xarray.py

# Copyright (c) 2018,2019 MetPy Developers.
# Distributed under the terms of the BSD 3-Clause License.
# SPDX-License-Identifier: BSD-3-Clause
"""Provide accessors to enhance interoperability between xarray and MetPy.

MetPy relies upon the `CF Conventions <http://cfconventions.org/>`_. to provide helpful
attributes and methods on xarray DataArrays and Dataset for working with
coordinate-related metadata. Also included are several attributes and methods for unit
operations.

These accessors will be activated with any import of MetPy. Do not use the
``MetPyDataArrayAccessor`` or ``MetPyDatasetAccessor`` classes directly, instead, utilize the
applicable properties and methods via the ``.metpy`` attribute on an xarray DataArray or
Dataset.

See Also: :doc:`xarray with MetPy Tutorial </tutorials/xarray_tutorial>`.
"""
import contextlib
import functools
from inspect import signature
from itertools import chain
import logging
import re

import numpy as np
from pyproj import CRS, Proj
import xarray as xr

from . import _warnings
from ._vendor.xarray import either_dict_or_kwargs, expanded_indexer, is_dict_like
from .units import (_mutate_arguments, DimensionalityError, is_quantity, UndefinedUnitError,
                    units)

__all__ = ('MetPyDataArrayAccessor', 'MetPyDatasetAccessor', 'grid_deltas_from_dataarray')
metpy_axes = ['time', 'vertical', 'y', 'latitude', 'x', 'longitude']

# Define the criteria for coordinate matches
coordinate_criteria = {
    'standard_name': {
        'time': 'time',
        'vertical': {'air_pressure', 'height', 'geopotential_height', 'altitude',
                     'model_level_number', 'atmosphere_ln_pressure_coordinate',
                     'atmosphere_sigma_coordinate',
                     'atmosphere_hybrid_sigma_pressure_coordinate',
                     'atmosphere_hybrid_height_coordinate', 'atmosphere_sleve_coordinate',
                     'height_above_geopotential_datum', 'height_above_reference_ellipsoid',
                     'height_above_mean_sea_level'},
        'y': 'projection_y_coordinate',
        'latitude': 'latitude',
        'x': 'projection_x_coordinate',
        'longitude': 'longitude'
    },
    '_CoordinateAxisType': {
        'time': 'Time',
        'vertical': {'GeoZ', 'Height', 'Pressure'},
        'y': 'GeoY',
        'latitude': 'Lat',
        'x': 'GeoX',
        'longitude': 'Lon'
    },
    'axis': {
        'time': 'T',
        'vertical': 'Z',
        'y': 'Y',
        'x': 'X'
    },
    'positive': {
        'vertical': {'up', 'down'}
    },
    'units': {
        'vertical': {
            'match': 'dimensionality',
            'units': 'Pa'
        },
        'latitude': {
            'match': 'name',
            'units': {'degree_north', 'degree_N', 'degreeN', 'degrees_north', 'degrees_N',
                      'degreesN'}
        },
        'longitude': {
            'match': 'name',
            'units': {'degree_east', 'degree_E', 'degreeE', 'degrees_east', 'degrees_E',
                      'degreesE'}
        },
    },
    'regular_expression': {
        'time': re.compile(r'^(x?)time(s?)[0-9]*$'),
        'vertical': re.compile(
            r'^(z|lv_|bottom_top|sigma|h(ei)?ght|altitude|depth|isobaric|pres|isotherm)'
            r'[a-z_]*[0-9]*$'
        ),
        'y': re.compile(r'^y(_?)[a-z0-9]*$'),
        'latitude': re.compile(r'^(x?)lat[a-z0-9_]*$'),
        'x': re.compile(r'^x(?!lon|lat|time).*(_?)[a-z0-9]*$'),
        'longitude': re.compile(r'^(x?)lon[a-z0-9_]*$')
    }
}

log = logging.getLogger(__name__)

_axis_identifier_error = ('Given axis is not valid. Must be an axis number, a dimension '
                          'coordinate name, or a standard axis type.')


@xr.register_dataarray_accessor('metpy')
class MetPyDataArrayAccessor:
    r"""Provide custom attributes and methods on xarray DataArrays for MetPy functionality.

    This accessor provides several convenient attributes and methods through the ``.metpy``
    attribute on a DataArray. For example, MetPy can identify the coordinate corresponding
    to a particular axis (given sufficient metadata):

        >>> import xarray as xr
        >>> from metpy.units import units
        >>> temperature = xr.DataArray([[0, 1], [2, 3]] * units.degC, dims=('lat', 'lon'),
        ...                            coords={'lat': [40, 41], 'lon': [-105, -104]})
        >>> temperature.metpy.x
        <xarray.DataArray 'lon' (lon: 2)> Size: 16B
        array([-105, -104])
        Coordinates:
          * lon      (lon) int64 16B -105 -104
        Attributes:
            _metpy_axis:  x,longitude

    """

    def __init__(self, data_array):  # noqa: D107
        # Initialize accessor with a DataArray. (Do not use directly).
        self._data_array = data_array

    @property
    def units(self):
        """Return the units of this DataArray as a `pint.Unit`."""
        if is_quantity(self._data_array.variable._data):
            return self._data_array.variable._data.units
        else:
            axis = self._data_array.attrs.get('_metpy_axis', '')
            if 'latitude' in axis or 'longitude' in axis:
                default_unit = 'degrees'
            else:
                default_unit = 'dimensionless'
            return units.parse_units(self._data_array.attrs.get('units', default_unit))

    @property
    def magnitude(self):
        """Return the magnitude of the data values of this DataArray (i.e., without units)."""
        if is_quantity(self._data_array.data):
            return self._data_array.data.magnitude
        else:
            return self._data_array.data

    @property
    def unit_array(self):
        """Return the data values of this DataArray as a `pint.Quantity`.

        Notes
        -----
        If not already existing as a `pint.Quantity` or Dask array, the data of this DataArray
        will be loaded into memory by this operation. Do not utilize on moderate- to
        large-sized remote datasets before subsetting!
        """
        if is_quantity(self._data_array.data):
            return self._data_array.data
        else:
            return units.Quantity(self._data_array.data, self.units)

    def convert_units(self, units):
        """Return new DataArray with values converted to different units.

        See Also
        --------
        convert_coordinate_units

        Notes
        -----
        Any cached/lazy-loaded data (except that in a Dask array) will be loaded into memory
        by this operation. Do not utilize on moderate- to large-sized remote datasets before
        subsetting!

        """
        return self.quantify().copy(data=self.unit_array.to(units))

    def convert_to_base_units(self):
        """Return new DataArray with values converted to base units.

        See Also
        --------
        convert_units

        Notes
        -----
        Any cached/lazy-loaded data (except that in a Dask array) will be loaded into memory
        by this operation. Do not utilize on moderate- to large-sized remote datasets before
        subsetting!

        """
        return self.quantify().copy(data=self.unit_array.to_base_units())

    def convert_coordinate_units(self, coord, units):
        """Return new DataArray with specified coordinate converted to different units.

        This operation differs from ``.convert_units`` since xarray coordinate indexes do not
        yet support unit-aware arrays (even though unit-aware *data* arrays are).

        See Also
        --------
        convert_units

        Notes
        -----
        Any cached/lazy-loaded coordinate data (except that in a Dask array) will be loaded
        into memory by this operation.

        """
        new_coord_var = self._data_array[coord].copy(
            data=self._data_array[coord].metpy.unit_array.m_as(units)
        )
        new_coord_var.attrs['units'] = str(units)
        return self._data_array.assign_coords(coords={coord: new_coord_var})

    def quantify(self):
        """Return a new DataArray with the data converted to a `pint.Quantity`.

        Notes
        -----
        Any cached/lazy-loaded data (except that in a Dask array) will be loaded into memory
        by this operation. Do not utilize on moderate- to large-sized remote datasets before
        subsetting!
        """
        if (
            not is_quantity(self._data_array.data)
            and np.issubdtype(self._data_array.data.dtype, np.number)
        ):
            # Only quantify if not already quantified and is quantifiable
            quantified_dataarray = self._data_array.copy(data=self.unit_array)
            if 'units' in quantified_dataarray.attrs:
                del quantified_dataarray.attrs['units']
        else:
            quantified_dataarray = self._data_array
        return quantified_dataarray

    def dequantify(self):
        """Return a new DataArray with the data as magnitude and the units as an attribute."""
        if is_quantity(self._data_array.data):
            # Only dequantify if quantified
            dequantified_dataarray = self._data_array.copy(
                data=self._data_array.data.magnitude
            )
            dequantified_dataarray.attrs['units'] = str(self.units)
        else:
            dequantified_dataarray = self._data_array
        return dequantified_dataarray

    @property
    def crs(self):
        """Return the coordinate reference system (CRS) as a CFProjection object."""
        if 'metpy_crs' in self._data_array.coords:
            return self._data_array.coords['metpy_crs'].item()
        raise AttributeError('crs attribute is not available. You may need to use the'
                             ' `parse_cf` or `assign_crs` methods. Consult the "xarray'
                             ' with MetPy Tutorial" for more details.')

    @property
    def cartopy_crs(self):
        """Return the coordinate reference system (CRS) as a cartopy object."""
        return self.crs.to_cartopy()

    @property
    def cartopy_globe(self):
        """Return the globe belonging to the coordinate reference system (CRS)."""
        return self.crs.cartopy_globe

    @property
    def cartopy_geodetic(self):
        """Return the cartopy Geodetic CRS associated with the native CRS globe."""
        return self.crs.cartopy_geodetic

    @property
    def pyproj_crs(self):
        """Return the coordinate reference system (CRS) as a pyproj object."""
        return self.crs.to_pyproj()

    @property
    def pyproj_proj(self):
        """Return the Proj object corresponding to the coordinate reference system (CRS)."""
        return Proj(self.pyproj_crs)

    def _fixup_coordinate_map(self, coord_map):
        """Ensure sure we have coordinate variables in map, not coordinate names."""
        new_coord_map = {}
        for axis in coord_map:
            if coord_map[axis] is not None and not isinstance(coord_map[axis], xr.DataArray):
                new_coord_map[axis] = self._data_array[coord_map[axis]]
            else:
                new_coord_map[axis] = coord_map[axis]

        return new_coord_map

    def assign_coordinates(self, coordinates):
        """Return new DataArray with given coordinates assigned to the given MetPy axis types.

        Parameters
        ----------
        coordinates : dict or None
            Mapping from axis types ('time', 'vertical', 'y', 'latitude', 'x', 'longitude') to
            coordinates of this DataArray. Coordinates can either be specified directly or by
            their name. If ``None``, clears the `_metpy_axis` attribute on all coordinates,
            which will trigger reparsing of all coordinates on next access.

        """
        coord_updates = {}
        if coordinates:
            # Assign the _metpy_axis attributes according to supplied mapping
            coordinates = self._fixup_coordinate_map(coordinates)
            for axis in coordinates:
                if coordinates[axis] is not None:
                    coord_updates[coordinates[axis].name] = (
                        coordinates[axis].assign_attrs(
                            _assign_axis(coordinates[axis].attrs.copy(), axis)
                        )
                    )
        else:
            # Clear _metpy_axis attribute on all coordinates
            for coord_name, coord_var in self._data_array.coords.items():
                coord_updates[coord_name] = coord_var.copy(deep=False)

                # Some coordinates remained linked in old form under other coordinates. We
                # need to remove from these.
                sub_coords = coord_updates[coord_name].coords
                for sub_coord in sub_coords:
                    coord_updates[coord_name].coords[sub_coord].attrs.pop('_metpy_axis', None)

                # Now we can remove the _metpy_axis attr from the coordinate itself
                coord_updates[coord_name].attrs.pop('_metpy_axis', None)

        return self._data_array.assign_coords(coord_updates)

    def _generate_coordinate_map(self):
        """Generate a coordinate map via CF conventions and other methods."""
        coords = self._data_array.coords.values()
        # Parse all the coordinates, attempting to identify longitude, latitude, x, y,
        # time, vertical, in that order.
        coord_lists = {'longitude': [], 'latitude': [], 'x': [], 'y': [], 'time': [],
                       'vertical': []}
        for coord_var in coords:
            # Identify the coordinate type using check_axis helper
            for axis in coord_lists:
                if check_axis(coord_var, axis):
                    coord_lists[axis].append(coord_var)
                    break  # Ensure a coordinate variable only goes to one axis

        # Fill in x/y with longitude/latitude if x/y not otherwise present
        for geometric, graticule in (('y', 'latitude'), ('x', 'longitude')):
            if len(coord_lists[geometric]) == 0 and len(coord_lists[graticule]) > 0:
                coord_lists[geometric] = coord_lists[graticule]

        # Filter out multidimensional coordinates where not allowed
        require_1d_coord = ['time', 'vertical', 'y', 'x']
        for axis in require_1d_coord:
            coord_lists[axis] = [coord for coord in coord_lists[axis] if coord.ndim <= 1]

        # Resolve any coordinate type duplication
        axis_duplicates = [axis for axis in coord_lists if len(coord_lists[axis]) > 1]
        for axis in axis_duplicates:
            self._resolve_axis_duplicates(axis, coord_lists)

        # Collapse the coord_lists to a coord_map
        return {axis: (coord_lists[axis][0] if len(coord_lists[axis]) > 0 else None)
                for axis in coord_lists}

    def _resolve_axis_duplicates(self, axis, coord_lists):
        """Handle coordinate duplication for an axis type if it arises."""
        # If one and only one of the possible axes is a dimension, use it
        dimension_coords = [coord_var for coord_var in coord_lists[axis] if
                            coord_var.name in coord_var.dims]
        if len(dimension_coords) == 1:
            coord_lists[axis] = dimension_coords
            return

        # Ambiguous axis, raise warning and do not parse
        varname = (' "' + self._data_array.name + '"'
                   if self._data_array.name is not None else '')
        _warnings.warn(f'More than one {axis} coordinate present for variable {varname}.')
        coord_lists[axis] = []

    def _metpy_axis_search(self, metpy_axis):
        """Search for cached _metpy_axis attribute on the coordinates, otherwise parse."""
        # Search for coord with proper _metpy_axis
        coords = self._data_array.coords.values()
        for coord_var in coords:
            if metpy_axis in coord_var.attrs.get('_metpy_axis', '').split(','):
                return coord_var

        # Opportunistically parse all coordinates, and assign if not already assigned
        # Note: since this is generally called by way of the coordinate properties, to cache
        # the coordinate parsing results in coord_map on the coordinates means modifying the
        # DataArray in-place (an exception to the usual behavior of MetPy's accessor). This is
        # considered safe because it only effects the "_metpy_axis" attribute on the
        # coordinates, and nothing else.
        coord_map = self._generate_coordinate_map()
        for axis, coord_var in coord_map.items():
            if coord_var is not None and all(
                axis not in coord.attrs.get('_metpy_axis', '').split(',')
                for coord in coords
            ):

                _assign_axis(coord_var.attrs, axis)

        # Return parsed result (can be None if none found)
        return coord_map[metpy_axis]

    def _axis(self, axis):
        """Return the coordinate variable corresponding to the given individual axis type."""
        if axis not in metpy_axes:
            raise AttributeError("'" + axis + "' is not an interpretable axis.")

        coord_var = self._metpy_axis_search(axis)
        if coord_var is None:
            raise AttributeError(axis + ' attribute is not available.')
        else:
            return coord_var

    def coordinates(self, *args):
        """Return the coordinate variables corresponding to the given axes types.

        Parameters
        ----------
        args : str
            Strings describing the axes type(s) to obtain. Currently understood types are
            'time', 'vertical', 'y', 'latitude', 'x', and 'longitude'.

        Notes
        -----
        This method is designed for use with multiple coordinates; it returns a generator. To
        access a single coordinate, use the appropriate attribute on the accessor, or use tuple
        unpacking.

        If latitude and/or longitude are requested here, and yet are not present on the
        DataArray, an on-the-fly computation from the CRS and y/x dimension coordinates is
        attempted.

        """
        latitude = None
        longitude = None
        for arg in args:
            try:
                yield self._axis(arg)
            except AttributeError as exc:
                if (
                    (arg == 'latitude' and latitude is None)
                    or (arg == 'longitude' and longitude is None)
                ):
                    # Try to compute on the fly
                    try:
                        latitude, longitude = _build_latitude_longitude(self._data_array)
                    except Exception:
                        # Attempt failed, re-raise original error
                        raise exc from None
                    # Otherwise, warn and yield result
                    _warnings.warn(
                        'Latitude and longitude computed on-demand, which may be an '
                        'expensive operation. To avoid repeating this computation, assign '
                        'these coordinates ahead of time with '
                        '.metpy.assign_latitude_longitude().'
                    )
                    if arg == 'latitude':
                        yield latitude
                    else:
                        yield longitude
                elif arg == 'latitude' and latitude is not None:
                    # We have this from previous computation
                    yield latitude
                elif arg == 'longitude' and longitude is not None:
                    # We have this from previous computation
                    yield longitude
                else:
                    raise exc

    @property
    def time(self):
        """Return the time coordinate."""
        return self._axis('time')

    @property
    def vertical(self):
        """Return the vertical coordinate."""
        return self._axis('vertical')

    @property
    def y(self):
        """Return the y coordinate."""
        return self._axis('y')

    @property
    def latitude(self):
        """Return the latitude coordinate (if it exists)."""
        return self._axis('latitude')

    @property
    def x(self):
        """Return the x coordinate."""
        return self._axis('x')

    @property
    def longitude(self):
        """Return the longitude coordinate (if it exists)."""
        return self._axis('longitude')

    def coordinates_identical(self, other):
        """Return whether the coordinates of other match this DataArray's."""
        return (len(self._data_array.coords) == len(other.coords)
                and all(coord_name in other.coords and other[coord_name].identical(coord_var)
                        for coord_name, coord_var in self._data_array.coords.items()))

    @property
    def time_deltas(self):
        """Return the time difference of the data in seconds (to microsecond precision)."""
        us_diffs = np.diff(self._data_array.values).astype('timedelta64[us]').astype('int64')
        return units.Quantity(us_diffs / 1e6, 's')

    @property
    def grid_deltas(self):
        """Return the horizontal dimensional grid deltas suitable for vector derivatives."""
        if (
                (hasattr(self, 'crs')
                 and self.crs._attrs['grid_mapping_name'] == 'latitude_longitude')
                or (hasattr(self, 'longitude') and self.longitude.squeeze().ndim == 1
                    and hasattr(self, 'latitude') and self.latitude.squeeze().ndim == 1)
        ):
            # Calculate dx and dy on ellipsoid (on equator and 0 deg meridian, respectively)
            from .calc.tools import nominal_lat_lon_grid_deltas
            crs = getattr(self, 'pyproj_crs', CRS('+proj=latlon'))
            dx, dy = nominal_lat_lon_grid_deltas(
                self.longitude.metpy.unit_array,
                self.latitude.metpy.unit_array,
                crs.get_geod()
            )
        else:
            # Calculate dx and dy in projection space
            try:
                dx = np.diff(self.x.metpy.unit_array)
                dy = np.diff(self.y.metpy.unit_array)
            except AttributeError:
                raise AttributeError(
                    'Grid deltas cannot be calculated since horizontal dimension coordinates '
                    'cannot be found.'
                ) from None

        return {'dx': dx, 'dy': dy}

    def find_axis_name(self, axis):
        """Return the name of the axis corresponding to the given identifier.

        Parameters
        ----------
        axis : str or int
            Identifier for an axis. Can be an axis number (integer), dimension coordinate
            name (string) or a standard axis type (string).

        """
        if isinstance(axis, int):
            # If an integer, use the corresponding dimension
            return self._data_array.dims[axis]
        elif axis not in self._data_array.dims and axis in metpy_axes:
            # If not a dimension name itself, but a valid axis type, get the name of the
            # coordinate corresponding to that axis type
            return self._axis(axis).name
        elif axis in self._data_array.dims and axis in self._data_array.coords:
            # If this is a dimension coordinate name, use it directly
            return axis
        else:
            # Otherwise, not valid
            raise ValueError(_axis_identifier_error)

    def find_axis_number(self, axis):
        """Return the dimension number of the axis corresponding to the given identifier.

        Parameters
        ----------
        axis : str or int
            Identifier for an axis. Can be an axis number (integer), dimension coordinate
            name (string) or a standard axis type (string).

        """
        if isinstance(axis, int):
            # If an integer, use it directly
            return axis
        elif axis in self._data_array.dims:
            # Simply index into dims
            return self._data_array.dims.index(axis)
        elif axis in metpy_axes:
            # If not a dimension name itself, but a valid axis type, first determine if this
            # standard axis type is present as a dimension coordinate
            try:
                name = self._axis(axis).name
                return self._data_array.dims.index(name)
            except AttributeError as exc:
                # If x, y, or vertical requested, but not available, attempt to interpret dim
                # names using regular expressions from coordinate parsing to allow for
                # multidimensional lat/lon without y/x dimension coordinates, and basic
                # vertical dim recognition
                if axis in ('vertical', 'y', 'x'):
                    for i, dim in enumerate(self._data_array.dims):
                        if coordinate_criteria['regular_expression'][axis].match(dim.lower()):
                            return i
                raise exc
            except ValueError:
                # Intercept ValueError when axis type found but not dimension coordinate
                raise AttributeError(f'Requested {axis} dimension coordinate but {axis} '
                                     f'coordinate {name} is not a dimension') from None
        else:
            # Otherwise, not valid
            raise ValueError(_axis_identifier_error)

    class _LocIndexer:
        """Provide the unit-wrapped .loc indexer for data arrays."""

        def __init__(self, data_array):
            self.data_array = data_array

        def expand(self, key):
            """Parse key using xarray utils to ensure we have dimension names."""
            if not is_dict_like(key):
                labels = expanded_indexer(key, self.data_array.ndim)
                key = dict(zip(self.data_array.dims, labels))
            return key

        def __getitem__(self, key):
            key = _reassign_quantity_indexer(self.data_array, self.expand(key))
            return self.data_array.loc[key]

        def __setitem__(self, key, value):
            key = _reassign_quantity_indexer(self.data_array, self.expand(key))
            self.data_array.loc[key] = value

    @property
    def loc(self):
        """Wrap DataArray.loc with an indexer to handle units and coordinate types."""
        return self._LocIndexer(self._data_array)

    def sel(self, indexers=None, method=None, tolerance=None, drop=False, **indexers_kwargs):
        """Wrap DataArray.sel to handle units and coordinate types."""
        indexers = either_dict_or_kwargs(indexers, indexers_kwargs, 'sel')
        indexers = _reassign_quantity_indexer(self._data_array, indexers)
        return self._data_array.sel(indexers, method=method, tolerance=tolerance, drop=drop)

    def assign_crs(self, cf_attributes=None, **kwargs):
        """Assign a CRS to this DataArray based on CF projection attributes.

        Specify a coordinate reference system/grid mapping following the Climate and
        Forecasting (CF) conventions (see `Appendix F: Grid Mappings
        <http://cfconventions.org/Data/cf-conventions/cf-conventions-1.7/cf-conventions.html#appendix-grid-mappings>`_
        ) and store in the ``metpy_crs`` coordinate.

        This method is only required if your data do not come from a dataset that follows CF
        conventions with respect to grid mappings (in which case the ``.parse_cf`` method will
        parse for the CRS metadata automatically).

        Parameters
        ----------
        cf_attributes : dict, optional
            Dictionary of CF projection attributes
        kwargs : optional
            CF projection attributes specified as keyword arguments

        Returns
        -------
        `xarray.DataArray`
            New xarray DataArray with CRS coordinate assigned

        Notes
        -----
        CF projection arguments should be supplied as a dictionary or collection of kwargs,
        but not both.

        """
        return _assign_crs(self._data_array, cf_attributes, kwargs)

    def assign_latitude_longitude(self, force=False):
        """Assign 2D latitude and longitude coordinates derived from 1D y and x coordinates.

        Parameters
        ----------
        force : bool, optional
            If force is true, overwrite latitude and longitude coordinates if they exist,
            otherwise, raise a RuntimeError if such coordinates exist.

        Returns
        -------
        `xarray.DataArray`
            New xarray DataArray with latitude and longtiude auxiliary coordinates assigned.

        Notes
        -----
        A valid CRS coordinate must be present (as assigned by ``.parse_cf`` or
        ``.assign_crs``). PyProj is used for the coordinate transformations.

        """
        # Check for existing latitude and longitude coords
        if (not force and (self._metpy_axis_search('latitude') is not None
                           or self._metpy_axis_search('longitude'))):
            raise RuntimeError('Latitude/longitude coordinate(s) are present. If you wish to '
                               'overwrite these, specify force=True.')

        # Build new latitude and longitude DataArrays
        latitude, longitude = _build_latitude_longitude(self._data_array)

        # Assign new coordinates, refresh MetPy's parsed axis attribute, and return result
        new_dataarray = self._data_array.assign_coords(latitude=latitude, longitude=longitude)
        return new_dataarray.metpy.assign_coordinates(None)

    def assign_y_x(self, force=False, tolerance=None):
        """Assign 1D y and x dimension coordinates derived from 2D latitude and longitude.

        Parameters
        ----------
        force : bool, optional
            If force is true, overwrite y and x coordinates if they exist, otherwise, raise a
            RuntimeError if such coordinates exist.
        tolerance : `pint.Quantity`
            Maximum range tolerated when collapsing projected y and x coordinates from 2D to
            1D. Defaults to 1 meter.

        Returns
        -------
        `xarray.DataArray`
            New xarray DataArray with y and x dimension coordinates assigned.

        Notes
        -----
        A valid CRS coordinate must be present (as assigned by ``.parse_cf`` or
        ``.assign_crs``) for the y/x projection space. PyProj is used for the coordinate
        transformations.

        """
        # Check for existing latitude and longitude coords
        if (not force and (self._metpy_axis_search('y') is not None
                           or self._metpy_axis_search('x'))):
            raise RuntimeError('y/x coordinate(s) are present. If you wish to overwrite '
                               'these, specify force=True.')

        # Build new y and x DataArrays
        y, x = _build_y_x(self._data_array, tolerance)

        # Assign new coordinates, refresh MetPy's parsed axis attribute, and return result
        new_dataarray = self._data_array.assign_coords(**{y.name: y, x.name: x})
        return new_dataarray.metpy.assign_coordinates(None)


@xr.register_dataset_accessor('metpy')
class MetPyDatasetAccessor:
    """Provide custom attributes and methods on XArray Datasets for MetPy functionality.

    This accessor provides parsing of CF grid mapping metadata, generating missing coordinate
    types, and unit-/coordinate-type-aware operations.

        >>> import xarray as xr
        >>> from metpy.cbook import get_test_data
        >>> ds = xr.open_dataset(get_test_data('narr_example.nc', False)).metpy.parse_cf()
        >>> print(ds['metpy_crs'].item())
        Projection: lambert_conformal_conic

    """

    def __init__(self, dataset):  # noqa: D107
        # Initialize accessor with a Dataset. (Do not use directly).
        self._dataset = dataset

    def parse_cf(self, varname=None, coordinates=None):
        """Parse dataset for coordinate system metadata according to CF conventions.

        Interpret the grid mapping metadata in the dataset according to the Climate and
        Forecasting (CF) conventions (see `Appendix F: Grid Mappings
        <http://cfconventions.org/Data/cf-conventions/cf-conventions-1.7/cf-conventions.html#appendix-grid-mappings>`_
        ) and store in the ``metpy_crs`` coordinate. Also, gives option to manually specify
        coordinate types with the ``coordinates`` keyword argument.

        If your dataset does not follow the CF conventions, you can manually supply the grid
        mapping metadata with the ``.assign_crs`` method.

        This method operates on individual data variables within the dataset, so do not be
        surprised if information not associated with individual data variables is not
        preserved.

        Parameters
        ----------
        varname : str or Sequence[str], optional
            Name of the variable(s) to extract from the dataset while parsing for CF metadata.
            Defaults to all variables.
        coordinates : dict, optional
            Dictionary mapping CF axis types to coordinates of the variable(s). Only specify
            if you wish to override MetPy's automatic parsing of some axis type(s).

        Returns
        -------
        `xarray.DataArray` or `xarray.Dataset`
            Parsed DataArray (if varname is a string) or Dataset

        See Also
        --------
        assign_crs

        """
        from .plots.mapping import CFProjection

        if varname is None:
            # If no varname is given, parse all variables in the dataset
            varname = list(self._dataset.data_vars)

        if np.iterable(varname) and not isinstance(varname, str):
            # If non-string iterable is given, apply recursively across the varnames
            subset = xr.merge([self.parse_cf(single_varname, coordinates=coordinates)
                               for single_varname in varname])
            subset.attrs = self._dataset.attrs
            return subset

        var = self._dataset[varname]

        # Check for crs conflict
        if varname == 'metpy_crs':
            _warnings.warn(
                'Attempting to parse metpy_crs as a data variable. Unexpected merge conflicts '
                'may occur.'
            )
        elif 'metpy_crs' in var.coords and (var.coords['metpy_crs'].size > 1 or not isinstance(
                var.coords['metpy_crs'].item(), CFProjection)):
            _warnings.warn(
                'metpy_crs already present as a non-CFProjection coordinate. Unexpected '
                'merge conflicts may occur.'
            )

        # Assign coordinates if the coordinates argument is given
        if coordinates is not None:
            var = var.metpy.assign_coordinates(coordinates)

        # Attempt to build the crs coordinate
        crs = None
        if 'grid_mapping' in var.attrs:
            # Use given CF grid_mapping
            proj_name = var.attrs['grid_mapping']
            try:
                proj_var = self._dataset.variables[proj_name]
            except KeyError:
                log.warning(
                    'Could not find variable corresponding to the value of grid_mapping: %s',
                    proj_name)
            else:
                crs = CFProjection(proj_var.attrs)

        if crs is None:
            # This isn't a lat or lon coordinate itself, so determine if we need to fall back
            # to creating a latitude_longitude CRS. We do so if there exists valid *at most
            # 1D* coordinates for latitude and longitude (usually dimension coordinates, but
            # that is not strictly required, for example, for DSG's). What is required is that
            # x == latitude and y == latitude (so that all assumptions about grid coordinates
            # and CRS line up).
            try:
                latitude, y, longitude, x = var.metpy.coordinates(
                    'latitude',
                    'y',
                    'longitude',
                    'x'
                )
            except AttributeError:
                # This means that we don't even have sufficient coordinates, so skip
                pass
            else:
                if latitude.identical(y) and longitude.identical(x):
                    crs = CFProjection({'grid_mapping_name': 'latitude_longitude'})
                    log.debug('Found valid latitude/longitude coordinates, assuming '
                              'latitude_longitude for projection grid_mapping variable')

        # Rebuild the coordinates of the dataarray, and return quantified DataArray
        var = self._rebuild_coords(var, crs)
        if crs is not None:
            var = var.assign_coords(coords={'metpy_crs': crs})
        return var

    def _rebuild_coords(self, var, crs):
        """Clean up the units on the coordinate variables."""
        for coord_name, coord_var in var.coords.items():
            if (check_axis(coord_var, 'x', 'y')
                    and not check_axis(coord_var, 'longitude', 'latitude')):
                try:
                    var = var.metpy.convert_coordinate_units(coord_name, 'meters')
                except DimensionalityError:
                    # Radians! Attempt to use perspective point height conversion
                    if crs is not None:
                        height = crs['perspective_point_height']
                        new_coord_var = coord_var.copy(
                            data=(
                                coord_var.metpy.unit_array
                                * units.Quantity(height, 'meter')
                            ).m_as('meter')
                        )
                        new_coord_var.attrs['units'] = 'meter'
                        var = var.assign_coords(coords={coord_name: new_coord_var})

        return var

    class _LocIndexer:
        """Provide the unit-wrapped .loc indexer for datasets."""

        def __init__(self, dataset):
            self.dataset = dataset

        def __getitem__(self, key):
            parsed_key = _reassign_quantity_indexer(self.dataset, key)
            return self.dataset.loc[parsed_key]

    @property
    def loc(self):
        """Wrap Dataset.loc with an indexer to handle units and coordinate types."""
        return self._LocIndexer(self._dataset)

    def sel(self, indexers=None, method=None, tolerance=None, drop=False, **indexers_kwargs):
        """Wrap Dataset.sel to handle units."""
        indexers = either_dict_or_kwargs(indexers, indexers_kwargs, 'sel')
        indexers = _reassign_quantity_indexer(self._dataset, indexers)
        return self._dataset.sel(indexers, method=method, tolerance=tolerance, drop=drop)

    def assign_crs(self, cf_attributes=None, **kwargs):
        """Assign a CRS to this Dataset based on CF projection attributes.

        Specify a coordinate reference system/grid mapping following the Climate and
        Forecasting (CF) conventions (see `Appendix F: Grid Mappings
        <http://cfconventions.org/Data/cf-conventions/cf-conventions-1.7/cf-conventions.html#appendix-grid-mappings>`_
        ) and store in the ``metpy_crs`` coordinate.

        This method is only required if your dataset does not already follow CF conventions
        with respect to grid mappings (in which case the ``.parse_cf`` method will parse for
        the CRS metadata automatically).

        Parameters
        ----------
        cf_attributes : dict, optional
            Dictionary of CF projection attributes
        kwargs : optional
            CF projection attributes specified as keyword arguments

        Returns
        -------
        `xarray.Dataset`
            New xarray Dataset with CRS coordinate assigned

        See Also
        --------
        parse_cf

        Notes
        -----
        CF projection arguments should be supplied as a dictionary or collection of kwargs,
        but not both.

        """
        return _assign_crs(self._dataset, cf_attributes, kwargs)

    def assign_latitude_longitude(self, force=False):
        """Assign latitude and longitude coordinates derived from y and x coordinates.

        Parameters
        ----------
        force : bool, optional
            If force is true, overwrite latitude and longitude coordinates if they exist,
            otherwise, raise a RuntimeError if such coordinates exist.

        Returns
        -------
        `xarray.Dataset`
            New xarray Dataset with latitude and longitude coordinates assigned to all
            variables with y and x coordinates.

        Notes
        -----
        A valid CRS coordinate must be present (as assigned by ``.parse_cf`` or
        ``.assign_crs``). PyProj is used for the coordinate transformations.

        """
        # Determine if there is a valid grid prototype from which to compute the coordinates,
        # while also checking for existing lat/lon coords
        grid_prototype = None
        for data_var in self._dataset.data_vars.values():
            if hasattr(data_var.metpy, 'y') and hasattr(data_var.metpy, 'x'):
                if grid_prototype is None:
                    grid_prototype = data_var
                if (not force and (hasattr(data_var.metpy, 'latitude')
                                   or hasattr(data_var.metpy, 'longitude'))):
                    raise RuntimeError('Latitude/longitude coordinate(s) are present. If you '
                                       'wish to overwrite these, specify force=True.')

        # Calculate latitude and longitude from grid_prototype, if it exists, and assign
        if grid_prototype is None:
            _warnings.warn('No latitude and longitude assigned since horizontal coordinates '
                           'were not found')
            return self._dataset
        else:
            latitude, longitude = _build_latitude_longitude(grid_prototype)
            return self._dataset.assign_coords(latitude=latitude, longitude=longitude)

    def assign_y_x(self, force=False, tolerance=None):
        """Assign y and x dimension coordinates derived from 2D latitude and longitude.

        Parameters
        ----------
        force : bool, optional
            If force is true, overwrite y and x coordinates if they exist, otherwise, raise a
            RuntimeError if such coordinates exist.
        tolerance : `pint.Quantity`
            Maximum range tolerated when collapsing projected y and x coordinates from 2D to
            1D. Defaults to 1 meter.

        Returns
        -------
        `xarray.Dataset`
            New xarray Dataset with y and x dimension coordinates assigned to all variables
            with valid latitude and longitude coordinates.

        Notes
        -----
        A valid CRS coordinate must be present (as assigned by ``.parse_cf`` or
        ``.assign_crs``). PyProj is used for the coordinate transformations.

        """
        # Determine if there is a valid grid prototype from which to compute the coordinates,
        # while also checking for existing y and x coords
        grid_prototype = None
        for data_var in self._dataset.data_vars.values():
            if hasattr(data_var.metpy, 'latitude') and hasattr(data_var.metpy, 'longitude'):
                if grid_prototype is None:
                    grid_prototype = data_var
                if (not force and (hasattr(data_var.metpy, 'y')
                                   or hasattr(data_var.metpy, 'x'))):
                    raise RuntimeError('y/x coordinate(s) are present. If you wish to '
                                       'overwrite these, specify force=True.')

        # Calculate y and x from grid_prototype, if it exists, and assign
        if grid_prototype is None:
            _warnings.warn('No y and x coordinates assigned since horizontal coordinates '
                           'were not found')
            return self._dataset
        else:
            y, x = _build_y_x(grid_prototype, tolerance)
            return self._dataset.assign_coords(**{y.name: y, x.name: x})

    def update_attribute(self, attribute, mapping):
        """Return new Dataset with specified attribute updated on all Dataset variables.

        Parameters
        ----------
        attribute : str,
            Name of attribute to update
        mapping : dict or Callable
            Either a dict, with keys as variable names and values as attribute values to set,
            or a callable, which must accept one positional argument (variable name) and
            arbitrary keyword arguments (all existing variable attributes). If a variable name
            is not present/the callable returns None, the attribute will not be updated.

        Returns
        -------
        `xarray.Dataset`
            New Dataset with attribute updated

        """
        # Make mapping uniform
        if not callable(mapping):
            old_mapping = mapping

            def mapping(varname, **kwargs):
                return old_mapping.get(varname)

        # Define mapping function for Dataset.map
        def mapping_func(da):
            new_value = mapping(da.name, **da.attrs)
            if new_value is None:
                return da
            else:
                return da.assign_attrs(**{attribute: new_value})

        # Apply across all variables and coordinates
        return (
            self._dataset
            .map(mapping_func)
            .assign_coords({
                coord_name: mapping_func(coord_var)
                for coord_name, coord_var in self._dataset.coords.items()
            })
        )

    def quantify(self):
        """Return new dataset with all numeric variables quantified and cached data loaded.

        Notes
        -----
        Any cached/lazy-loaded data (except that in a Dask array) will be loaded into memory
        by this operation. Do not utilize on moderate- to large-sized remote datasets before
        subsetting!
        """
        return self._dataset.map(lambda da: da.metpy.quantify()).assign_attrs(
            self._dataset.attrs
        )

    def dequantify(self):
        """Return new dataset with variables cast to magnitude and units on attribute."""
        return self._dataset.map(lambda da: da.metpy.dequantify()).assign_attrs(
            self._dataset.attrs
        )


def _assign_axis(attributes, axis):
    """Assign the given axis to the _metpy_axis attribute."""
    existing_axes = attributes.get('_metpy_axis', '').split(',')
    if ((axis == 'y' and 'latitude' in existing_axes)
            or (axis == 'latitude' and 'y' in existing_axes)):
        # Special case for combined y/latitude handling
        attributes['_metpy_axis'] = 'y,latitude'
    elif ((axis == 'x' and 'longitude' in existing_axes)
            or (axis == 'longitude' and 'x' in existing_axes)):
        # Special case for combined x/longitude handling
        attributes['_metpy_axis'] = 'x,longitude'
    else:
        # Simply add it/overwrite past value
        attributes['_metpy_axis'] = axis
    return attributes


def check_axis(var, *axes):
    """Check if the criteria for any of the given axes are satisfied.

    Parameters
    ----------
    var : `xarray.DataArray`
        DataArray belonging to the coordinate to be checked
    axes : str
        Axis type(s) to check for. Currently can check for 'time', 'vertical', 'y', 'latitude',
        'x', and 'longitude'.

    """
    for axis in axes:
        # Check for
        #   - standard name (CF option)
        #   - _CoordinateAxisType (from THREDDS)
        #   - axis (CF option)
        #   - positive (CF standard for non-pressure vertical coordinate)
        if any(var.attrs.get(criterion, 'absent')
               in coordinate_criteria[criterion].get(axis, set())
               for criterion in ('standard_name', '_CoordinateAxisType', 'axis', 'positive')):
            return True

        # Check for units, either by dimensionality or name
        with contextlib.suppress(UndefinedUnitError):
            if (axis in coordinate_criteria['units'] and (
                    (
                        coordinate_criteria['units'][axis]['match'] == 'dimensionality'
                        and (units.get_dimensionality(var.metpy.units)
                             == units.get_dimensionality(
                                 coordinate_criteria['units'][axis]['units']))
                    ) or (
                        coordinate_criteria['units'][axis]['match'] == 'name'
                        and str(var.metpy.units)
                        in coordinate_criteria['units'][axis]['units']
                    ))):
                return True

        # Check if name matches regular expression (non-CF failsafe)
        if coordinate_criteria['regular_expression'][axis].match(var.name.lower()):
            return True

    # If no match has been made, return False (rather than None)
    return False


def _assign_crs(xarray_object, cf_attributes, cf_kwargs):
    from .plots.mapping import CFProjection

    # Handle argument options
    if cf_attributes is not None and len(cf_kwargs) > 0:
        raise ValueError('Cannot specify both attribute dictionary and kwargs.')
    elif cf_attributes is None and len(cf_kwargs) == 0:
        raise ValueError('Must specify either attribute dictionary or kwargs.')
    attrs = cf_attributes if cf_attributes is not None else cf_kwargs

    # Assign crs coordinate to xarray object
    return xarray_object.assign_coords(metpy_crs=CFProjection(attrs))


def _build_latitude_longitude(da):
    """Build latitude/longitude coordinates from DataArray's y/x coordinates."""
    y, x = da.metpy.coordinates('y', 'x')
    xx, yy = np.meshgrid(x.values, y.values)
    lonlats = np.stack(da.metpy.pyproj_proj(xx, yy, inverse=True, radians=False), axis=-1)
    longitude = xr.DataArray(lonlats[..., 0], dims=(y.name, x.name),
                             coords={y.name: y, x.name: x},
                             attrs={'units': 'degrees_east', 'standard_name': 'longitude'})
    latitude = xr.DataArray(lonlats[..., 1], dims=(y.name, x.name),
                            coords={y.name: y, x.name: x},
                            attrs={'units': 'degrees_north', 'standard_name': 'latitude'})
    return latitude, longitude


def _build_y_x(da, tolerance):
    """Build y/x coordinates from DataArray's latitude/longitude coordinates."""
    # Initial sanity checks
    latitude, longitude = da.metpy.coordinates('latitude', 'longitude')
    if latitude.dims != longitude.dims:
        raise ValueError('Latitude and longitude must have same dimensionality')
    elif latitude.ndim != 2:
        raise ValueError('To build 1D y/x coordinates via assign_y_x, latitude/longitude '
                         'must be 2D')

    # Convert to projected y/x
    xxyy = np.stack(da.metpy.pyproj_proj(
        longitude.values,
        latitude.values,
        inverse=False,
        radians=False
    ), axis=-1)

    # Handle tolerance
    tolerance = 1 if tolerance is None else tolerance.m_as('m')

    # If within tolerance, take median to collapse to 1D
    try:
        y_dim = latitude.metpy.find_axis_number('y')
        x_dim = latitude.metpy.find_axis_number('x')
    except AttributeError:
        _warnings.warn('y and x dimensions unable to be identified. Assuming [..., y, x] '
                       'dimension order.')
        y_dim, x_dim = 0, 1
    if (np.all(np.ptp(xxyy[..., 0], axis=y_dim) < tolerance)
            and np.all(np.ptp(xxyy[..., 1], axis=x_dim) < tolerance)):
        x = np.median(xxyy[..., 0], axis=y_dim)
        y = np.median(xxyy[..., 1], axis=x_dim)
        x = xr.DataArray(x, name=latitude.dims[x_dim], dims=(latitude.dims[x_dim],),
                         coords={latitude.dims[x_dim]: x},
                         attrs={'units': 'meter', 'standard_name': 'projection_x_coordinate'})
        y = xr.DataArray(y, name=latitude.dims[y_dim], dims=(latitude.dims[y_dim],),
                         coords={latitude.dims[y_dim]: y},
                         attrs={'units': 'meter', 'standard_name': 'projection_y_coordinate'})
        return y, x
    else:
        raise ValueError('Projected y and x coordinates cannot be collapsed to 1D within '
                         'tolerance. Verify that your latitude and longitude coordinates '
                         'correspond to your CRS coordinate.')


def preprocess_and_wrap(broadcast=None, wrap_like=None, match_unit=False, to_magnitude=False):
    """Return decorator to wrap array calculations for type flexibility.

    Assuming you have a calculation that works internally with `pint.Quantity` or
    `numpy.ndarray`, this will wrap the function to be able to handle `xarray.DataArray` and
    `pint.Quantity` as well (assuming appropriate match to one of the input arguments).

    Parameters
    ----------
    broadcast : Sequence[str] or None
        Iterable of string labels for arguments to broadcast against each other using xarray,
        assuming they are supplied as `xarray.DataArray`. No automatic broadcasting will occur
        with default of None.
    wrap_like : str or array-like or tuple of str or tuple of array-like or None
        Wrap the calculation output following a particular input argument (if str) or data
        object (if array-like). If tuple, will assume output is in the form of a tuple,
        and wrap iteratively according to the str or array-like contained within. If None,
        will not wrap output.
    match_unit : bool
        If true, force the unit of the final output to be that of wrapping object (as
        determined by wrap_like), no matter the original calculation output. Defaults to
        False.
    to_magnitude : bool
        If true, downcast xarray and Pint arguments to their magnitude. If false, downcast
        xarray arguments to Quantity, and do not change other array-like arguments.
    """
    def decorator(func):
        sig = signature(func)
        if broadcast is not None:
            for arg_name in broadcast:
                if arg_name not in sig.parameters:
                    raise ValueError(
                        f'Cannot broadcast argument {arg_name} as it is not in function '
                        'signature'
                    )

        @functools.wraps(func)
        def wrapper(*args, **kwargs):
            bound_args = sig.bind(*args, **kwargs)

            # Auto-broadcast select xarray arguments, and update bound_args
            if broadcast is not None:
                arg_names_to_broadcast = tuple(
                    arg_name for arg_name in broadcast
                    if arg_name in bound_args.arguments
                    and isinstance(
                        bound_args.arguments[arg_name],
                        (xr.DataArray, xr.Variable)
                    )
                )
                broadcasted_args = xr.broadcast(
                    *(bound_args.arguments[arg_name] for arg_name in arg_names_to_broadcast)
                )
                for i, arg_name in enumerate(arg_names_to_broadcast):
                    bound_args.arguments[arg_name] = broadcasted_args[i]

            # Cast all Variables to their data and warn
            # (need to do before match finding, since we don't want to rewrap as Variable)
            def cast_variables(arg, arg_name):
                _warnings.warn(
                    f'Argument {arg_name} given as xarray Variable...casting to its data. '
                    'xarray DataArrays are recommended instead.'
                )
                return arg.data
            _mutate_arguments(bound_args, xr.Variable, cast_variables)

            # Obtain proper match if referencing an input
            match = list(wrap_like) if isinstance(wrap_like, tuple) else wrap_like
            if isinstance(wrap_like, str):
                match = bound_args.arguments[wrap_like]
            elif isinstance(wrap_like, tuple):
                for i, arg in enumerate(wrap_like):
                    if isinstance(arg, str):
                        match[i] = bound_args.arguments[arg]
                match = tuple(match)

            # Cast all DataArrays to Pint Quantities
            _mutate_arguments(bound_args, xr.DataArray, lambda arg, _: arg.metpy.unit_array)

            # Optionally cast all Quantities to their magnitudes
            if to_magnitude:
                _mutate_arguments(bound_args, units.Quantity, lambda arg, _: arg.m)

            # Evaluate inner calculation
            result = func(*bound_args.args, **bound_args.kwargs)

            # Wrap output based on match and match_unit
            if match is None:
                return result
            else:
                if match_unit:
                    wrapping = _wrap_output_like_matching_units
                else:
                    wrapping = _wrap_output_like_not_matching_units

                if isinstance(match, tuple):
                    return tuple(wrapping(*args) for args in zip(result, match))
                else:
                    return wrapping(result, match)
        return wrapper
    return decorator


def _wrap_output_like_matching_units(result, match):
    """Convert result to be like match with matching units for output wrapper."""
    output_xarray = isinstance(match, xr.DataArray)
    match_units = str(match.metpy.units if output_xarray else getattr(match, 'units', ''))

    if isinstance(result, xr.DataArray):
        result = result.metpy.convert_units(match_units)
        return result if output_xarray else result.metpy.unit_array
    else:
        result = (
            result.to(match_units) if is_quantity(result)
            else units.Quantity(result, match_units)
        )
        return (
            xr.DataArray(result, coords=match.coords, dims=match.dims) if output_xarray
            else result
        )


def _wrap_output_like_not_matching_units(result, match):
    """Convert result to be like match without matching units for output wrapper."""
    output_xarray = isinstance(match, xr.DataArray)
    if isinstance(result, xr.DataArray):
        return result if output_xarray else result.metpy.unit_array
    # Determine if need to upcast to Quantity
    if (
        not is_quantity(result) and (
            is_quantity(match) or (output_xarray and is_quantity(match.data))
        )
    ):
        result = units.Quantity(result)
    return (
        xr.DataArray(result, coords=match.coords, dims=match.dims)
        if output_xarray and result is not None
        else result
    )


def check_matching_coordinates(func):
    """Decorate a function to make sure all given DataArrays have matching coordinates."""
    @functools.wraps(func)
    def wrapper(*args, **kwargs):
        data_arrays = ([a for a in args if isinstance(a, xr.DataArray)]
                       + [a for a in kwargs.values() if isinstance(a, xr.DataArray)])
        if len(data_arrays) > 1:
            first = data_arrays[0]
            for other in data_arrays[1:]:
                if not first.metpy.coordinates_identical(other):
                    raise ValueError('Input DataArray arguments must be on same coordinates.')
        return func(*args, **kwargs)
    return wrapper


def _reassign_quantity_indexer(data, indexers):
    """Reassign a units.Quantity indexer to units of relevant coordinate."""
    def _to_magnitude(val, unit):
        try:
            return val.m_as(unit)
        except AttributeError:
            return val

    # Update indexers keys for axis type -> coord name replacement
    indexers = {(key if not isinstance(data, xr.DataArray) or key in data.dims
                 or key not in metpy_axes else
                 next(data.metpy.coordinates(key)).name): indexers[key]
                for key in indexers}

    # Update indexers to handle quantities and slices of quantities
    reassigned_indexers = {}
    for coord_name in indexers:
        coord_units = data[coord_name].metpy.units
        if isinstance(indexers[coord_name], slice):
            # Handle slices of quantities
            start = _to_magnitude(indexers[coord_name].start, coord_units)
            stop = _to_magnitude(indexers[coord_name].stop, coord_units)
            step = _to_magnitude(indexers[coord_name].step, coord_units)
            reassigned_indexers[coord_name] = slice(start, stop, step)
        else:
            # Handle quantities
            reassigned_indexers[coord_name] = _to_magnitude(indexers[coord_name], coord_units)

    return reassigned_indexers


def grid_deltas_from_dataarray(f, kind='default'):
    """Calculate the horizontal deltas between grid points of a DataArray.

    Calculate the signed delta distance between grid points of a DataArray in the horizontal
    directions, using actual (real distance) or nominal (in projection space) deltas.

    Parameters
    ----------
    f : `xarray.DataArray`
        Parsed DataArray (``metpy_crs`` coordinate must be available for kind="actual")
    kind : str
        Type of grid delta to calculate. "actual" returns true distances as calculated from
        longitude and latitude via `lat_lon_grid_deltas`. "nominal" returns horizontal
        differences in the data's coordinate space, either in degrees (for lat/lon CRS) or
        meters (for y/x CRS). "default" behaves like "actual" for datasets with a lat/lon CRS
        and like "nominal" for all others. Defaults to "default".

    Returns
    -------
    dx, dy:
        arrays of signed deltas between grid points in the x and y directions with dimensions
        matching those of `f`.

    See Also
    --------
    `~metpy.calc.lat_lon_grid_deltas`

    """
    from metpy.calc import lat_lon_grid_deltas

    # Determine behavior
    if (
        kind == 'default'
        and (
            not hasattr(f.metpy, 'crs')
            or f.metpy.crs['grid_mapping_name'] == 'latitude_longitude'
        )
    ):
        # Use actual grid deltas by default with latitude_longitude or missing CRS
        kind = 'actual'
    elif kind == 'default':
        # Otherwise, use grid deltas in projected grid space by default
        kind = 'nominal'
    elif kind not in ('actual', 'nominal'):
        raise ValueError('"kind" argument must be specified as "default", "actual", or '
                         '"nominal"')

    if kind == 'actual':
        # Get latitude/longitude coordinates and find dim order
        latitude, longitude = xr.broadcast(*f.metpy.coordinates('latitude', 'longitude'))
        try:
            y_dim = latitude.metpy.find_axis_number('y')
            x_dim = latitude.metpy.find_axis_number('x')
        except AttributeError:
            _warnings.warn('y and x dimensions unable to be identified. Assuming [..., y, x] '
                           'dimension order.')
            y_dim, x_dim = -2, -1

        # Get geod if it exists, otherwise fall back to PyProj default
        try:
            geod = f.metpy.pyproj_crs.get_geod()
        except AttributeError:
            geod = CRS.from_cf({'grid_mapping_name': 'latitude_longitude'}).get_geod()
        # Obtain grid deltas as xarray Variables
        (dx_var, dx_units), (dy_var, dy_units) = (
            (xr.Variable(dims=latitude.dims, data=deltas.magnitude), deltas.units)
            for deltas in lat_lon_grid_deltas(longitude, latitude, x_dim=x_dim, y_dim=y_dim,
                                              geod=geod))
    else:
        # Obtain y/x coordinate differences
        y, x = f.metpy.coordinates('y', 'x')
        dx_var = x.diff(x.dims[0]).variable
        dx_units = units(x.attrs.get('units'))
        dy_var = y.diff(y.dims[0]).variable
        dy_units = units(y.attrs.get('units'))

    # Broadcast to input and attach units
    dx_var = dx_var.set_dims(f.dims, shape=[dx_var.sizes[dim] if dim in dx_var.dims else 1
                                            for dim in f.dims])
    dx = units.Quantity(dx_var.data, dx_units)
    dy_var = dy_var.set_dims(f.dims, shape=[dy_var.sizes[dim] if dim in dy_var.dims else 1
                                            for dim in f.dims])
    dy = units.Quantity(dy_var.data, dy_units)

    return dx, dy


def dataarray_arguments(bound_args):
    """Get any dataarray arguments in the bound function arguments."""
    for value in chain(bound_args.args, bound_args.kwargs.values()):
        if isinstance(value, xr.DataArray):
            yield value


def add_vertical_dim_from_xarray(func):
    """Fill in optional vertical_dim from DataArray argument."""
    @functools.wraps(func)
    def wrapper(*args, **kwargs):
        bound_args = signature(func).bind(*args, **kwargs)
        bound_args.apply_defaults()

        # Fill in vertical_dim
        if 'vertical_dim' in bound_args.arguments:
            a = next(dataarray_arguments(bound_args), None)
            if a is not None:
                try:
                    bound_args.arguments['vertical_dim'] = a.metpy.find_axis_number('vertical')
                except AttributeError:
                    # If axis number not found, fall back to default but warn.
                    _warnings.warn(
                        'Vertical dimension number not found. Defaulting to initial dimension.'
                    )

        return func(*bound_args.args, **bound_args.kwargs)
    return wrapper