# -*- coding: utf-8 -*-
# Licensed under a 3-clause BSD style license - see LICENSE.rst
"""
Framework and base classes for coordinate frames/"low-level" coordinate
classes.
"""

from __future__ import (absolute_import, unicode_literals, division,
                        print_function)

# Standard library
import inspect
import warnings
from copy import deepcopy
from collections import namedtuple

# Dependencies
import numpy as np

# Project
from ..utils.compat.misc import override__dir__
from ..extern import six
from ..utils.exceptions import AstropyDeprecationWarning, AstropyWarning
from .. import units as u
from ..utils import OrderedDict
from .transformations import TransformGraph
from .representation import (BaseRepresentation, CartesianRepresentation,
                             SphericalRepresentation,
                             UnitSphericalRepresentation,
                             REPRESENTATION_CLASSES)


__all__ = ['BaseCoordinateFrame', 'frame_transform_graph', 'GenericFrame',
           'FrameAttribute', 'TimeFrameAttribute', 'QuantityFrameAttribute',
           'EarthLocationAttribute', 'RepresentationMapping']


# the graph used for all transformations between frames
frame_transform_graph = TransformGraph()


def _get_repr_cls(value):
    """
    Return a valid representation class from ``value`` or raise exception.
    """

    if value in REPRESENTATION_CLASSES:
        value = REPRESENTATION_CLASSES[value]
    try:
        # value might not be a class, so use try
        assert issubclass(value, BaseRepresentation)
    except (TypeError, AssertionError):
        raise ValueError(
            'Representation is {0!r} but must be a BaseRepresentation class '
            'or one of the string aliases {1}'.format(
                value, list(REPRESENTATION_CLASSES)))
    return value


class FrameMeta(type):
    def __new__(mcls, name, bases, members):
        if 'default_representation' in members:
            default_repr = members.pop('default_representation')
            found_default_repr = True
        else:
            default_repr = None
            found_default_repr = False

        if 'frame_specific_representation_info' in members:
            repr_info = members.pop('frame_specific_representation_info')
            found_repr_info = True
        else:
            repr_info = None
            found_repr_info = False

        # somewhat hacky, but this is the best way to get the MRO according to
        # https://mail.python.org/pipermail/python-list/2002-December/167861.html
        tmp_cls = super(FrameMeta, mcls).__new__(mcls, name, bases, members)

        # now look through the whole MRO for the class attributes, raw for
        # frame_attr_names, and leading underscore for others
        for m in (c.__dict__ for c in tmp_cls.__mro__):
            if not found_default_repr and '_default_representation' in m:
                default_repr = m['_default_representation']
                found_default_repr = True
            if (not found_repr_info and
                    '_frame_specific_representation_info' in m):
                repr_info = m['_frame_specific_representation_info']
                found_repr_info = True

            if found_default_repr and found_repr_info:
                break
        else:
            raise ValueError(
                'Could not find all expected BaseCoordinateFrame class '
                'attributes.  Are you mis-using FrameMeta?')

        # Make read-only properties for the frame class attributes that should
        # be read-only to make them immutable after creation.
        # We copy attributes instead of linking to make sure there's no
        # accidental cross-talk between classes
        mcls.readonly_prop_factory(members, 'default_representation',
                                   default_repr)
        mcls.readonly_prop_factory(members,
                                   'frame_specific_representation_info',
                                   deepcopy(repr_info))

        # now set the frame name as lower-case class name, if it isn't explicit
        if 'name' not in members:
            members['name'] = name.lower()

        return super(FrameMeta, mcls).__new__(mcls, name, bases, members)

    @staticmethod
    def readonly_prop_factory(members, attr, value):
        private_attr = '_' + attr

        def getter(self):
            return getattr(self, private_attr)

        members[private_attr] = value
        members[attr] = property(getter)


class FrameAttribute(object):
    """A non-mutable data descriptor to hold a frame attribute.

    This class must be used to define frame attributes (e.g. ``equinox`` or
    ``obstime``) that are included in a frame class definition.

    Examples
    --------
    The `~astropy.coordinates.FK4` class uses the following class attributes::

      class FK4(BaseCoordinateFrame):
          equinox = TimeFrameAttribute(default=_EQUINOX_B1950)
          obstime = TimeFrameAttribute(default=None,
                                       secondary_attribute='equinox')

    This means that ``equinox`` and ``obstime`` are available to be set as
    keyword arguments when creating an ``FK4`` class instance and are then
    accessible as instance attributes.  The instance value for the attribute
    must be stored in ``'_' + <attribute_name>`` by the frame ``__init__``
    method.

    Note in this example that ``equinox`` and ``obstime`` are time attributes
    and use the ``TimeAttributeFrame`` class.  This subclass overrides the
    ``convert_input`` method to validate and convert inputs into a ``Time``
    object.

    Parameters
    ----------
    default : object
        Default value for the attribute if not provided
    secondary_attribute : str
        Name of a secondary instance attribute which supplies the value if
        ``default is None`` and no value was supplied during initialization.
    """

    _nextid = 1
    """
    Used to ascribe some ordering to FrameAttribute instances so that the
    order they were assigned in a class body can be determined.
    """

    def __init__(self, default=None, secondary_attribute=''):
        self.default = default
        self.secondary_attribute = secondary_attribute

        # Use FrameAttribute._nextid explicitly so that subclasses of
        # FrameAttribute use the same counter
        self._order = FrameAttribute._nextid
        FrameAttribute._nextid += 1

    def convert_input(self, value):
        """
        Validate the input ``value`` and convert to expected attribute class.

        The base method here does nothing, but subclasses can implement this
        as needed.  The method should catch any internal exceptions and raise
        ValueError with an informative message.

        The method returns the validated input along with a boolean that
        indicates whether the input value was actually converted.  If the input
        value was already the correct type then the ``converted`` return value
        should be ``False``.

        Parameters
        ----------
        value : object
            Input value to be converted.

        Returns
        -------
        output_value
            The ``value`` converted to the correct type (or just ``value`` if
            ``converted`` is False)
        converted : bool
            True if the conversion was actually performed, False otherwise.

        Raises
        ------
        ValueError
            If the input is not valid for this attribute.
        """
        return value, False

    def __get__(self, instance, frame_cls=None):
        if not hasattr(self, 'name'):
            # Find attribute name of self by finding this object in the frame
            # class which is requesting this attribute or any of its
            # superclasses.
            for mro_cls in frame_cls.__mro__:
                for name, val in mro_cls.__dict__.items():
                    if val is self:
                        self.name = name
                        break
                if hasattr(self, 'name'):  # Can't nicely break out of two loops
                    break
            else:
                # Cannot think of a way to actually raise this exception.  This
                # instance containing this code must be in the class dict in
                # order to get excecuted by attribute access.  But leave this
                # here just in case...
                raise AttributeError(
                        'Unexpected inability to locate descriptor')

        out = None

        if instance is not None:
            out = getattr(instance, '_' + self.name, None)
            if out is None and self.default is None:
                out = getattr(instance, self.secondary_attribute, None)

        if out is None:
            out = self.default

        out, converted = self.convert_input(out)
        if instance is not None and converted:
            setattr(instance, '_' + self.name, out)

        return out

    def __set__(self, instance, val):
        raise AttributeError('Cannot set frame attribute')


class TimeFrameAttribute(FrameAttribute):
    """
    Frame attribute descriptor for quantities that are Time objects.
    See the `~astropy.coordinates.FrameAttribute` API doc for further
    information.

    Parameters
    ----------
    default : object
        Default value for the attribute if not provided
    secondary_attribute : str
        Name of a secondary instance attribute which supplies the value if
        ``default is None`` and no value was supplied during initialization.
    """

    def convert_input(self, value):
        """
        Convert input value to a Time object and validate by running through
        the Time constructor.  Also check that the input was a scalar.

        Parameters
        ----------
        value : object
            Input value to be converted.

        Returns
        -------
        out, converted : correctly-typed object, boolean
            Tuple consisting of the correctly-typed object and a boolean which
            indicates if conversion was actually performed.

        Raises
        ------
        ValueError
            If the input is not valid for this attribute.
        """

        from ..time import Time

        if value is None:
            return None, False

        if isinstance(value, Time):
            out = value
            converted = False
        else:
            try:
                out = Time(value)
            except Exception as err:
                raise ValueError(
                    'Invalid time input {0}={1!r}\n{2}'.format(self.name,
                                                               value, err))
            converted = True

        return out, converted


class QuantityFrameAttribute(FrameAttribute):
    """
    A frame attribute that is a quantity with specified units and shape
    (optionally).

    Parameters
    ----------
    default : object
        Default value for the attribute if not provided
    secondary_attribute : str
        Name of a secondary instance attribute which supplies the value if
        ``default is None`` and no value was supplied during initialization.
    unit : unit object or None
        Name of a unit that the input will be converted into. If None, no
        unit-checking or conversion is performed
    shape : tuple or None
        If given, specifies the shape the attribute must be
    """
    def __init__(self, default=None, secondary_attribute='', unit=None, shape=None):
        super(QuantityFrameAttribute, self).__init__(default, secondary_attribute)
        self.unit = unit
        self.shape = shape

    def convert_input(self, value):
        """
        Checks that the input is a Quantity with the necessary units (or the
        special value ``0``).

        Parameters
        ----------
        value : object
            Input value to be converted.

        Returns
        -------
        out, converted : correctly-typed object, boolean
            Tuple consisting of the correctly-typed object and a boolean which
            indicates if conversion was actually performed.

        Raises
        ------
        ValueError
            If the input is not valid for this attribute.
        """
        if np.all(value == 0) and self.unit is not None and self.unit is not None:
            return u.Quantity(np.zeros(self.shape), self.unit), True
        else:
            converted = True
            if not (hasattr(value, 'unit') ):
                raise TypeError('Tried to set a QuantityFrameAttribute with '
                                'something that does not have a unit.')
            oldvalue = value
            value = u.Quantity(oldvalue, copy=False).to(self.unit)
            if self.shape is not None and value.shape != self.shape:
                raise ValueError('The provided value has shape "{0}", but '
                                 'should have shape "{1}"'.format(value.shape,
                                                                  self.shape))
            if (oldvalue.unit == value.unit and hasattr(oldvalue, 'value') and
                np.all(oldvalue.value == value.value)):
                converted = False
            return value, converted


class EarthLocationAttribute(FrameAttribute):
    """
    A frame attribute that can act as a `~astropy.coordinates.EarthLocation`.
    It can be created as anything that can be transformed to the
    `~astropy.coordinates.ITRS` frame, but always presents as an `EarthLocation`
    when accessed after creation.

    Parameters
    ----------
    default : object
        Default value for the attribute if not provided
    secondary_attribute : str
        Name of a secondary instance attribute which supplies the value if
        ``default is None`` and no value was supplied during initialization.
    """

    def convert_input(self, value):
        """
        Checks that the input is a Quantity with the necessary units (or the
        special value ``0``).

        Parameters
        ----------
        value : object
            Input value to be converted.

        Returns
        -------
        out, converted : correctly-typed object, boolean
            Tuple consisting of the correctly-typed object and a boolean which
            indicates if conversion was actually performed.

        Raises
        ------
        ValueError
            If the input is not valid for this attribute.
        """
        if value is None:
            return None, False
        elif isinstance(value, EarthLocation):
            return value, False
        else:
            #we have to do the import here because of some tricky circular deps
            from .builtin_frames import ITRS

            if not hasattr(value, 'transform_to'):
                raise ValueError('"{0}" was passed into an '
                                 'EarthLocationAttribute, but it does not have '
                                 '"transform_to" method'.format(value))
            itrsobj = value.transform_to(ITRS)
            return itrsobj.earth_location, True

_RepresentationMappingBase = \
    namedtuple('RepresentationMapping',
               ('reprname', 'framename', 'defaultunit'))


class RepresentationMapping(_RepresentationMappingBase):
    """
    This `~collections.namedtuple` is used with the
    ``frame_specific_representation_info`` attribute to tell frames what
    attribute names (and default units) to use for a particular representation.
    ``reprname`` and ``framename`` should be strings, while ``defaultunit`` can
    be either an astropy unit, the string ``'recommended'`` (to use whatever
    the representation's ``recommended_units`` is), or None (to indicate that
    no unit mapping should be done).
    """

    def __new__(cls, reprname, framename, defaultunit='recommended'):
        # this trick just provides some defaults
        return super(RepresentationMapping, cls).__new__(cls, reprname,
                                                         framename,
                                                         defaultunit)


@six.add_metaclass(FrameMeta)
class BaseCoordinateFrame(object):
    """
    The base class for coordinate frames.

    This class is intended to be subclassed to create instances of specific
    systems.  Subclasses can implement the following attributes:

    * `default_representation`
        A subclass of `~astropy.coordinates.BaseRepresentation` that will be
        treated as the default representation of this frame.  This is the
        representation assumed by default when the frame is created.

    * `~astropy.coordinates.FrameAttribute` class attributes
       Frame attributes such as ``FK4.equinox`` or ``FK4.obstime`` are defined
       using a descriptor class.  See the narrative documentation or
       built-in classes code for details.

    * `frame_specific_representation_info`
        A dictionary mapping the name or class of a representation to a list of
        `~astropy.coordinates.RepresentationMapping` objects that tell what
        names and default units should be used on this frame for the components
        of that representation.
    """

    default_representation = None
    # specifies special names/units for representation attributes
    frame_specific_representation_info = {}

    # This __new__ provides for backward-compatibility with pre-0.4 API.
    # TODO: remove in 1.0
    def __new__(cls, *args, **kwargs):

        # Only do backward-compatibility if frame is previously defined one
        frame_name = cls.__name__.lower()
        if frame_name not in ['altaz', 'fk4', 'fk4noeterms', 'fk5',
                              'galactic', 'icrs']:
            return super(BaseCoordinateFrame, cls).__new__(cls)

        use_skycoord = False

        if (len(args) > 1 or (len(args) == 1 and
                not isinstance(args[0], BaseRepresentation))):
            for arg in args:
                if (not isinstance(arg, u.Quantity)
                    and not isinstance(arg, BaseRepresentation)):
                    msg = ('Initializing frame classes like "{0}" using string '
                           'or other non-Quantity arguments is deprecated, and '
                           'will be removed in the next version of Astropy.  '
                           'Instead, you probably want to use the SkyCoord '
                           'class with the "frame={1}" keyword, or if you '
                           'really want to use the low-level frame classes, '
                           'create it with an Angle or Quantity.')

                    warnings.warn(msg.format(cls.__name__,
                                             cls.__name__.lower()),
                                  AstropyDeprecationWarning)
                    use_skycoord = True
                    break

        if 'unit' in kwargs and not use_skycoord:
            warnings.warn(
                "Initializing frames using the ``unit`` argument is "
                "now deprecated. Use SkyCoord or pass Quantity "
                "instances to frames instead.", AstropyDeprecationWarning)
            use_skycoord = True

        if not use_skycoord:
            representation = kwargs.get('representation',
                                        cls._default_representation)
            representation = _get_repr_cls(representation)

            repr_info = cls._get_representation_info()

            for key in repr_info[representation]['names']:
                if key in kwargs:
                    if not isinstance(kwargs[key], u.Quantity):
                        warnings.warn(
                            "Initializing frames using non-Quantity arguments "
                            "is now deprecated. Use SkyCoord or pass Quantity "
                            "instances instead.", AstropyDeprecationWarning)
                        use_skycoord = True
                        break

        if use_skycoord:
            kwargs['frame'] = frame_name
            from .sky_coordinate import SkyCoord
            return SkyCoord(*args, **kwargs)
        else:
            return super(BaseCoordinateFrame, cls).__new__(cls)

    def __init__(self, *args, **kwargs):
        self._attr_names_with_defaults = []

        if 'representation' in kwargs:
            self.representation = kwargs.pop('representation')

        # if not set below, this is a frame with no data
        representation_data = None

        for fnm, fdefault in self.get_frame_attr_names().items():
            # Read-only frame attributes are defined as FrameAttribue
            # descriptors which are not settable, so set 'real' attributes as
            # the name prefaced with an underscore.

            if fnm in kwargs:
                value = kwargs.pop(fnm)
                setattr(self, '_' + fnm, value)
            else:
                setattr(self, '_' + fnm, fdefault)
                self._attr_names_with_defaults.append(fnm)

            # Validate input by getting the attribute here.
            getattr(self, fnm)

        pref_rep = self.representation

        args = list(args)  # need to be able to pop them
        if (len(args) > 0) and (isinstance(args[0], BaseRepresentation) or
                                args[0] is None):
            representation_data = args.pop(0)
            if len(args) > 0:
                raise TypeError(
                    'Cannot create a frame with both a representation and '
                    'other positional arguments')

        elif self.representation:
            repr_kwargs = {}
            for nmkw, nmrep in self.representation_component_names.items():
                if len(args) > 0:
                    #first gather up positional args
                    repr_kwargs[nmrep] = args.pop(0)
                elif nmkw in kwargs:
                    repr_kwargs[nmrep] = kwargs.pop(nmkw)

            #special-case the Spherical->UnitSpherical if no `distance`
            #TODO: possibly generalize this somehow?

            if repr_kwargs:
                if repr_kwargs.get('distance', True) is None:
                    del repr_kwargs['distance']
                if (issubclass(self.representation, SphericalRepresentation) and
                        'distance' not in repr_kwargs):
                    representation_data = UnitSphericalRepresentation(**repr_kwargs)
                else:
                    representation_data = self.representation(**repr_kwargs)

        if len(args) > 0:
            raise TypeError(
                '{0}.__init__ had {1} remaining unhandled arguments'.format(
                    self.__class__.__name__, len(args)))
        if kwargs:
            raise TypeError(
                'Coordinate frame got unexpected keywords: {0}'.format(
                    list(kwargs)))

        self._data = representation_data

        # We do ``is not None`` because self._data might evaluate to false for
        # empty arrays or data == 0
        if self._data is not None:
            self._rep_cache = dict()
            self._rep_cache[self._data.__class__.__name__, False] = self._data

    @property
    def data(self):
        """
        The coordinate data for this object.  If this frame has no data, an
        `~.exceptions.ValueError` will be raised.  Use `has_data` to
        check if data is present on this frame object.
        """
        if self._data is None:
            raise ValueError('The frame object "{0}" does not have associated '
                             'data'.format(repr(self)))
        return self._data

    @property
    def has_data(self):
        """
        True if this frame has `data`, False otherwise.
        """
        return self._data is not None

    def __len__(self):
        return len(self.data)

    def __nonzero__(self):  # Py 2.x
        return self.isscalar or len(self) != 0

    def __bool__(self):  # Py 3.x
        return self.isscalar or len(self) != 0

    @property
    def shape(self):
        return self.data.shape

    @property
    def isscalar(self):
        return self.data.isscalar

    @classmethod
    def get_frame_attr_names(cls):
        seen = set()
        attributes = []
        for mro_cls in cls.__mro__:
            for name, val in mro_cls.__dict__.items():
                if isinstance(val, FrameAttribute) and name not in seen:
                    seen.add(name)
                    # Add the sort order, name, and actual value of the frame
                    # attribute in question
                    attributes.append((val._order, name,
                                       getattr(mro_cls, name)))

        # Sort by the frame attribute order
        attributes.sort(key=lambda a: a[0])
        return OrderedDict((a[1], a[2]) for a in attributes)

    @property
    def representation(self):
        """
        The representation of the data in this frame, as a class that is
        subclassed from `~astropy.coordinates.BaseRepresentation`.  Can
        also be *set* using the string name of the representation.
        """
        if not hasattr(self, '_representation'):
            self._representation = self.default_representation
        return self._representation

    @representation.setter
    def representation(self, value):
        self._representation = _get_repr_cls(value)

    @classmethod
    def _get_representation_info(cls):
        # This exists as a class method only to support handling frame inputs
        # without units, which are deprecated and will be removed.  This can be
        # moved into the representation_info property at that time.

        repr_attrs = {}
        for repr_cls in REPRESENTATION_CLASSES.values():
            repr_attrs[repr_cls] = {'names': [], 'units': []}
            for c in repr_cls.attr_classes.keys():
                repr_attrs[repr_cls]['names'].append(c)
                rec_unit = repr_cls.recommended_units.get(c, None)
                repr_attrs[repr_cls]['units'].append(rec_unit)

        for repr_cls, mappings in cls._frame_specific_representation_info.items():
            # keys may be a class object or a name
            repr_cls = _get_repr_cls(repr_cls)

            # take the 'names' and 'units' tuples from repr_attrs,
            # and then use the RepresentationMapping objects
            # to update as needed for this frame.
            nms = repr_attrs[repr_cls]['names']
            uns = repr_attrs[repr_cls]['units']
            comptomap = dict([(m.reprname, m) for m in mappings])
            for i, c in enumerate(repr_cls.attr_classes.keys()):
                if c in comptomap:
                    mapp = comptomap[c]
                    nms[i] = mapp.framename
                    # need the isinstance because otherwise if it's a unit it
                    # will try to compare to the unit string representation
                    if not (isinstance(mapp.defaultunit, six.string_types) and
                            mapp.defaultunit == 'recommended'):
                        uns[i] = mapp.defaultunit
                        # else we just leave it as recommended_units says above
            # Convert to tuples so that this can't mess with frame internals
            repr_attrs[repr_cls]['names'] = tuple(nms)
            repr_attrs[repr_cls]['units'] = tuple(uns)

        return repr_attrs

    @property
    def representation_info(self):
        """
        A dictionary with the information of what attribute names for this frame
        apply to particular representations.
        """
        return self._get_representation_info()

    @property
    def representation_component_names(self):
        out = OrderedDict()
        if self.representation is None:
            return out
        data_names = self.representation.attr_classes.keys()
        repr_names = self.representation_info[self.representation]['names']
        for repr_name, data_name in zip(repr_names, data_names):
            out[repr_name] = data_name
        return out

    @property
    def representation_component_units(self):
        out = OrderedDict()
        if self.representation is None:
            return out
        repr_attrs = self.representation_info[self.representation]
        repr_names = repr_attrs['names']
        repr_units = repr_attrs['units']
        for repr_name, repr_unit in zip(repr_names, repr_units):
            if repr_unit:
                out[repr_name] = repr_unit
        return out


    def realize_frame(self, representation):
        """
        Generates a new frame *with new data* from another frame (which may or
        may not have data).

        Parameters
        ----------
        representation : BaseRepresentation
            The representation to use as the data for the new frame.

        Returns
        -------
        frameobj : same as this frame
            A new object with the same frame attributes as this one, but
            with the ``representation`` as the data.
        """
        frattrs = dict([(attr, getattr(self, attr))
                        for attr in self.get_frame_attr_names()
                        if attr not in self._attr_names_with_defaults])
        return self.__class__(representation, **frattrs)

    def represent_as(self, new_representation, in_frame_units=False):
        """
        Generate and return a new representation of this frame's `data`
        as a Representation object.

        Note: In order to make an in-place change of the representation
        of a Frame or SkyCoord object, set the ``representation``
        attribute of that object to the desired new representation.

        Parameters
        ----------
        new_representation : subclass of BaseRepresentation or string
            The type of representation to generate.  May be a *class*
            (not an instance), or the string name of the representation
            class.

        in_frame_units : bool
            Force the representation units to match the specified units
            particular to this frame

        Returns
        -------
        newrep : BaseRepresentation-derived object
            A new representation object of this frame's `data`.

        Raises
        ------
        AttributeError
            If this object had no `data`

        Examples
        --------
        >>> from astropy import units as u
        >>> from astropy.coordinates import SkyCoord, CartesianRepresentation
        >>> coord = SkyCoord(0*u.deg, 0*u.deg)
        >>> coord.represent_as(CartesianRepresentation)
        <CartesianRepresentation (x, y, z) [dimensionless]
                (1.0, 0.0, 0.0)>

        >>> coord.representation = CartesianRepresentation
        >>> coord
        <SkyCoord (ICRS): (x, y, z) [dimensionless]
            (1.0, 0.0, 0.0)>
        """
        new_representation = _get_repr_cls(new_representation)

        cached_repr = self._rep_cache.get((new_representation.__name__,
                                           in_frame_units))
        if not cached_repr:
            data = self.data.represent_as(new_representation)

            # If the new representation is known to this frame and has a defined
            # set of names and units, then use that.
            new_attrs = self.representation_info.get(new_representation)
            if new_attrs and in_frame_units:
                datakwargs = dict((comp, getattr(data, comp))
                                  for comp in data.components)
                for comp, new_attr_unit in zip(data.components, new_attrs['units']):
                    if new_attr_unit:
                        datakwargs[comp] = datakwargs[comp].to(new_attr_unit)
                data = data.__class__(**datakwargs)

            self._rep_cache[new_representation.__name__, in_frame_units] = data

        return self._rep_cache[new_representation.__name__, in_frame_units]

    def transform_to(self, new_frame):
        """
        Transform this object's coordinate data to a new frame.

        Parameters
        ----------
        new_frame : class or frame object or SkyCoord object
            The frame to transform this coordinate frame into.

        Returns
        -------
        transframe
            A new object with the coordinate data represented in the
            ``newframe`` system.

        Raises
        ------
        ValueError
            If there is no possible transformation route.
        """
        from .errors import ConvertError

        if self._data is None:
            raise ValueError('Cannot transform a frame with no data')

        if inspect.isclass(new_frame):
            #means use the defaults for this class
            new_frame = new_frame()

        if hasattr(new_frame, '_sky_coord_frame'):
            # Input new_frame is not a frame instance or class and is most
            # likely a SkyCoord object.
            new_frame = new_frame._sky_coord_frame

        trans = frame_transform_graph.get_transform(self.__class__,
                                                    new_frame.__class__)
        if trans is None:
            if new_frame is self.__class__:
                # no special transform needed, but should update frame info
                return new_frame.realize_frame(self.data)
            msg = 'Cannot transform from {0} to {1}'
            raise ConvertError(msg.format(self.__class__, new_frame.__class__))
        return trans(self, new_frame)

    def is_transformable_to(self, new_frame):
        """
        Determines if this coordinate frame can be transformed to another
        given frame.

        Parameters
        ----------
        new_frame : class or frame object
            The proposed frame to transform into.

        Returns
        -------
        transformable : bool or str
            `True` if this can be transformed to ``new_frame``, `False` if
            not, or the string 'same' if ``new_frame`` is the same system as
            this object but no transformation is defined.

        Notes
        -----
        A return value of 'same' means the transformation will work, but it will
        just give back a copy of this object.  The intended usage is::

            if coord.is_transformable_to(some_unknown_frame):
                coord2 = coord.transform_to(some_unknown_frame)

        This will work even if ``some_unknown_frame``  turns out to be the same
        frame class as ``coord``.  This is intended for cases where the frame
        is the same regardless of the frame attributes (e.g. ICRS), but be
        aware that it *might* also indicate that someone forgot to define the
        transformation between two objects of the same frame class but with
        different attributes.
        """

        new_frame_cls = new_frame if inspect.isclass(new_frame) else new_frame.__class__
        trans = frame_transform_graph.get_transform(self.__class__, new_frame_cls)

        if trans is None:
            if new_frame_cls is self.__class__:
                return 'same'
            else:
                return False
        else:
            return True

    def is_frame_attr_default(self, attrnm):
        """
        Determine whether or not a frame attribute has its value because it's
        the default value, or because this frame was created with that value
        explicitly requested.

        Parameters
        ----------
        attrnm : str
            The name of the attribute to check.

        Returns
        -------
        isdefault : bool
            True if the attribute ``attrnm`` has its value by default, False if
            it was specified at creation of this frame.
        """
        return attrnm in self._attr_names_with_defaults

    def is_equivalent_frame(self, other):
        """
        Checks if this object is the same frame as the ``other`` object.

        To be the same frame, two objects must be the same frame class and have
        the same frame attributes.  Note that it does *not* matter what, if any,
        data either object has.

        Parameters
        ----------
        other : BaseCoordinateFrame
            the other frame to check

        Returns
        -------
        isequiv : bool
            True if the frames are the same, False if not.

        Raises
        ------
        TypeError
            If ``other`` isn't a `BaseCoordinateFrame` or subclass.
        """
        if self.__class__ == other.__class__:
            for frame_attr_name in self.get_frame_attr_names():
                if getattr(self, frame_attr_name) != getattr(other, frame_attr_name):
                    return False
            return True
        elif not isinstance(other, BaseCoordinateFrame):
            raise TypeError("Tried to do is_equivalent_frame on something that "
                            "isn't a frame")
        else:
            return False

    def __repr__(self):
        frameattrs = self._frame_attrs_repr()
        data_repr = self._data_repr()

        if frameattrs:
            frameattrs = ' ({0})'.format(frameattrs)

        if data_repr:
            return '<{0} Coordinate{1}: {2}>'.format(self.__class__.__name__,
                                                     frameattrs, data_repr)
        else:
            return '<{0} Frame{1}>'.format(self.__class__.__name__,
                                            frameattrs)

    def _data_repr(self):
        """Returns a string representation of the coordinate data."""

        if not self.has_data:
            return ''

        if self.representation:
            if (issubclass(self.representation, SphericalRepresentation) and
                    isinstance(self.data, UnitSphericalRepresentation)):
                data = self.represent_as(self.data.__class__,
                                         in_frame_units=True)
            else:
                data = self.represent_as(self.representation,
                                         in_frame_units=True)

            data_repr = repr(data)
            for nmpref, nmrepr in self.representation_component_names.items():
                data_repr = data_repr.replace(nmrepr, nmpref)

        else:
            data = self.data
            data_repr = repr(self.data)

        if data_repr.startswith('<' + data.__class__.__name__):
            # remove both the leading "<" and the space after the name, as well
            # as the trailing ">"
            data_repr = data_repr[(len(data.__class__.__name__) + 2):-1]
        else:
            data_repr = 'Data:\n' + data_repr

        return data_repr

    def _frame_attrs_repr(self):
        """
        Returns a string representation of the frame's attributes, if any.
        """
        return ', '.join([attrnm + '=' + str(getattr(self, attrnm))
                          for attrnm in self.get_frame_attr_names()])

    def __getitem__(self, view):
        if self.has_data:
            out = self.realize_frame(self.data[view])
            out.representation = self.representation
            return out
        else:
            raise ValueError('Cannot index a frame with no data')

    @override__dir__
    def __dir__(self):
        """
        Override the builtin `dir` behavior to include representation
        names.

        TODO: dynamic representation transforms (i.e. include cylindrical et al.).
        """
        dir_values = set(self.representation_component_names)

        return dir_values

    def __getattr__(self, attr):
        """
        Allow access to attributes defined in
        ``self.representation_component_names``.

        TODO: dynamic representation transforms (i.e. include cylindrical et
        al.).
        """

        # attr == '_representation' is likely from the hasattr() test in the
        # representation property which is used for
        # self.representation_component_names.
        #
        # Prevent infinite recursion here.
        if (attr == '_representation' or
                attr not in self.representation_component_names):
            raise AttributeError("'{0}' object has no attribute '{1}'"
                                 .format(self.__class__.__name__, attr))

        rep = self.represent_as(self.representation, in_frame_units=True)
        val = getattr(rep, self.representation_component_names[attr])
        return val

    def __setattr__(self, attr, value):
        repr_attr_names = []
        if hasattr(self, 'representation_info'):
            for representation_attr in self.representation_info.values():
                repr_attr_names.extend(representation_attr['names'])
        if attr in repr_attr_names:
            raise AttributeError(
                'Cannot set any frame attribute {0}'.format(attr))
        else:
            super(BaseCoordinateFrame, self).__setattr__(attr, value)

    def separation(self, other):
        """
        Computes on-sky separation between this coordinate and another.

        Parameters
        ----------
        other : `~astropy.coordinates.BaseCoordinateFrame`
            The coordinate to get the separation to.

        Returns
        -------
        sep : `~astropy.coordinates.Angle`
            The on-sky separation between this and the ``other`` coordinate.

        Notes
        -----
        The separation is calculated using the Vincenty formula, which
        is stable at all locations, including poles and antipodes [1]_.

        .. [1] http://en.wikipedia.org/wiki/Great-circle_distance

        """
        from .angle_utilities import angular_separation
        from .angles import Angle

        self_unit_sph = self.represent_as(UnitSphericalRepresentation)
        other_transformed = other.transform_to(self)
        other_unit_sph = other_transformed.represent_as(UnitSphericalRepresentation)

        # Get the separation as a Quantity, convert to Angle in degrees
        sep = angular_separation(self_unit_sph.lon, self_unit_sph.lat,
                                 other_unit_sph.lon, other_unit_sph.lat)
        return Angle(sep, unit=u.degree)

    def separation_3d(self, other):
        """
        Computes three dimensional separation between this coordinate
        and another.

        Parameters
        ----------
        other : `~astropy.coordinates.BaseCoordinateFrame`
            The coordinate system to get the distance to.

        Returns
        -------
        sep : `~astropy.coordinates.Distance`
            The real-space distance between these two coordinates.

        Raises
        ------
        ValueError
            If this or the other coordinate do not have distances.
        """

        from .distances import Distance

        if self.data.__class__ == UnitSphericalRepresentation:
            raise ValueError('This object does not have a distance; cannot '
                             'compute 3d separation.')

        # do this first just in case the conversion somehow creates a distance
        other_in_self_system = other.transform_to(self)

        if other_in_self_system.__class__ == UnitSphericalRepresentation:
            raise ValueError('The other object does not have a distance; '
                             'cannot compute 3d separation.')

        dx = self.cartesian.x - other_in_self_system.cartesian.x
        dy = self.cartesian.y - other_in_self_system.cartesian.y
        dz = self.cartesian.z - other_in_self_system.cartesian.z

        distval = (dx.value ** 2 + dy.value ** 2 + dz.value ** 2) ** 0.5
        return Distance(distval, dx.unit)

    @property
    def cartesian(self):
        """
        Shorthand for a cartesian representation of the coordinates in this
        object.
        """

        # TODO: if representations are updated to use a full transform graph,
        #       the representation aliases should not be hard-coded like this
        return self.represent_as(CartesianRepresentation, in_frame_units=True)

    @property
    def spherical(self):
        """
        Shorthand for a spherical representation of the coordinates in this object.
        """

        # TODO: if representations are updated to use a full transform graph,
        #       the representation aliases should not be hard-coded like this
        return self.represent_as(SphericalRepresentation, in_frame_units=True)


class GenericFrame(BaseCoordinateFrame):
    """
    A frame object that can't store data but can hold any arbitrary frame
    attributes. Mostly useful as a utility for the high-level class to store
    intermediate frame attributes.

    Parameters
    ----------
    frame_attrs : dict
        A dictionary of attributes to be used as the frame attributes for this
        frame.
    """
    name = None  # it's not a "real" frame so it doesn't have a name

    def __init__(self, frame_attrs):
        super(GenericFrame, self).__setattr__('_frame_attr_names', frame_attrs)
        super(GenericFrame, self).__init__(None)

        for attrnm, attrval in frame_attrs.items():
            setattr(self, '_' + attrnm, attrval)

    def get_frame_attr_names(self):
        return self._frame_attr_names

    def __getattr__(self, name):
        if '_' + name in self.__dict__:
            return getattr(self, '_' + name)
        else:
            raise AttributeError('no {0}'.format(name))

    def __setattr__(self, name, value):
        if name in self._frame_attr_names:
            raise AttributeError("can't set frame attribute '{0}'".format(name))
        else:
            super(GenericFrame, self).__setattr__(name, value)

# doing this import at the bottom prevents a circular import issue that is
# otherwise present due to EarthLocation needing to import ITRS
from .earth import EarthLocation
