cftime 1.3.0 has value error that 1.2.0 doesn not

[guidov]

When updating to cftime=1.3.0, xarray plot throws a value error now. (downgrading to 1.2.0 doesn not have the same problem)

datetime variable looks like
array(cftime.DatetimeNoLeap(2114, 12, 16, 0, 0, 0, 0), dtype=object)

---

ValueError Traceback (most recent call last)
~/anaconda3/lib/python3.8/site-packages/ipykernel/pylab/backend_inline.py in show(close, block)
41 display(
42 figure_manager.canvas.figure,
---> 43 metadata=_fetch_figure_metadata(figure_manager.canvas.figure)
44 )
45 finally:

~/anaconda3/lib/python3.8/site-packages/ipykernel/pylab/backend_inline.py in _fetch_figure_metadata(fig)
178 if _is_transparent(fig.get_facecolor()):
179 # the background is transparent
--> 180 ticksLight = _is_light([label.get_color()
181 for axes in fig.axes
182 for axis in (axes.xaxis, axes.yaxis)

~/anaconda3/lib/python3.8/site-packages/ipykernel/pylab/backend_inline.py in (.0)
181 for axes in fig.axes
182 for axis in (axes.xaxis, axes.yaxis)
--> 183 for label in axis.get_ticklabels()])
184 if ticksLight.size and (ticksLight == ticksLight[0]).all():
185 # there are one or more tick labels, all with the same lightness

~/anaconda3/lib/python3.8/site-packages/matplotlib/axis.py in get_ticklabels(self, minor, which)
1318 if minor:
1319 return self.get_minorticklabels()
-> 1320 return self.get_majorticklabels()
1321
1322 def get_majorticklines(self):

~/anaconda3/lib/python3.8/site-packages/matplotlib/axis.py in get_majorticklabels(self)
1274 def get_majorticklabels(self):
1275 'Return a list of Text instances for the major ticklabels.'
-> 1276 ticks = self.get_major_ticks()
1277 labels1 = [tick.label1 for tick in ticks if tick.label1.get_visible()]
1278 labels2 = [tick.label2 for tick in ticks if tick.label2.get_visible()]

~/anaconda3/lib/python3.8/site-packages/matplotlib/axis.py in get_major_ticks(self, numticks)
1429 'Get the tick instances; grow as necessary.'
1430 if numticks is None:
-> 1431 numticks = len(self.get_majorticklocs())
1432
1433 while len(self.majorTicks) < numticks:

~/anaconda3/lib/python3.8/site-packages/matplotlib/axis.py in get_majorticklocs(self)
1346 def get_majorticklocs(self):
1347 """Get the array of major tick locations in data coordinates."""
-> 1348 return self.major.locator()
1349
1350 def get_minorticklocs(self):

~/anaconda3/lib/python3.8/site-packages/nc_time_axis/init.py in call(self)
136 def call(self):
137 vmin, vmax = self.axis.get_view_interval()
--> 138 return self.tick_values(vmin, vmax)
139
140 def tick_values(self, vmin, vmax):

~/anaconda3/lib/python3.8/site-packages/nc_time_axis/init.py in tick_values(self, vmin, vmax)
154 # appropriate.
155 years = self._max_n_locator.tick_values(lower.year, upper.year)
--> 156 ticks = [cftime.datetime(int(year), 1, 1) for year in years]
157 elif resolution == 'MONTHLY':
158 # TODO START AT THE BEGINNING OF A DECADE/CENTURY/MILLENIUM as

~/anaconda3/lib/python3.8/site-packages/nc_time_axis/init.py in (.0)
154 # appropriate.
155 years = self._max_n_locator.tick_values(lower.year, upper.year)
--> 156 ticks = [cftime.datetime(int(year), 1, 1) for year in years]
157 elif resolution == 'MONTHLY':
158 # TODO START AT THE BEGINNING OF A DECADE/CENTURY/MILLENIUM as

cftime/_cftime.pyx in cftime._cftime.datetime.init()

cftime/_cftime.pyx in cftime._cftime.assert_valid_date()

ValueError: invalid year provided in cftime.datetime(0, 1, 1, 0, 0, 0, 0, calendar='gregorian')

[spencerkclark]

This looks to be related to #202, which caused the cftime.datetime constructor to create calendar-aware datetimes (with a Gregorian calendar by default). @jswhit started on a PR in nc-time-axis to fix plotting issues related to this, SciTools/nc-time-axis#51, but I think some additional changes will be required there (e.g. a calendar argument will need to be added here).

[jswhit]

You don't say what code actually triggers this error, but as @spencerkclark suggested it looks to be coming from matplotlib, or the nc-time-axis toolkit. That code appears to trying to create a calendar instance with year zero for a calendar in which there is no year zero. cftime-1.3.0 checks for this, but previous versions did not.

[guidov]

Hi , Sorry I didn't provide more info.

Here is a test notebook that throws the error , only for a particular time sequence (the third plot throws the error which is odd)

The netcdf file can be downloaded here:
https://www.dropbox.com/s/j3zw2d9hs5zxzbg/cftime_test_MOC.nc?dl=0

####################################
import numpy as np
import matplotlib.pyplot as plt
import xarray as xr
import cftime
import nc_time_axis
import datetime

cftime.version
cftime_file = xr.open_dataset("cftime_test_MOC.nc", engine='netcdf4')
cftime_file.time
cftime_file.MOC.isel(transport_reg=0, moc_comp=2,moc_z=20,lat_aux_grid=70).isel(time=slice(0,4)).plot()
cftime_file.MOC.isel(transport_reg=0, moc_comp=2,moc_z=20,lat_aux_grid=70).isel(time=slice(1,5)).plot()
cftime_file.MOC.isel(transport_reg=0, moc_comp=2,moc_z=20,lat_aux_grid=70).isel(time=slice(0,5)).plot()

Thanks

[jswhit]

this confirms that the problem is related to SciTools/nc-time-axis#51. nc-time-axis needs be updated to work with cftime >= 1.3.0