, f)
            return rv
        return decorator

    def format_commands(self, ctx, formatter):
        """Extra format methods for multi methods that adds all the commands
        after the options.
        """
        rows = []
        for subcommand in self.list_commands(ctx):
            cmd = self.get_command(ctx, subcommand)
            # What is this, the tool lied about a command.  Ignore it
            if cmd is None:
                continue

            help = cmd.short_help or ''
            rows.append((subcommand, help))

        if rows:
            with formatter.section('Commands'):
                formatter.write_dl(rows)

    def parse_args(self, ctx, args):
        if not args and self.no_args_is_help and not ctx.resilient_parsing:
            echo(ctx.get_help(), color=ctx.color)
            ctx.exit()

        rest = Command.parse_args(self, ctx, args)
        if self.chain:
            ctx.protected_args = rest
            ctx.args = []
        elif rest:
            ctx.protected_args, ctx.args = rest[:1], rest[1:]

        return ctx.args

    def invoke(self, ctx):
        def _process_result(value):
            if self.result_callback is not None:
                value = ctx.invoke(self.result_callback, value,
                                   **ctx.params)
            return value

        if not ctx.protected_args:
            # If we are invoked without command the chain flag controls
            # how this happens.  If we are not in chain mode, the return
            # value here is the return value of the command.
            # If however we are in chain mode, the return value is the
            # return value of the result processor invoked with an empty
            # list (which means that no subcommand actually was executed).
            if self.invoke_without_command:
                if not self.chain:
                    return Command.invoke(self, ctx)
                with ctx:
                    Command.invoke(self, ctx)
                    return _process_result([])
            ctx.fail('Missing command.')

        # Fetch args back out
        args = ctx.protected_args + ctx.args
        ctx.args = []
        ctx.protected_args = []

        # If we're not in chain mode, we only allow the invocation of a
        # single command but we also inform the current context about the
        # name of the command to invoke.
        if not self.chain:
            # Make sure the context is entered so we do not clean up
            # resources until the result processor has worked.
            with ctx:
                cmd_name, cmd, args = self.resolve_command(ctx, args)
                ctx.invoked_subcommand = cmd_name
                Command.invoke(self, ctx)
                sub_ctx = cmd.make_context(cmd_name, args, parent=ctx)
                with sub_ctx:
                    return _process_result(sub_ctx.command.invoke(sub_ctx))

        # In chain mode we create the contexts step by step, but after the
        # base command has been invoked.  Because at that point we do not
        # know the subcommands yet, the invoked subcommand attribute is
        # set to ``*`` to inform the command that subcommands are executed
        # but nothing else.
        with ctx:
            ctx.invoked_subcommand = args and '*' or None
            Command.invoke(self, ctx)

            # Otherwise we make every single context and invoke them in a
            # chain.  In that case the return value to the result processor
            # is the list of all invoked subcommand's results.
            contexts = []
            while args:
                cmd_name, cmd, args = self.resolve_command(ctx, args)
                sub_ctx = cmd.make_context(cmd_name, args, parent=ctx,
                                           allow_extra_args=True,
                                           allow_interspersed_args=False)
                contexts.append(sub_ctx)
                args, sub_ctx.args = sub_ctx.args, []

            rv = []
            for sub_ctx in contexts:
                with sub_ctx:
                    rv.append(sub_ctx.command.invoke(sub_ctx))
            return _process_result(rv)

    def resolve_command(self, ctx, args):
        cmd_name = make_str(args[0])
        original_cmd_name = cmd_name

        # Get the command
        cmd = self.get_command(ctx, cmd_name)

        # If we can't find the command but there is a normalization
        # function available, we try with that one.
        if cmd is None and ctx.token_normalize_func is not None:
            cmd_name = ctx.token_normalize_func(cmd_name)
            cmd = self.get_command(ctx, cmd_name)

        # If we don't find the command we want to show an error message
        # to the user that it was not provided.  However, there is
        # something else we should do: if the first argument looks like
        # an option we want to kick off parsing again for arguments to
        # resolve things like --help which now should go to the main
        # place.
        if cmd is None:
            if split_opt(cmd_name)[0]:
                self.parse_args(ctx, ctx.args)
            ctx.fail('No such command "%s".' % original_cmd_name)

        return cmd_name, cmd, args[1:]

    def get_command(self, ctx, cmd_name):
        """Given a context and a command name, this returns a
        :class:`Command` object if it exists or returns `None`.
        """
        raise NotImplementedError()

    def list_commands(self, ctx):
        """Returns a list of subcommand names in the order they should
        appear.
        """
        return []


class Group(MultiCommand):
    """A group allows a command to have subcommands attached.  This is the
    most common way to implement nesting in Click.

    :param commands: a dictionary of commands.
    """

    def __init__(self, name=None, commands=None, **attrs):
        MultiCommand.__init__(self, name, **attrs)
        #: the registered subcommands by their exported names.
        self.commands = commands or {}

    def add_command(self, cmd, name=None):
        """Registers another :class:`Command` with this group.  If the name
        is not provided, the name of the command is used.
        """
        name = name or cmd.name
        if name is None:
            raise TypeError('Command has no name.')
        _check_multicommand(self, name, cmd, register=True)
        self.commands[name] = cmd

    def command(self, *args, **kwargs):
        """A shortcut decorator for declaring and attaching a command to
        the group.  This takes the same arguments as :func:`command` but
        immediately registers the created command with this instance by
        calling into :meth:`add_command`.
        """
        def decorator(f):
            cmd = command(*args, **kwargs)(f)
            self.add_command(cmd)
            return cmd
        return decorator

    def group(self, *args, **kwargs):
        """A shortcut decorator for declaring and attaching a group to
        the group.  This takes the same arguments as :func:`group` but
        immediately registers the created command with this instance by
        calling into :meth:`add_command`.
        """
        def decorator(f):
            cmd = group(*args, **kwargs)(f)
            self.add_command(cmd)
            return cmd
        return decorator

    def get_command(self, ctx, cmd_name):
        return self.commands.get(cmd_name)

    def list_commands(self, ctx):
        return sorted(self.commands)


class CommandCollection(MultiCommand):
    """A command collection is a multi command that merges multiple multi
    commands together into one.  This is a straightforward implementation
    that accepts a list of different multi commands as sources and
    provides all the commands for each of them.
    """

    def __init__(self, name=None, sources=None, **attrs):
        MultiCommand.__init__(self, name, **attrs)
        #: The list of registered multi commands.
        self.sources = sources or []

    def add_source(self, multi_cmd):
        """Adds a new multi command to the chain dispatcher."""
        self.sources.append(multi_cmd)

    def get_command(self, ctx, cmd_name):
        for source in self.sources:
            rv = source.get_command(ctx, cmd_name)
            if rv is not None:
                if self.chain:
                    _check_multicommand(self, cmd_name, rv)
                return rv

    def list_commands(self, ctx):
        rv = set()
        for source in self.sources:
            rv.update(source.list_commands(ctx))
        return sorted(rv)


class Parameter(object):
    """A parameter to a command comes in two versions: they are either
    :class:`Option`\s or :class:`Argument`\s.  Other subclasses are currently
    not supported by design as some of the internals for parsing are
    intentionally not finalized.

    Some settings are supported by both options and arguments.

    .. versionchanged:: 2.0
       Changed signature for parameter callback to also be passed the
       parameter.  In Click 2.0, the old callback format will still work,
       but it will raise a warning to give you change to migrate the
       code easier.

    :param param_decls: the parameter declarations for this option or
                        argument.  This is a list of flags or argument
                        names.
    :param type: the type that should be used.  Either a :class:`ParamType`
                 or a Python type.  The later is converted into the former
                 automatically if supported.
    :param required: controls if this is optional or not.
    :param default: the default value if omitted.  This can also be a callable,
                    in which case it's invoked when the default is needed
                    without any arguments.
    :param callback: a callback that should be executed after the parameter
                     was matched.  This is called as ``fn(ctx, param,
                     value)`` and needs to return the value.  Before Click
                     2.0, the signature was ``(ctx, value)``.
    :param nargs: the number of arguments to match.  If not ``1`` the return
                  value is a tuple instead of single value.  The default for
                  nargs is ``1`` (except if the type is a tuple, then it's
                  the arity of the tuple).
    :param metavar: how the value is represented in the help page.
    :param expose_value: if this is `True` then the value is passed onwards
                         to the command callback and stored on the context,
                         otherwise it's skipped.
    :param is_eager: eager values are processed before non eager ones.  This
                     should not be set for arguments or it will inverse the
                     order of processing.
    :param envvar: a string or list of strings that are environment variables
                   that should be checked.
    """
    param_type_name = 'parameter'

    def __init__(self, param_decls=None, type=None, required=False,
                 default=None, callback=None, nargs=None, metavar=None,
                 expose_value=True, is_eager=False, envvar=None):
        self.name, self.opts, self.secondary_opts = \
            self._parse_decls(param_decls or (), expose_value)

        self.type = convert_type(type, default)

        # Default nargs to what the type tells us if we have that
        # information available.
        if nargs is None:
            if self.type.is_composite:
                nargs = self.type.arity
            else:
                nargs = 1

        self.required = required
        self.callback = callback
        self.nargs = nargs
        self.multiple = False
        self.expose_value = expose_value
        self.default = default
        self.is_eager = is_eager
        self.metavar = metavar
        self.envvar = envvar

    @property
    def human_readable_name(self):
        """Returns the human readable name of this parameter.  This is the
        same as the name for options, but the metavar for arguments.
        """
        return self.name

    def make_metavar(self):
        if self.metavar is not None:
            return self.metavar
        metavar = self.type.get_metavar(self)
        if metavar is None:
            metavar = self.type.name.upper()
        if self.nargs != 1:
            metavar += '...'
        return metavar

    def get_default(self, ctx):
        """Given a context variable this calculates the default value."""
        # Otherwise go with the regular default.
        if callable(self.default):
            rv = self.default()
        else:
            rv = self.default
        return self.type_cast_value(ctx, rv)

    def add_to_parser(self, parser, ctx):
        pass

    def consume_value(self, ctx, opts):
        value = opts.get(self.name)
        if value is None:
            value = ctx.lookup_default(self.name)
        if value is None:
            value = self.value_from_envvar(ctx)
        return value

    def type_cast_value(self, ctx, value):
        """Given a value this runs it properly through the type system.
        This automatically handles things like `nargs` and `multiple` as
        well as composite types.
        """
        if self.type.is_composite:
            if self.nargs <= 1:
                raise TypeError('Attempted to invoke composite type '
                                'but nargs has been set to %s.  This is '
                                'not supported; nargs needs to be set to '
                                'a fixed value > 1.' % self.nargs)
            if self.multiple:
                return tuple(self.type(x or (), self, ctx) for x in value or ())
            return self.type(value or (), self, ctx)

        def _convert(value, level):
            if level == 0:
                return self.type(value, self, ctx)
            return tuple(_convert(x, level - 1) for x in value or ())
        return _convert(value, (self.nargs != 1) + bool(self.multiple))

    def process_value(self, ctx, value):
        """Given a value and context this runs the logic to convert the
        value as necessary.
        """
        # If the value we were given is None we do nothing.  This way
        # code that calls this can easily figure out if something was
        # not provided.  Otherwise it would be converted into an empty
        # tuple for multiple invocations which is inconvenient.
        if value is not None:
            return self.type_cast_value(ctx, value)

    def value_is_missing(self, value):
        if value is None:
            return True
        if (self.nargs != 1 or self.multiple) and value == ():
            return True
        return False

    def full_process_value(self, ctx, value):
        value = self.process_value(ctx, value)

        if value is None:
            value = self.get_default(ctx)

        if self.required and self.value_is_missing(value):
            raise MissingParameter(ctx=ctx, param=self)

        return value

    def resolve_envvar_value(self, ctx):
        if self.envvar is None:
            return
        if isinstance(self.envvar, (tuple, list)):
            for envvar in self.envvar:
                rv = os.environ.get(envvar)
                if rv is not None:
                    return rv
        else:
            return os.environ.get(self.envvar)

    def value_from_envvar(self, ctx):
        rv = self.resolve_envvar_value(ctx)
        if rv is not None and self.nargs != 1:
            rv = self.type.split_envvar_value(rv)
        return rv

    def handle_parse_result(self, ctx, opts, args):
        with augment_usage_errors(ctx, param=self):
            value = self.consume_value(ctx, opts)
            try:
                value = self.full_process_value(ctx, value)
            except Exception:
                if not ctx.resilient_parsing:
                    raise
                value = None
            if self.callback is not None:
                try:
                    value = invoke_param_callback(
                        self.callback, ctx, self, value)
                except Exception:
                    if not ctx.resilient_parsing:
                        raise

        if self.expose_value:
            ctx.params[self.name] = value
        return value, args

    def get_help_record(self, ctx):
        pass

    def get_usage_pieces(self, ctx):
        return []


class Option(Parameter):
    """Options are usually optional values on the command line and
    have some extra features that arguments don't have.

    All other parameters are passed onwards to the parameter constructor.

    :param show_default: controls if the default value should be shown on the
                         help page.  Normally, defaults are not shown.
    :param prompt: if set to `True` or a non empty string then the user will
                   be prompted for input if not set.  If set to `True` the
                   prompt will be the option name capitalized.
    :param confirmation_prompt: if set then the value will need to be confirmed
                                if it was prompted for.
    :param hide_input: if this is `True` then the input on the prompt will be
                       hidden from the user.  This is useful for password
                       input.
    :param is_flag: forces this option to act as a flag.  The default is
                    auto detection.
    :param flag_value: which value should be used for this flag if it's
                       enabled.  This is set to a boolean automatically if
                       the option string contains a slash to mark two options.
    :param multiple: if this is set to `True` then the argument is accepted
                     multiple times and recorded.  This is similar to ``nargs``
                     in how it works but supports arbitrary number of
                     arguments.
    :param count: this flag makes an option increment an integer.
    :param allow_from_autoenv: if this is enabled then the value of this
                               parameter will be pulled from an environment
                               variable in case a prefix is defined on the
                               context.
    :param help: the help string.
    """
    param_type_name = 'option'

    def __init__(self, param_decls=None, show_default=False,
                 prompt=False, confirmation_prompt=False,
                 hide_input=False, is_flag=None, flag_value=None,
                 multiple=False, count=False, allow_from_autoenv=True,
                 type=None, help=None, **attrs):
        default_is_missing = attrs.get('default', _missing) is _missing
        Parameter.__init__(self, param_decls, type=type, **attrs)

        if prompt is True:
            prompt_text = self.name.replace('_', ' ').capitalize()
        elif prompt is False:
            prompt_text = None
        else:
            prompt_text = prompt
        self.prompt = prompt_text
        self.confirmation_prompt = confirmation_prompt
        self.hide_input = hide_input

        # Flags
        if is_flag is None:
            if flag_value is not None:
                is_flag = True
            else:
                is_flag = bool(self.secondary_opts)
        if is_flag and default_is_missing:
            self.default = False
        if flag_value is None:
            flag_value = not self.default
        self.is_flag = is_flag
        self.flag_value = flag_value
        if self.is_flag and isinstance(self.flag_value, bool) \
           and type is None:
            self.type = BOOL
            self.is_bool_flag = True
        else:
            self.is_bool_flag = False

        # Counting
        self.count = count
        if count:
            if type is None:
                self.type = IntRange(min=0)
            if default_is_missing:
                self.default = 0

        self.multiple = multiple
        self.allow_from_autoenv = allow_from_autoenv
        self.help = help
        self.show_default = show_default

        # Sanity check for stuff we don't support
        if __debug__:
            if self.nargs < 0:
                raise TypeError('Options cannot have nargs < 0')
            if self.prompt and self.is_flag and not self.is_bool_flag:
                raise TypeError('Cannot prompt for flags that are not bools.')
            if not self.is_bool_flag and self.secondary_opts:
                raise TypeError('Got secondary option for non boolean flag.')
            if self.is_bool_flag and self.hide_input \
               and self.prompt is not None:
                raise TypeError('Hidden input does not work with boolean '
                                'flag prompts.')
            if self.count:
                if self.multiple:
                    raise TypeError('Options cannot be multiple and count '
                                    'at the same time.')
                elif self.is_flag:
                    raise TypeError('Options cannot be count and flags at '
                                    'the same time.')

    def _parse_decls(self, decls, expose_value):
        opts = []
        secondary_opts = []
        name = None
        possible_names = []

        for decl in decls:
            if isidentifier(decl):
                if name is not None:
                    raise TypeError('Name defined twice')
                name = decl
            else:
                split_char = decl[:1] == '/' and ';' or '/'
                if split_char in decl:
                    first, second = decl.split(split_char, 1)
                    first = first.rstrip()
                    if first:
                        possible_names.append(split_opt(first))
                        opts.append(first)
                    second = second.lstrip()
                    if second:
                        secondary_opts.append(second.lstrip())
                else:
                    possible_names.append(split_opt(decl))
                    opts.append(decl)

        if name is None and possible_names:
            possible_names.sort(key=lambda x: len(x[0]))
            name = possible_names[-1][1].replace('-', '_').lower()
            if not isidentifier(name):
                name = None

        if name is None:
            if not expose_value:
                return None, opts, secondary_opts
            raise TypeError('Could not determine name for option')

        if not opts and not secondary_opts:
            raise TypeError('No options defined but a name was passed (%s). '
                            'Did you mean to declare an argument instead '
                            'of an option?' % name)

        return name, opts, secondary_opts

    def add_to_parser(self, parser, ctx):
        kwargs = {
            'dest': self.name,
            'nargs': self.nargs,
            'obj': self,
        }

        if self.multiple:
            action = 'append'
        elif self.count:
            action = 'count'
        else:
            action = 'store'

        if self.is_flag:
            kwargs.pop('nargs', None)
            if self.is_bool_flag and self.secondary_opts:
                parser.add_option(self.opts, action=action + '_const',
                                  const=True, **kwargs)
                parser.add_option(self.secondary_opts, action=action +
                                  '_const', const=False, **kwargs)
            else:
                parser.add_option(self.opts, action=action + '_const',
                                  const=self.flag_value,
                                  **kwargs)
        else:
            kwargs['action'] = action
            parser.add_option(self.opts, **kwargs)

    def get_help_record(self, ctx):
        any_prefix_is_slash = []

        def _write_opts(opts):
            rv, any_slashes = join_options(opts)
            if any_slashes:
                any_prefix_is_slash[:] = [True]
            if not self.is_flag and not self.count:
                rv += ' ' + self.make_metavar()
            return rv

        rv = [_write_opts(self.opts)]
        if self.secondary_opts:
            rv.append(_write_opts(self.secondary_opts))

        help = self.help or ''
        extra = []
        if self.default is not None and self.show_default:
            extra.append('default: %s' % (
                         ', '.join('%s' % d for d in self.default)
                         if isinstance(self.default, (list, tuple))
                         else self.default, ))
        if self.required:
            extra.append('required')
        if extra:
            help = '%s[%s]' % (help and help + '  ' or '', '; '.join(extra))

        return ((any_prefix_is_slash and '; ' or ' / ').join(rv), help)

    def get_default(self, ctx):
        # If we're a non boolean flag out default is more complex because
        # we need to look at all flags in the same group to figure out
        # if we're the the default one in which case we return the flag
        # value as default.
        if self.is_flag and not self.is_bool_flag:
            for param in ctx.command.params:
                if param.name == self.name and param.default:
                    return param.flag_value
            return None
        return Parameter.get_default(self, ctx)

    def prompt_for_value(self, ctx):
        """This is an alternative flow that can be activated in the full
        value processing if a value does not exist.  It will prompt the
        user until a valid value exists and then returns the processed
        value as result.
        """
        # Calculate the default before prompting anything to be stable.
        default = self.get_default(ctx)

        # If this is a prompt for a flag we need to handle this
        # differently.
        if self.is_bool_flag:
            return confirm(self.prompt, default)

        return prompt(self.prompt, default=default,
                      hide_input=self.hide_input,
                      confirmation_prompt=self.confirmation_prompt,
                      value_proc=lambda x: self.process_value(ctx, x))

    def resolve_envvar_value(self, ctx):
        rv = Parameter.resolve_envvar_value(self, ctx)
        if rv is not None:
            return rv
        if self.allow_from_autoenv and \
           ctx.auto_envvar_prefix is not None:
            envvar = '%s_%s' % (ctx.auto_envvar_prefix, self.name.upper())
            return os.environ.get(envvar)

    def value_from_envvar(self, ctx):
        rv = self.resolve_envvar_value(ctx)
        if rv is None:
            return None
        value_depth = (self.nargs != 1) + bool(self.multiple)
        if value_depth > 0 and rv is not None:
            rv = self.type.split_envvar_value(rv)
            if self.multiple and self.nargs != 1:
                rv = batch(rv, self.nargs)
        return rv

    def full_process_value(self, ctx, value):
        if value is None and self.prompt is not None \
           and not ctx.resilient_parsing:
            return self.prompt_for_value(ctx)
        return Parameter.full_process_value(self, ctx, value)


class Argument(Parameter):
    """Arguments are positional parameters to a command.  They generally
    provide fewer features than options but can have infinite ``nargs``
    and are required by default.

    All parameters are passed onwards to the parameter constructor.
    """
    param_type_name = 'argument'

    def __init__(self, param_decls, required=None, **attrs):
        if required is None:
            if attrs.get('default') is not None:
                required = False
            else:
                required = attrs.get('nargs', 1) > 0
        Parameter.__init__(self, param_decls, required=required, **attrs)
        if self.default is not None and self.nargs < 0:
            raise TypeError('nargs=-1 in combination with a default value '
                            'is not supported.')

    @property
    def human_readable_name(self):
        if self.metavar is not None:
            return self.metavar
        return self.name.upper()

    def make_metavar(self):
        if self.metavar is not None:
            return self.metavar
        var = self.name.upper()
        if not self.required:
            var = '[%s]' % var
        if self.nargs != 1:
            var += '...'
        return var

    def _parse_decls(self, decls, expose_value):
        if not decls:
            if not expose_value:
                return None, [], []
            raise TypeError('Could not determine name for argument')
        if len(decls) == 1:
            name = arg = decls[0]
            name = name.replace('-', '_').lower()
        elif len(decls) == 2:
            name, arg = decls
        else:
            raise TypeError('Arguments take exactly one or two '
                            'parameter declarations, got %d' % len(decls))
        return name, [arg], []

    def get_usage_pieces(self, ctx):
        return [self.make_metavar()]

    def add_to_parser(self, parser, ctx):
        parser.add_argument(dest=self.name, nargs=self.nargs,
                            obj=self)


# Circular dependency between decorators and core
from .decorators import command, group
