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Refactor parse_cf to not modify coordinates inplace #1285

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Jan 12, 2020
Merged

Refactor parse_cf to not modify coordinates inplace #1285

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78c642c
Refactor parse_cf to not modify coordinates inplace
jthielen Jan 10, 2020
01e28e0
Change call to assign_coords to work on old xarray
dopplershift Jan 12, 2020
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86 changes: 49 additions & 37 deletions src/metpy/xarray.py
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Original file line number Diff line number Diff line change
Expand Up @@ -141,6 +141,7 @@ def unit_array(self, values):
def convert_units(self, units):
"""Convert the data values to different units in-place."""
self.unit_array = self.unit_array.to(units)
return self._data_array # allow method chaining

@property
def crs(self):
Expand Down Expand Up @@ -190,6 +191,8 @@ def assign_coordinates(self, coordinates):
for coord_var in self._data_array.coords.values():
coord_var.attrs.pop('_metpy_axis', None)

return self._data_array # allow method chaining

def _generate_coordinate_map(self):
"""Generate a coordinate map via CF conventions and other methods."""
coords = self._data_array.coords.values()
Expand Down Expand Up @@ -445,7 +448,15 @@ def parse_cf(self, varname=None, coordinates=None):
return subset

var = self._dataset[varname]

# Assign coordinates if the coordinates argument is given
if coordinates is not None:
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]
Expand All @@ -454,45 +465,46 @@ def parse_cf(self, varname=None, coordinates=None):
'Could not find variable corresponding to the value of '
'grid_mapping: {}'.format(proj_name))
else:
var.coords['crs'] = CFProjection(proj_var.attrs)

self._fixup_coords(var)

# Trying to guess whether we should be adding a crs to this variable's coordinates
# First make sure it's missing CRS but isn't lat/lon itself
if not check_axis(var, 'latitude', 'longitude') and 'crs' not in var.coords:
# Look for both lat/lon in the coordinates
has_lat = has_lon = False
for coord_var in var.coords.values():
has_lat = has_lat or check_axis(coord_var, 'latitude')
has_lon = has_lon or check_axis(coord_var, 'longitude')

# If we found them, create a lat/lon projection as the crs coord
if has_lat and has_lon:
var.coords['crs'] = CFProjection({'grid_mapping_name': 'latitude_longitude'})
log.warning('Found latitude/longitude values, assuming latitude_longitude '
'for projection grid_mapping variable')

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

return var

def _fixup_coords(self, var):
crs = CFProjection(proj_var.attrs)

if crs is None and not check_axis(var, 'latitude', 'longitude'):
# 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 coordinates
# for latitude and longitude, even if they are not the dimension coordinates of
# the variable.
def _has_coord(coord_type):
return any(check_axis(coord_var, coord_type)
for coord_var in var.coords.values())
if _has_coord('latitude') and _has_coord('longitude'):
crs = CFProjection({'grid_mapping_name': 'latitude_longitude'})
log.warning('Found valid latitude/longitude coordinates, assuming '
'latitude_longitude for projection grid_mapping variable')

# Rebuild the coordinates of the dataarray, and return
coords = dict(self._rebuild_coords(var, crs))
if crs is not None:
coords['crs'] = crs
return var.assign_coords(coords)

def _rebuild_coords(self, var, crs):
"""Clean up the units on the coordinate variables."""
for coord_name, data_array in var.coords.items():
if (check_axis(data_array, 'x', 'y')
and not check_axis(data_array, 'longitude', 'latitude')):
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.coords[coord_name].metpy.convert_units('meters')
except DimensionalityError: # Radians!
if 'crs' in var.coords:
new_data_array = data_array.copy()
height = var.coords['crs'].item()['perspective_point_height']
scaled_vals = new_data_array.metpy.unit_array * (height * units.meters)
new_data_array.metpy.unit_array = scaled_vals.to('meters')
var.coords[coord_name] = new_data_array
# Cannot modify an index inplace, so use copy
yield coord_name, coord_var.copy().metpy.convert_units('meters')
except DimensionalityError:
# Radians! Attempt to use perspective point height conversion
if crs is not None:
new_coord_var = coord_var.copy()
height = crs['perspective_point_height']
scaled_vals = new_coord_var.metpy.unit_array * (height * units.meters)
new_coord_var.metpy.unit_array = scaled_vals.to('meters')
yield coord_name, new_coord_var
else:
# Do nothing
yield coord_name, coord_var

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