Add some CF support for lon and lat

xesmf/cf.py

@@ -0,0 +1,158 @@
+"""
+Decode longitude and latitude of an `xarray.Dataset` or a `xarray.DataArray`
+according to CF conventions.
+"""
+
+import warnings
+
+CF_SPECS = {
+    'lon': {
+        'name': ['lon', 'longitude'],
+        'units': ['degrees_east', 'degree_east', 'degree_E', 'degrees_E',
+                  'degreeE', 'degreesE'],
+        'standard_name': ['longitude'],
+        'axis': 'X'
+        },
+    'lat': {
+        'name': ['lat', 'latitude'],
+        'units': ['degrees_north', 'degree_north', 'degree_N', 'degrees_N',
+                  'degreeN', 'degreesN'],
+        'standard_name': ['latitude'],
+        'axis': 'Y'
+        }
+    }
+
+
+def get_coord_name_from_specs(ds, specs):
+    '''
+    Get the name of a `xarray.DataArray` according to search specifications
+
+    Parameters
+    ----------
+    ds: xarray.DataArray or xarray.Dataset
+    specs: dict
+
+    Returns
+    -------
+    str: Name of the xarray.DataArray
+    '''
+    # Targets
+    ds_names = []
+    if hasattr(ds, 'coords'):
+        ds_names.extend(list(ds.coords))
+    if hasattr(ds, 'variables'):
+        ds_names.extend(list(ds.variables))
+
+    # Search in names first
+    if 'name' in specs:
+        for specs_name in specs['name']:
+            if specs_name in ds_names:
+                return specs_name
+
+    # Search in units attributes
+    if 'units' in specs:
+        for ds_name in ds_names:
+            if hasattr(ds[ds_name], 'units'):
+                for specs_units in specs['units']:
+                    if ds[ds_name].units == specs_units:
+                        return ds_name
+
+    # Search in standard_name attributes
+    if 'standard_name' in specs:
+        for ds_name in ds_names:
+            if hasattr(ds[ds_name], 'standard_name'):
+                for specs_sn in specs['standard_name']:
+                    if (ds[ds_name].standard_name == specs_sn or
+                            ds[ds_name].standard_name.startswith(
+                                specs_sn+'_at_')):
+                        return ds_name
+
+    # Search in axis attributes
+    if 'axis' in specs:
+        for ds_name in ds_names:
+            if (hasattr(ds[ds_name], 'axis') and
+                    ds[ds_name].axis.lower() == specs['axis'].lower()):
+                return ds_name
+
+
+def get_lon_name(ds):
+    '''
+    Get the longitude name in a `xarray.Dataset` or a `xarray.DataArray`
+
+    Parameters
+    ----------
+    ds: xarray.DataArray or xarray.Dataset
+
+    Returns
+    -------
+    str: Name of the xarray.DataArray
+    '''
+    lon_name = get_coord_name_from_specs(ds, CF_SPECS['lon'])
+    if lon_name is not None:
+        return lon_name
+    warnings.warn('longitude not found in dataset')
+
+
+def get_lat_name(ds):
+    '''
+    Get the latitude name in a `xarray.Dataset` or a `xarray.DataArray`
+
+    Parameters
+    ----------
+    ds: xarray.DataArray or xarray.Dataset
+
+    Returns
+    -------
+    str: Name of the xarray.DataArray
+    '''
+    lat_name = get_coord_name_from_specs(ds, CF_SPECS['lat'])
+    if lat_name is not None:
+        return lat_name
+    warnings.warn('latitude not found in dataset')
+
+
+def decode_cf(ds, mapping=None):
+    '''
+    Search for longitude and latitude and rename them
+
+    Parameters
+    ----------
+    ds: xarray.DataArray or xarray.Dataset
+    mapping: None or dict
+        When a `dict` is provided, it is filled with keys that are the new
+        names and values that are the old names, so that the output dataset
+        can have its coordinates be renamed back with
+        :meth:`~xarray.Dataset.rename`.
+
+    Returns
+    -------
+    ds: xarray.DataArray or xarray.Dataset
+    '''
+    # Suffixes for bounds
+    bsuffixes = ('_b', '_bounds', '_bnds')
+
+    # Longitude
+    lon_name = get_lon_name(ds)
+    if lon_name is not None and lon_name != 'lon':
+        ds = ds.rename({lon_name: 'lon'})
+        if isinstance(mapping, dict):
+            mapping['lon'] = lon_name
+        for suffix in bsuffixes:
+            if lon_name+suffix in ds:
+                ds = ds.rename({lon_name+suffix: 'lon_b'})
+                if isinstance(mapping, dict):
+                    mapping['lon_b'] = lon_name + suffix
+
+    # Latitude
+    lat_name = get_lat_name(ds)
+    if lat_name is not None and lat_name != 'lat':
+        ds = ds.rename({lat_name: 'lat'})
+        if isinstance(mapping, dict):
+            mapping['lat'] = lat_name
+        for suffix in bsuffixes:
+            if lat_name+suffix in ds:
+                ds = ds.rename({lat_name+suffix: 'lat_b'})
+                if isinstance(mapping, dict):
+                    mapping['lat_b'] = lat_name + suffix
+
+    return ds

xesmf/frontend.py

@@ -10,6 +10,8 @@
 from . backend import (esmf_grid, add_corner,
                        esmf_regrid_build, esmf_regrid_finalize)
 
+from .cf import decode_cf as decode_cf_
+
 from . smm import read_weights, apply_weights
 
 try:
@@ -30,7 +32,8 @@ def as_2d_mesh(lon, lat):
     return lon, lat
 
 
-def ds_to_ESMFgrid(ds, need_bounds=False, periodic=None, append=None):
+def ds_to_ESMFgrid(ds, need_bounds=False, periodic=None, append=None,
+                   decode_cf=True):
     '''
     Convert xarray DataSet or dictionary to ESMF.Grid object.
 
@@ -49,12 +52,22 @@ def ds_to_ESMFgrid(ds, need_bounds=False, periodic=None, append=None):
     periodic : bool, optional
         Periodic in longitude?
 
+    decode_cf: bool, dict, optional
+        Search for lon and lat according to CF conventions and rename them.
+        When a dict is provided, it is filled so it can be used to rename
+        lon and lat back.
+
     Returns
     -------
     grid : ESMF.Grid object
 
     '''
 
+    # rename lon and lat?
+    if decode_cf or isinstance(decode_cf, dict):
+        mapping = decode_cf if isinstance(decode_cf, dict) else None
+        ds = decode_cf_(ds, mapping)
+
     # use np.asarray(dr) instead of dr.values, so it also works for dictionary
     lon = np.asarray(ds['lon'])
     lat = np.asarray(ds['lat'])
@@ -74,7 +87,8 @@ def ds_to_ESMFgrid(ds, need_bounds=False, periodic=None, append=None):
 
 class Regridder(object):
     def __init__(self, ds_in, ds_out, method, periodic=False,
-                 filename=None, reuse_weights=False, ignore_degenerate=None):
+                 filename=None, reuse_weights=False, ignore_degenerate=None,
+                 decode_cf=True):
         """
         Make xESMF regridder
 
@@ -118,6 +132,10 @@ def __init__(self, ds_in, ds_out, method, periodic=False,
             If False (default), raise error if grids contain degenerated cells
             (i.e. triangles or lines, instead of quadrilaterals)
 
+
+        decode_cf: bool, optional
+            Search for lon and lat according to CF conventions and rename them
+
         Returns
         -------
         regridder : xESMF regridder object
@@ -139,12 +157,16 @@ def __init__(self, ds_in, ds_out, method, periodic=False,
         # construct ESMF grid, with some shape checking
         self._grid_in, shape_in = ds_to_ESMFgrid(ds_in,
                                                  need_bounds=self.need_bounds,
-                                                 periodic=periodic
+                                                 periodic=periodic,
+                                                 decode_cf=decode_cf
                                                  )
+        self._cf_mapping = {}
+        if decode_cf:
+            ds_out = decode_cf_(ds_out, self._cf_mapping)
         self._grid_out, shape_out = ds_to_ESMFgrid(ds_out,
-                                                   need_bounds=self.need_bounds
+                                                   need_bounds=self.need_bounds,
+                                                   decode_cf=False,
                                                    )
-
         # record output grid and metadata
         self._lon_out = np.asarray(ds_out['lon'])
         self._lat_out = np.asarray(ds_out['lat'])
@@ -367,6 +389,10 @@ def regrid_dataarray(self, dr_in, keep_attrs=False):
         dr_out.coords['lon'] = xr.DataArray(self._lon_out, dims=self.lon_dim)
         dr_out.coords['lat'] = xr.DataArray(self._lat_out, dims=self.lat_dim)
 
+        # rename coords back to original output grid
+        if self._cf_mapping:
+            dr_out = dr_out.rename(self._cf_mapping)
+
         dr_out.attrs['regrid_method'] = self.method
 
         return dr_out
@@ -414,6 +440,10 @@ def regrid_dataset(self, ds_in, keep_attrs=False):
         ds_out.coords['lon'] = xr.DataArray(self._lon_out, dims=self.lon_dim)
         ds_out.coords['lat'] = xr.DataArray(self._lat_out, dims=self.lat_dim)
 
+        # rename coords back to original output grid
+        if self._cf_mapping:
+            ds_out = ds_out.rename(self._cf_mapping)
+
         ds_out.attrs['regrid_method'] = self.method
 
         return ds_out

xesmf/tests/test_cf.py

@@ -0,0 +1,63 @@
+import pytest
+import numpy as np
+import xarray as xr
+import xesmf.cf as xecf
+
+
+coord_base = xr.DataArray(np.arange(2), dims='d')
+da = xr.DataArray(np.ones(2), dims='d', name='temp')
+
+
+def test_cf_get_lon_name_warns():
+    with pytest.warns(UserWarning):
+        xecf.get_lon_name(da.to_dataset())
+
+
+@pytest.mark.parametrize("name,coord_specs", [
+    ('lon', coord_base),
+    ('longitude', coord_base),
+    ('x', ('d', coord_base, {'units': 'degrees_east'})),
+    ('x', ('d', coord_base, {'standard_name': 'longitude'})),
+    ('xu', ('d', coord_base, {'standard_name': 'longitude_at_U_location'})),
+    ('x', ('d', coord_base, {'axis': 'X'})),
+    ('x', ('d', coord_base, {'units': 'degrees_east', 'axis': 'Y'})),
+    ])
+def test_cf_get_lon_name(name, coord_specs):
+    assert xecf.get_lon_name(da.assign_coords(**{name: coord_specs})) == name
+
+
+def test_cf_get_lat_name_warns():
+    with pytest.warns(UserWarning):
+        xecf.get_lat_name(da.to_dataset())
+
+
+@pytest.mark.parametrize("name,coord_specs", [
+    ('lat', coord_base),
+    ('latitude', coord_base),
+    ('y', ('d', coord_base, {'units': 'degrees_north'})),
+    ('y', ('d', coord_base, {'standard_name': 'latitude'})),
+    ('yu', ('d', coord_base, {'standard_name': 'latitude_at_V_location'})),
+    ('y', ('d', coord_base, {'axis': 'Y'})),
+    ('y', ('d', coord_base, {'units': 'degrees_north', 'axis': 'X'})),
+    ])
+def test_cf_get_lat_name(name, coord_specs):
+    assert xecf.get_lat_name(da.assign_coords(**{name: coord_specs})) == name
+
+
+def test_cf_decode_cf():
+
+    yy, xx = np.mgrid[:3, :4].astype('d')
+    yyb, xxb = np.mgrid[:4, :5] - .5
+
+    xx = xr.DataArray(xx, dims=['ny', 'nx'], attrs={'units': "degrees_east"})
+    xxb = xr.DataArray(xxb, dims=['nyb', 'nxb'])
+    yy = xr.DataArray(yy, dims=['ny', 'nx'])
+    yyb = xr.DataArray(yyb, dims=['nyb', 'nxb'])
+
+    ds = xr.Dataset({
+        'xx': xx,
+        'xx_b': xxb,
+        'latitude': yy,
+        'latitude_bounds': yyb})
+    ds_decoded = xecf.decode_cf(ds)
+    assert sorted(list(ds_decoded)) == ['lat', 'lat_b', 'lon', 'lon_b']

xesmf/tests/test_frontend.py

@@ -260,3 +260,29 @@ def test_regrid_dataset():
 
     # clean-up
     regridder.clean_weight_file()
+
+
+def test_regrid_dataset_renamed_back():
+
+    # renaming
+    ds_out2 = ds_out.rename(lon='X', lat='Y', lon_b='X_b', lat_b='Y_bnds')
+    ds_out2.X.attrs['units'] = 'degrees_east'
+    ds_out2.Y.attrs['standard_name'] = 'latitude'
+    print(ds_out2.X.units)
+
+    regridder = xe.Regridder(ds_in, ds_out2, 'conservative')
+
+    ds_result = regridder(ds_in)
+
+    # compare with analytical solution
+    rel_err = (ds_out2['data_ref'] - ds_result['data'])/ds_out2['data_ref']
+    assert np.max(np.abs(rel_err)) < 0.05
+
+    # check that name are restored
+    assert 'X' in ds_result.coords
+    assert 'Y' in ds_result.coords
+    assert 'X_b' in ds_result.variables
+    assert 'Y_bnds' in ds_result.variables
+
+    # clean-up
+    regridder.clean_weight_file()