I get the example code from this website: https://unidata.github.io/MetPy/latest/examples/cross_section.html#sphx-glr-examples-cross-section-py
I wonder why the example programs can't work on my computer with either xarray-0.15.0 or with xarray-0.15.1.
the original code is below
import cartopy.crs as ccrs
import cartopy.feature as cfeature
import matplotlib.pyplot as plt
import numpy as np
import xarray as xr
import metpy.calc as mpcalc
from metpy.cbook import get_test_data
from metpy.interpolate import cross_section
data = xr.open_dataset(get_test_data('narr_example.nc', False))
data = data.metpy.parse_cf().squeeze()
print(data)
start = (37.0, -105.0)
end = (35.5, -65.0)
cross = cross_section(data, start, end).set_coords(('lat', 'lon'))
print(cross)
temperature, pressure, specific_humidity = xr.broadcast(cross['Temperature'],
cross['isobaric'],
cross['Specific_humidity'])
theta = mpcalc.potential_temperature(pressure, temperature)
rh = mpcalc.relative_humidity_from_specific_humidity(pressure, temperature, specific_humidity)
# These calculations return unit arrays, so put those back into DataArrays in our Dataset
cross['Potential_temperature'] = xr.DataArray(theta,
coords=temperature.coords,
dims=temperature.dims,
attrs={'units': theta.units})
cross['Relative_humidity'] = xr.DataArray(rh,
coords=specific_humidity.coords,
dims=specific_humidity.dims,
attrs={'units': rh.units})
cross['u_wind'].metpy.convert_units('knots')
cross['v_wind'].metpy.convert_units('knots')
cross['t_wind'], cross['n_wind'] = mpcalc.cross_section_components(cross['u_wind'],
cross['v_wind'])
print(cross)
the error information is below (with xarray-0.15.1)
---------------------------------------------------------------------------
ValueError Traceback (most recent call last)
<ipython-input-3-506a7aabdc3e> in <module>
1 data = xr.open_dataset(get_test_data('narr_example.nc', False))
----> 2 data = data.metpy.parse_cf().squeeze()
3 print(data)
/opt/anaconda3/lib/python3.7/site-packages/metpy/xarray.py in parse_cf(self, varname, coordinates)
502 # If non-string iterable is given, apply recursively across the varnames
503 subset = xr.merge([self.parse_cf(single_varname, coordinates=coordinates)
--> 504 for single_varname in varname])
505 subset.attrs = self._dataset.attrs
506 return subset
/opt/anaconda3/lib/python3.7/site-packages/metpy/xarray.py in <listcomp>(.0)
502 # If non-string iterable is given, apply recursively across the varnames
503 subset = xr.merge([self.parse_cf(single_varname, coordinates=coordinates)
--> 504 for single_varname in varname])
505 subset.attrs = self._dataset.attrs
506 return subset
/opt/anaconda3/lib/python3.7/site-packages/metpy/xarray.py in parse_cf(self, varname, coordinates)
518 var.coords['crs'] = CFProjection(proj_var.attrs)
519
--> 520 self._fixup_coords(var)
521
522 # Trying to guess whether we should be adding a crs to this variable's coordinates
/opt/anaconda3/lib/python3.7/site-packages/metpy/xarray.py in _fixup_coords(self, var)
547 and not check_axis(data_array, 'longitude', 'latitude')):
548 try:
--> 549 var.coords[coord_name].metpy.convert_units('meters')
550 except DimensionalityError: # Radians!
551 if 'crs' in var.coords:
/opt/anaconda3/lib/python3.7/site-packages/metpy/xarray.py in convert_units(self, units)
143 def convert_units(self, units):
144 """Convert the data values to different units in-place."""
--> 145 self.unit_array = self.unit_array.to(units)
146
147 @property
/opt/anaconda3/lib/python3.7/site-packages/metpy/xarray.py in unit_array(self, values)
138 def unit_array(self, values):
139 """Set data values from a `pint.Quantity`."""
--> 140 self._data_array.values = values.magnitude
141 self._units = self._data_array.attrs['units'] = str(values.units)
142
/opt/anaconda3/lib/python3.7/site-packages/xarray/core/common.py in __setattr__(self, name, value)
260 """
261 try:
--> 262 object.__setattr__(self, name, value)
263 except AttributeError as e:
264 # Don't accidentally shadow custom AttributeErrors, e.g.
/opt/anaconda3/lib/python3.7/site-packages/xarray/core/dataarray.py in values(self, value)
560 @values.setter
561 def values(self, value: Any) -> None:
--> 562 self.variable.values = value
563
564 @property
/opt/anaconda3/lib/python3.7/site-packages/xarray/core/variable.py in values(self, values)
2113 def values(self, values):
2114 raise ValueError(
-> 2115 f"Cannot assign to the .values attribute of dimension coordinate a.k.a IndexVariable {self.name!r}. "
2116 f"Please use DataArray.assign_coords, Dataset.assign_coords or Dataset.assign as appropriate."
2117 )
ValueError: Cannot assign to the .values attribute of dimension coordinate a.k.a IndexVariable 'y'. Please use DataArray.assign_coords, Dataset.assign_coords or Dataset.assign as appropriate.
the error information is below (with xarray-0.15.0)
Traceback (most recent call last):
File "cross_section.py", line 57, in <module>
rh = mpcalc.relative_humidity_from_specific_humidity(pressure, temperature, specific_humidity)
File "/opt/anaconda3/lib/python3.7/site-packages/metpy/xarray.py", line 655, in wrapper
return func(*args, **kwargs)
File "/opt/anaconda3/lib/python3.7/site-packages/metpy/units.py", line 319, in wrapper
raise ValueError(msg)
ValueError: `relative_humidity_from_specific_humidity` given arguments with incorrect units: `specific_humidity` requires "[dimensionless]" but given "hectopascal", `pressure` requires "[pressure]" but given "dimensionless".
The problem with xarray 0.15.1 is a known issue that should be fixed in the next release.
The error you get with 0.15.0 is certainly confusing. The problem is that the example code you're using is from the docs, which were pointing to the docs for 1.0, which is currently in release candidate status--which means you're most likely not running it, but MetPy 0.12. The problem is that MetPy 1.0 changes the order of arguments to the relative_humidity_from_specific_humidity. So the example code you downloaded wasn't appropriate for the version of MetPy you're running.
I have updated the MetPy docs to point the latest docs to the 0.12 release docs. You should now be able to download the correct example code for your version.