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# Source code for astropy.utils.decorators

    # -*- coding: utf-8 -*-
    # Licensed under a 3-clause BSD style license - see LICENSE.rst
    """Sundry function and class decorators."""

    import functools
    import inspect
    import textwrap
    import types
    import warnings
    from inspect import signature
    from functools import wraps

    from .exceptions import (AstropyDeprecationWarning, AstropyUserWarning,
                             AstropyPendingDeprecationWarning)

    __all__ = ['classproperty', 'deprecated', 'deprecated_attribute',
               'deprecated_renamed_argument', 'format_doc',
               'lazyproperty', 'sharedmethod', 'wraps']

    _NotFound = object()

    def deprecated(since, message='', name='', alternative='', pending=False,
                   obj_type=None, warning_type=AstropyDeprecationWarning):
        """
        Used to mark a function or class as deprecated.

        To mark an attribute as deprecated, use `deprecated_attribute`.

        Parameters
        ------------
        since : str
            The release at which this API became deprecated.  This is
            required.

        message : str, optional
            Override the default deprecation message.  The format
            specifier ``func`` may be used for the name of the function,
            and ``alternative`` may be used in the deprecation message
            to insert the name of an alternative to the deprecated
            function. ``obj_type`` may be used to insert a friendly name
            for the type of object being deprecated.

        name : str, optional
            The name of the deprecated function or class; if not provided
            the name is automatically determined from the passed in
            function or class, though this is useful in the case of
            renamed functions, where the new function is just assigned to
            the name of the deprecated function.  For example::

                def new_function():
                    ...
                oldFunction = new_function

        alternative : str, optional
            An alternative function or class name that the user may use in
            place of the deprecated object.  The deprecation warning will
            tell the user about this alternative if provided.

        pending : bool, optional
            If True, uses a AstropyPendingDeprecationWarning instead of a
            ``warning_type``.

        obj_type : str, optional
            The type of this object, if the automatically determined one
            needs to be overridden.

        warning_type : warning
            Warning to be issued.
            Default is `~astropy.utils.exceptions.AstropyDeprecationWarning`.
        """

        method_types = (classmethod, staticmethod, types.MethodType)

        def deprecate_doc(old_doc, message):
            """
            Returns a given docstring with a deprecation message prepended
            to it.
            """
            if not old_doc:
                old_doc = ''
            old_doc = textwrap.dedent(old_doc).strip('\n')
            new_doc = (('\n.. deprecated:: {since}'
                        '\n    {message}\n\n'.format(
                         **{'since': since, 'message': message.strip()})) + old_doc)
            if not old_doc:
                # This is to prevent a spurious 'unexpected unindent' warning from
                # docutils when the original docstring was blank.
                new_doc += r'\ '
            return new_doc

        def get_function(func):
            """
            Given a function or classmethod (or other function wrapper type), get
            the function object.
            """
            if isinstance(func, method_types):
                func = func.__func__
            return func

        def deprecate_function(func, message, warning_type=warning_type):
            """
            Returns a wrapped function that displays ``warning_type``
            when it is called.
            """

            if isinstance(func, method_types):
                func_wrapper = type(func)
            else:
                func_wrapper = lambda f: f

            func = get_function(func)

            def deprecated_func(*args, **kwargs):
                if pending:
                    category = AstropyPendingDeprecationWarning
                else:
                    category = warning_type

                warnings.warn(message, category, stacklevel=2)

                return func(*args, **kwargs)

            # If this is an extension function, we can't call
            # functools.wraps on it, but we normally don't care.
            # This crazy way to get the type of a wrapper descriptor is
            # straight out of the Python 3.3 inspect module docs.
            if type(func) is not type(str.__dict__['__add__']):  # nopep8
                deprecated_func = functools.wraps(func)(deprecated_func)

            deprecated_func.__doc__ = deprecate_doc(
                deprecated_func.__doc__, message)

            return func_wrapper(deprecated_func)

        def deprecate_class(cls, message, warning_type=warning_type):
            """
            Update the docstring and wrap the ``__init__`` in-place (or ``__new__``
            if the class or any of the bases overrides ``__new__``) so it will give
            a deprecation warning when an instance is created.

            This won't work for extension classes because these can't be modified
            in-place and the alternatives don't work in the general case:

            - Using a new class that looks and behaves like the original doesn't
              work because the __new__ method of extension types usually makes sure
              that it's the same class or a subclass.
            - Subclassing the class and return the subclass can lead to problems
              with pickle and will look weird in the Sphinx docs.
            """
            cls.__doc__ = deprecate_doc(cls.__doc__, message)
            if cls.__new__ is object.__new__:
                cls.__init__ = deprecate_function(get_function(cls.__init__),
                                                  message, warning_type)
            else:
                cls.__new__ = deprecate_function(get_function(cls.__new__),
                                                 message, warning_type)
            return cls

        def deprecate(obj, message=message, name=name, alternative=alternative,
                      pending=pending, warning_type=warning_type):
            if obj_type is None:
                if isinstance(obj, type):
                    obj_type_name = 'class'
                elif inspect.isfunction(obj):
                    obj_type_name = 'function'
                elif inspect.ismethod(obj) or isinstance(obj, method_types):
                    obj_type_name = 'method'
                else:
                    obj_type_name = 'object'
            else:
                obj_type_name = obj_type

            if not name:
                name = get_function(obj).__name__

            altmessage = ''
            if not message or type(message) is type(deprecate):
                if pending:
                    message = ('The {func} {obj_type} will be deprecated in a '
                               'future version.')
                else:
                    message = ('The {func} {obj_type} is deprecated and may '
                               'be removed in a future version.')
                if alternative:
                    altmessage = '\n        Use {} instead.'.format(alternative)

            message = ((message.format(**{
                'func': name,
                'name': name,
                'alternative': alternative,
                'obj_type': obj_type_name})) +
                altmessage)

            if isinstance(obj, type):
                return deprecate_class(obj, message, warning_type)
            else:
                return deprecate_function(obj, message, warning_type)

        if type(message) is type(deprecate):
            return deprecate(message)

        return deprecate

    def deprecated_attribute(name, since, message=None, alternative=None,
                             pending=False, warning_type=AstropyDeprecationWarning):
        """
        Used to mark a public attribute as deprecated.  This creates a
        property that will warn when the given attribute name is accessed.
        To prevent the warning (i.e. for internal code), use the private
        name for the attribute by prepending an underscore
        (i.e. ``self._name``).

        Parameters
        ----------
        name : str
            The name of the deprecated attribute.

        since : str
            The release at which this API became deprecated.  This is
            required.

        message : str, optional
            Override the default deprecation message.  The format
            specifier ``name`` may be used for the name of the attribute,
            and ``alternative`` may be used in the deprecation message
            to insert the name of an alternative to the deprecated
            function.

        alternative : str, optional
            An alternative attribute that the user may use in place of the
            deprecated attribute.  The deprecation warning will tell the
            user about this alternative if provided.

        pending : bool, optional
            If True, uses a AstropyPendingDeprecationWarning instead of
            ``warning_type``.

        warning_type : warning
            Warning to be issued.
            Default is `~astropy.utils.exceptions.AstropyDeprecationWarning`.

        Examples
        --------

        ::

            class MyClass:
                # Mark the old_name as deprecated
                old_name = misc.deprecated_attribute('old_name', '0.1')

                def method(self):
                    self._old_name = 42
        """
        private_name = '_' + name

        @deprecated(since, name=name, obj_type='attribute', warning_type=warning_type)
        def get(self):
            return getattr(self, private_name)

        @deprecated(since, name=name, obj_type='attribute', warning_type=warning_type)
        def set(self, val):
            setattr(self, private_name, val)

        @deprecated(since, name=name, obj_type='attribute', warning_type=warning_type)
        def delete(self):
            delattr(self, private_name)

        return property(get, set, delete)

    def deprecated_renamed_argument(old_name, new_name, since,
                                    arg_in_kwargs=False, relax=False,
                                    pending=False,
                                    warning_type=AstropyDeprecationWarning,
                                    alternative=''):
        """Deprecate a _renamed_ or _removed_ function argument.

        The decorator assumes that the argument with the ``old_name`` was removed
        from the function signature and the ``new_name`` replaced it at the
        **same position** in the signature.  If the ``old_name`` argument is
        given when calling the decorated function the decorator will catch it and
        issue a deprecation warning and pass it on as ``new_name`` argument.

        Parameters
        ----------
        old_name : str or list/tuple thereof
            The old name of the argument.

        new_name : str or list/tuple thereof or `None`
            The new name of the argument. Set this to `None` to remove the
            argument ``old_name`` instead of renaming it.

        since : str or number or list/tuple thereof
            The release at which the old argument became deprecated.

        arg_in_kwargs : bool or list/tuple thereof, optional
            If the argument is not a named argument (for example it
            was meant to be consumed by ``**kwargs``) set this to
            ``True``.  Otherwise the decorator will throw an Exception
            if the ``new_name`` cannot be found in the signature of
            the decorated function.
            Default is ``False``.

        relax : bool or list/tuple thereof, optional
            If ``False`` a ``TypeError`` is raised if both ``new_name`` and
            ``old_name`` are given.  If ``True`` the value for ``new_name`` is used
            and a Warning is issued.
            Default is ``False``.

        pending : bool or list/tuple thereof, optional
            If ``True`` this will hide the deprecation warning and ignore the
            corresponding ``relax`` parameter value.
            Default is ``False``.

        warning_type : warning
            Warning to be issued.
            Default is `~astropy.utils.exceptions.AstropyDeprecationWarning`.

        alternative : str, optional
            An alternative function or class name that the user may use in
            place of the deprecated object if ``new_name`` is None. The deprecation
            warning will tell the user about this alternative if provided.

        Raises
        ------
        TypeError
            If the new argument name cannot be found in the function
            signature and arg_in_kwargs was False or if it is used to
            deprecate the name of the ``*args``-, ``**kwargs``-like arguments.
            At runtime such an Error is raised if both the new_name
            and old_name were specified when calling the function and
            "relax=False".

        Notes
        -----
        The decorator should be applied to a function where the **name**
        of an argument was changed but it applies the same logic.

        .. warning::
            If ``old_name`` is a list or tuple the ``new_name`` and ``since`` must
            also be a list or tuple with the same number of entries. ``relax`` and
            ``arg_in_kwarg`` can be a single bool (applied to all) or also a
            list/tuple with the same number of entries like ``new_name``, etc.

        Examples
        --------
        The deprecation warnings are not shown in the following examples.

        To deprecate a positional or keyword argument::

            >>> from astropy.utils.decorators import deprecated_renamed_argument
            >>> @deprecated_renamed_argument('sig', 'sigma', '1.0')
            ... def test(sigma):
            ...     return sigma

            >>> test(2)
            2
            >>> test(sigma=2)
            2
            >>> test(sig=2)
            2

        To deprecate an argument caught inside the ``**kwargs`` the
        ``arg_in_kwargs`` has to be set::

            >>> @deprecated_renamed_argument('sig', 'sigma', '1.0',
            ...                             arg_in_kwargs=True)
            ... def test(**kwargs):
            ...     return kwargs['sigma']

            >>> test(sigma=2)
            2
            >>> test(sig=2)
            2

        By default providing the new and old keyword will lead to an Exception. If
        a Warning is desired set the ``relax`` argument::

            >>> @deprecated_renamed_argument('sig', 'sigma', '1.0', relax=True)
            ... def test(sigma):
            ...     return sigma

            >>> test(sig=2)
            2

        It is also possible to replace multiple arguments. The ``old_name``,
        ``new_name`` and ``since`` have to be `tuple` or `list` and contain the
        same number of entries::

            >>> @deprecated_renamed_argument(['a', 'b'], ['alpha', 'beta'],
            ...                              ['1.0', 1.2])
            ... def test(alpha, beta):
            ...     return alpha, beta

            >>> test(a=2, b=3)
            (2, 3)

        In this case ``arg_in_kwargs`` and ``relax`` can be a single value (which
        is applied to all renamed arguments) or must also be a `tuple` or `list`
        with values for each of the arguments.

        """
        cls_iter = (list, tuple)
        if isinstance(old_name, cls_iter):
            n = len(old_name)
            # Assume that new_name and since are correct (tuple/list with the
            # appropriate length) in the spirit of the "consenting adults". But the
            # optional parameters may not be set, so if these are not iterables
            # wrap them.
            if not isinstance(arg_in_kwargs, cls_iter):
                arg_in_kwargs = [arg_in_kwargs] * n
            if not isinstance(relax, cls_iter):
                relax = [relax] * n
            if not isinstance(pending, cls_iter):
                pending = [pending] * n
        else:
            # To allow a uniform approach later on, wrap all arguments in lists.
            n = 1
            old_name = [old_name]
            new_name = [new_name]
            since = [since]
            arg_in_kwargs = [arg_in_kwargs]
            relax = [relax]
            pending = [pending]

        def decorator(function):
            # The named arguments of the function.
            arguments = signature(function).parameters
            keys = list(arguments.keys())
            position = [None] * n

            for i in range(n):
                # Determine the position of the argument.
                if arg_in_kwargs[i]:
                    pass
                else:
                    if new_name[i] is None:
                        continue
                    elif new_name[i] in arguments:
                        param = arguments[new_name[i]]
                    # In case the argument is not found in the list of arguments
                    # the only remaining possibility is that it should be caught
                    # by some kind of **kwargs argument.
                    # This case has to be explicitly specified, otherwise throw
                    # an exception!
                    else:
                        raise TypeError('"{}" was not specified in the function '
                                        'signature. If it was meant to be part of '
                                        '"**kwargs" then set "arg_in_kwargs" to "True"'
                                        '.'.format(new_name[i]))

                    # There are several possibilities now:

                    # 1.) Positional or keyword argument:
                    if param.kind == param.POSITIONAL_OR_KEYWORD:
                        position[i] = keys.index(new_name[i])

                    # 2.) Keyword only argument:
                    elif param.kind == param.KEYWORD_ONLY:
                        # These cannot be specified by position.
                        position[i] = None

                    # 3.) positional-only argument, varargs, varkwargs or some
                    #     unknown type:
                    else:
                        raise TypeError('cannot replace argument "{0}" of kind '
                                        '{1!r}.'.format(new_name[i], param.kind))

            @functools.wraps(function)
            def wrapper(*args, **kwargs):
                for i in range(n):
                    # The only way to have oldkeyword inside the function is
                    # that it is passed as kwarg because the oldkeyword
                    # parameter was renamed to newkeyword.
                    if old_name[i] in kwargs:
                        value = kwargs.pop(old_name[i])
                        # Display the deprecation warning only when it's not
                        # pending.
                        if not pending[i]:
                            message = ('"{0}" was deprecated in version {1} '
                                       'and will be removed in a future version. '
                                       .format(old_name[i], since[i]))
                            if new_name[i] is not None:
                                message += ('Use argument "{}" instead.'
                                            .format(new_name[i]))
                            elif alternative:
                                message += ('\n        Use {} instead.'
                                            .format(alternative))
                            warnings.warn(message, warning_type, stacklevel=2)

                        # Check if the newkeyword was given as well.
                        newarg_in_args = (position[i] is not None and
                                          len(args) > position[i])
                        newarg_in_kwargs = new_name[i] in kwargs

                        if newarg_in_args or newarg_in_kwargs:
                            if not pending[i]:
                                # If both are given print a Warning if relax is
                                # True or raise an Exception is relax is False.
                                if relax[i]:
                                    warnings.warn(
                                        '"{0}" and "{1}" keywords were set. '
                                        'Using the value of "{1}".'
                                        ''.format(old_name[i], new_name[i]),
                                        AstropyUserWarning)
                                else:
                                    raise TypeError(
                                        'cannot specify both "{}" and "{}"'
                                        '.'.format(old_name[i], new_name[i]))
                        else:
                            # Pass the value of the old argument with the
                            # name of the new argument to the function
                            if new_name[i] is not None:
                                kwargs[new_name[i]] = value

                return function(*args, **kwargs)

            return wrapper
        return decorator

    # TODO: This can still be made to work for setters by implementing an
    # accompanying metaclass that supports it; we just don't need that right this
    # second
    class classproperty(property):
        """
        Similar to `property`, but allows class-level properties.  That is,
        a property whose getter is like a `classmethod`.

        The wrapped method may explicitly use the `classmethod` decorator (which
        must become before this decorator), or the `classmethod` may be omitted
        (it is implicit through use of this decorator).

        .. note::

            classproperty only works for *read-only* properties.  It does not
            currently allow writeable/deletable properties, due to subtleties of how
            Python descriptors work.  In order to implement such properties on a class
            a metaclass for that class must be implemented.

        Parameters
        ----------
        fget : callable
            The function that computes the value of this property (in particular,
            the function when this is used as a decorator) a la `property`.

        doc : str, optional
            The docstring for the property--by default inherited from the getter
            function.

        lazy : bool, optional
            If True, caches the value returned by the first call to the getter
            function, so that it is only called once (used for lazy evaluation
            of an attribute).  This is analogous to `lazyproperty`.  The ``lazy``
            argument can also be used when `classproperty` is used as a decorator
            (see the third example below).  When used in the decorator syntax this
            *must* be passed in as a keyword argument.

        Examples
        --------

        ::

            >>> class Foo:
            ...     _bar_internal = 1
            ...     @classproperty
            ...     def bar(cls):
            ...         return cls._bar_internal + 1
            ...
            >>> Foo.bar
            2
            >>> foo_instance = Foo()
            >>> foo_instance.bar
            2
            >>> foo_instance._bar_internal = 2
            >>> foo_instance.bar  # Ignores instance attributes
            2

        As previously noted, a `classproperty` is limited to implementing
        read-only attributes::

            >>> class Foo:
            ...     _bar_internal = 1
            ...     @classproperty
            ...     def bar(cls):
            ...         return cls._bar_internal
            ...     @bar.setter
            ...     def bar(cls, value):
            ...         cls._bar_internal = value
            ...
            Traceback (most recent call last):
            ...
            NotImplementedError: classproperty can only be read-only; use a
            metaclass to implement modifiable class-level properties

        When the ``lazy`` option is used, the getter is only called once::

            >>> class Foo:
            ...     @classproperty(lazy=True)
            ...     def bar(cls):
            ...         print("Performing complicated calculation")
            ...         return 1
            ...
            >>> Foo.bar
            Performing complicated calculation
            1
            >>> Foo.bar
            1

        If a subclass inherits a lazy `classproperty` the property is still
        re-evaluated for the subclass::

            >>> class FooSub(Foo):
            ...     pass
            ...
            >>> FooSub.bar
            Performing complicated calculation
            1
            >>> FooSub.bar
            1
        """

        def __new__(cls, fget=None, doc=None, lazy=False):
            if fget is None:
                # Being used as a decorator--return a wrapper that implements
                # decorator syntax
                def wrapper(func):
                    return cls(func, lazy=lazy)

                return wrapper

            return super().__new__(cls)

        def __init__(self, fget, doc=None, lazy=False):
            self._lazy = lazy
            if lazy:
                self._cache = {}
            fget = self._wrap_fget(fget)

            super().__init__(fget=fget, doc=doc)

            # There is a buglet in Python where self.__doc__ doesn't
            # get set properly on instances of property subclasses if
            # the doc argument was used rather than taking the docstring
            # from fget
            # Related Python issue: https://bugs.python.org/issue24766
            if doc is not None:
                self.__doc__ = doc

        def __get__(self, obj, objtype):
            if self._lazy and objtype in self._cache:
                return self._cache[objtype]

            # The base property.__get__ will just return self here;
            # instead we pass objtype through to the original wrapped
            # function (which takes the class as its sole argument)
            val = self.fget.__wrapped__(objtype)

            if self._lazy:
                self._cache[objtype] = val

            return val

        def getter(self, fget):
            return super().getter(self._wrap_fget(fget))

        def setter(self, fset):
            raise NotImplementedError(
                "classproperty can only be read-only; use a metaclass to "
                "implement modifiable class-level properties")

        def deleter(self, fdel):
            raise NotImplementedError(
                "classproperty can only be read-only; use a metaclass to "
                "implement modifiable class-level properties")

        @staticmethod
        def _wrap_fget(orig_fget):
            if isinstance(orig_fget, classmethod):
                orig_fget = orig_fget.__func__

            # Using stock functools.wraps instead of the fancier version
            # found later in this module, which is overkill for this purpose

            @functools.wraps(orig_fget)
            def fget(obj):
                return orig_fget(obj.__class__)

            return fget

    class lazyproperty(property):
        """
        Works similarly to property(), but computes the value only once.

        This essentially memorizes the value of the property by storing the result
        of its computation in the ``__dict__`` of the object instance.  This is
        useful for computing the value of some property that should otherwise be
        invariant.  For example::

            >>> class LazyTest:
            ...     @lazyproperty
            ...     def complicated_property(self):
            ...         print('Computing the value for complicated_property...')
            ...         return 42
            ...
            >>> lt = LazyTest()
            >>> lt.complicated_property
            Computing the value for complicated_property...
            42
            >>> lt.complicated_property
            42

        As the example shows, the second time ``complicated_property`` is accessed,
        the ``print`` statement is not executed.  Only the return value from the
        first access off ``complicated_property`` is returned.

        By default, a setter and deleter are used which simply overwrite and
        delete, respectively, the value stored in ``__dict__``. Any user-specified
        setter or deleter is executed before executing these default actions.
        The one exception is that the default setter is not run if the user setter
        already sets the new value in ``__dict__`` and returns that value and the
        returned value is not ``None``.

        Adapted from the recipe at
        http://code.activestate.com/recipes/363602-lazy-property-evaluation
        """

        def __init__(self, fget, fset=None, fdel=None, doc=None):
            super().__init__(fget, fset, fdel, doc)
            self._key = self.fget.__name__

        def __get__(self, obj, owner=None):
            try:
                val = obj.__dict__.get(self._key, _NotFound)
                if val is not _NotFound:
                    return val
                else:
                    val = self.fget(obj)
                    obj.__dict__[self._key] = val
                    return val
            except AttributeError:
                if obj is None:
                    return self
                raise

        def __set__(self, obj, val):
            obj_dict = obj.__dict__
            if self.fset:
                ret = self.fset(obj, val)
                if ret is not None and obj_dict.get(self._key) is ret:
                    # By returning the value set the setter signals that it took
                    # over setting the value in obj.__dict__; this mechanism allows
                    # it to override the input value
                    return
            obj_dict[self._key] = val

        def __delete__(self, obj):
            if self.fdel:
                self.fdel(obj)
            if self._key in obj.__dict__:
                del obj.__dict__[self._key]

    class sharedmethod(classmethod):
        """
        This is a method decorator that allows both an instancemethod and a
        `classmethod` to share the same name.

        When using `sharedmethod` on a method defined in a class's body, it
        may be called on an instance, or on a class.  In the former case it
        behaves like a normal instance method (a reference to the instance is
        automatically passed as the first ``self`` argument of the method)::

            >>> class Example:
            ...     @sharedmethod
            ...     def identify(self, *args):
            ...         print('self was', self)
            ...         print('additional args were', args)
            ...
            >>> ex = Example()
            >>> ex.identify(1, 2)
            self was <astropy.utils.decorators.Example object at 0x...>
            additional args were (1, 2)

        In the latter case, when the `sharedmethod` is called directly from a
        class, it behaves like a `classmethod`::

            >>> Example.identify(3, 4)
            self was <class 'astropy.utils.decorators.Example'>
            additional args were (3, 4)

        This also supports a more advanced usage, where the `classmethod`
        implementation can be written separately.  If the class's *metaclass*
        has a method of the same name as the `sharedmethod`, the version on
        the metaclass is delegated to::

            >>> class ExampleMeta(type):
            ...     def identify(self):
            ...         print('this implements the {0}.identify '
            ...               'classmethod'.format(self.__name__))
            ...
            >>> class Example(metaclass=ExampleMeta):
            ...     @sharedmethod
            ...     def identify(self):
            ...         print('this implements the instancemethod')
            ...
            >>> Example().identify()
            this implements the instancemethod
            >>> Example.identify()
            this implements the Example.identify classmethod
        """

        def __get__(self, obj, objtype=None):
            if obj is None:
                mcls = type(objtype)
                clsmeth = getattr(mcls, self.__func__.__name__, None)
                if callable(clsmeth):
                    func = clsmeth
                else:
                    func = self.__func__

                return self._make_method(func, objtype)
            else:
                return self._make_method(self.__func__, obj)

        @staticmethod
        def _make_method(func, instance):
            return types.MethodType(func, instance)

    def format_doc(docstring, *args, **kwargs):
        """
        Replaces the docstring of the decorated object and then formats it.

        The formatting works like :meth:`str.format` and if the decorated object
        already has a docstring this docstring can be included in the new
        documentation if you use the ``{__doc__}`` placeholder.
        Its primary use is for reusing a *long* docstring in multiple functions
        when it is the same or only slightly different between them.

        Parameters
        ----------
        docstring : str or object or None
            The docstring that will replace the docstring of the decorated
            object. If it is an object like a function or class it will
            take the docstring of this object. If it is a string it will use the
            string itself. One special case is if the string is ``None`` then
            it will use the decorated functions docstring and formats it.

        args :
            passed to :meth:`str.format`.

        kwargs :
            passed to :meth:`str.format`. If the function has a (not empty)
            docstring the original docstring is added to the kwargs with the
            keyword ``'__doc__'``.

        Raises
        ------
        ValueError
            If the ``docstring`` (or interpreted docstring if it was ``None``
            or not a string) is empty.

        IndexError, KeyError
            If a placeholder in the (interpreted) ``docstring`` was not filled. see
            :meth:`str.format` for more information.

        Notes
        -----
        Using this decorator allows, for example Sphinx, to parse the
        correct docstring.

        Examples
        --------

        Replacing the current docstring is very easy::

            >>> from astropy.utils.decorators import format_doc
            >>> @format_doc('''Perform num1 + num2''')
            ... def add(num1, num2):
            ...     return num1+num2
            ...
            >>> help(add) # doctest: +SKIP
            Help on function add in module __main__:
            <BLANKLINE>
            add(num1, num2)
                Perform num1 + num2

        sometimes instead of replacing you only want to add to it::

            >>> doc = '''
            ...       {__doc__}
            ...       Parameters
            ...       ----------
            ...       num1, num2 : Numbers
            ...       Returns
            ...       -------
            ...       result: Number
            ...       '''
            >>> @format_doc(doc)
            ... def add(num1, num2):
            ...     '''Perform addition.'''
            ...     return num1+num2
            ...
            >>> help(add) # doctest: +SKIP
            Help on function add in module __main__:
            <BLANKLINE>
            add(num1, num2)
                Perform addition.
                Parameters
                ----------
                num1, num2 : Numbers
                Returns
                -------
                result : Number

        in case one might want to format it further::

            >>> doc = '''
            ...       Perform {0}.
            ...       Parameters
            ...       ----------
            ...       num1, num2 : Numbers
            ...       Returns
            ...       -------
            ...       result: Number
            ...           result of num1 {op} num2
            ...       {__doc__}
            ...       '''
            >>> @format_doc(doc, 'addition', op='+')
            ... def add(num1, num2):
            ...     return num1+num2
            ...
            >>> @format_doc(doc, 'subtraction', op='-')
            ... def subtract(num1, num2):
            ...     '''Notes: This one has additional notes.'''
            ...     return num1-num2
            ...
            >>> help(add) # doctest: +SKIP
            Help on function add in module __main__:
            <BLANKLINE>
            add(num1, num2)
                Perform addition.
                Parameters
                ----------
                num1, num2 : Numbers
                Returns
                -------
                result : Number
                    result of num1 + num2
            >>> help(subtract) # doctest: +SKIP
            Help on function subtract in module __main__:
            <BLANKLINE>
            subtract(num1, num2)
                Perform subtraction.
                Parameters
                ----------
                num1, num2 : Numbers
                Returns
                -------
                result : Number
                    result of num1 - num2
                Notes : This one has additional notes.

        These methods can be combined an even taking the docstring from another
        object is possible as docstring attribute. You just have to specify the
        object::

            >>> @format_doc(add)
            ... def another_add(num1, num2):
            ...     return num1 + num2
            ...
            >>> help(another_add) # doctest: +SKIP
            Help on function another_add in module __main__:
            <BLANKLINE>
            another_add(num1, num2)
                Perform addition.
                Parameters
                ----------
                num1, num2 : Numbers
                Returns
                -------
                result : Number
                    result of num1 + num2

        But be aware that this decorator *only* formats the given docstring not
        the strings passed as ``args`` or ``kwargs`` (not even the original
        docstring)::

            >>> @format_doc(doc, 'addition', op='+')
            ... def yet_another_add(num1, num2):
            ...    '''This one is good for {0}.'''
            ...    return num1 + num2
            ...
            >>> help(yet_another_add) # doctest: +SKIP
            Help on function yet_another_add in module __main__:
            <BLANKLINE>
            yet_another_add(num1, num2)
                Perform addition.
                Parameters
                ----------
                num1, num2 : Numbers
                Returns
                -------
                result : Number
                    result of num1 + num2
                This one is good for {0}.

        To work around it you could specify the docstring to be ``None``::

            >>> @format_doc(None, 'addition')
            ... def last_add_i_swear(num1, num2):
            ...    '''This one is good for {0}.'''
            ...    return num1 + num2
            ...
            >>> help(last_add_i_swear) # doctest: +SKIP
            Help on function last_add_i_swear in module __main__:
            <BLANKLINE>
            last_add_i_swear(num1, num2)
                This one is good for addition.

        Using it with ``None`` as docstring allows to use the decorator twice
        on an object to first parse the new docstring and then to parse the
        original docstring or the ``args`` and ``kwargs``.
        """
        def set_docstring(obj):
            if docstring is None:
                # None means: use the objects __doc__
                doc = obj.__doc__
                # Delete documentation in this case so we don't end up with
                # awkwardly self-inserted docs.
                obj.__doc__ = None
            elif isinstance(docstring, str):
                # String: use the string that was given
                doc = docstring
            else:
                # Something else: Use the __doc__ of this
                doc = docstring.__doc__

            if not doc:
                # In case the docstring is empty it's probably not what was wanted.
                raise ValueError('docstring must be a string or containing a '
                                 'docstring that is not empty.')

            # If the original has a not-empty docstring append it to the format
            # kwargs.
            kwargs['__doc__'] = obj.__doc__ or ''
            obj.__doc__ = doc.format(*args, **kwargs)
            return obj
        return set_docstring