What happened:

Setting invalid units for time axis encoding results in an exception AttributeError: 'NoneType' object has no attribute 'groups'

What you expected to happen:

It should say "invalid time units", like this (see commented out line below) ValueError: invalid time units: days after 1/3/2000

Minimal Complete Verifiable Example:

```python

Put your MCVE code here

import pandas as pd import xarray as xr ds = xr.Dataset(data_vars={'v': (('t',), [0,])}, coords={'t': [pd.Timestamp(2000, 1, 1)]}) ds.t.encoding['units'] = 'days since Big Bang'

ds.t.encoding['units'] = 'days after 1/3/2000'

ds.to_netcdf('/tmp/x.nc', mode='w')

---

AttributeError Traceback (most recent call last) <ipython-input-16-0d97a797be64> in <module> 5 ds.t.encoding['units'] = 'days since Big Bang' 6 #ds.t.encoding['units'] = 'days after 1/3/2000' ----> 7 ds.to_netcdf('/tmp/x.nc', mode='w')

~/miniconda3/envs/xarray/lib/python3.9/site-packages/xarray-0.0.0-py3.9.egg/xarray/core/dataset.py in to_netcdf(self, path, mode, format, group, engine, encoding, unlimited_dims, compute, invalid_netcdf) 1752 from ..backends.api import to_netcdf 1753 -> 1754 return to_netcdf( 1755 self, 1756 path,

~/miniconda3/envs/xarray/lib/python3.9/site-packages/xarray-0.0.0-py3.9.egg/xarray/backends/api.py in to_netcdf(dataset, path_or_file, mode, format, group, engine, encoding, unlimited_dims, compute, multifile, invalid_netcdf) 1066 # TODO: allow this work (setting up the file for writing array data) 1067 # to be parallelized with dask -> 1068 dump_to_store( 1069 dataset, store, writer, encoding=encoding, unlimited_dims=unlimited_dims 1070 )

~/miniconda3/envs/xarray/lib/python3.9/site-packages/xarray-0.0.0-py3.9.egg/xarray/backends/api.py in dump_to_store(dataset, store, writer, encoder, encoding, unlimited_dims) 1113 variables, attrs = encoder(variables, attrs) 1114 -> 1115 store.store(variables, attrs, check_encoding, writer, unlimited_dims=unlimited_dims) 1116 1117

~/miniconda3/envs/xarray/lib/python3.9/site-packages/xarray-0.0.0-py3.9.egg/xarray/backends/common.py in store(self, variables, attributes, check_encoding_set, writer, unlimited_dims) 261 writer = ArrayWriter() 262 --> 263 variables, attributes = self.encode(variables, attributes) 264 265 self.set_attributes(attributes)

~/miniconda3/envs/xarray/lib/python3.9/site-packages/xarray-0.0.0-py3.9.egg/xarray/backends/common.py in encode(self, variables, attributes) 350 # All NetCDF files get CF encoded by default, without this attempting 351 # to write times, for example, would fail. --> 352 variables, attributes = cf_encoder(variables, attributes) 353 variables = {k: self.encode_variable(v) for k, v in variables.items()} 354 attributes = {k: self.encode_attribute(v) for k, v in attributes.items()}

~/miniconda3/envs/xarray/lib/python3.9/site-packages/xarray-0.0.0-py3.9.egg/xarray/conventions.py in cf_encoder(variables, attributes) 841 _update_bounds_encoding(variables) 842 --> 843 new_vars = {k: encode_cf_variable(v, name=k) for k, v in variables.items()} 844 845 # Remove attrs from bounds variables (issue #2921)

~/miniconda3/envs/xarray/lib/python3.9/site-packages/xarray-0.0.0-py3.9.egg/xarray/conventions.py in <dictcomp>(.0) 841 _update_bounds_encoding(variables) 842 --> 843 new_vars = {k: encode_cf_variable(v, name=k) for k, v in variables.items()} 844 845 # Remove attrs from bounds variables (issue #2921)

~/miniconda3/envs/xarray/lib/python3.9/site-packages/xarray-0.0.0-py3.9.egg/xarray/conventions.py in encode_cf_variable(var, needs_copy, name) 267 variables.UnsignedIntegerCoder(), 268 ]: --> 269 var = coder.encode(var, name=name) 270 271 # TODO(shoyer): convert all of these to use coders, too:

~/miniconda3/envs/xarray/lib/python3.9/site-packages/xarray-0.0.0-py3.9.egg/xarray/coding/times.py in encode(self, variable, name) 510 variable 511 ): --> 512 (data, units, calendar) = encode_cf_datetime( 513 data, encoding.pop("units", None), encoding.pop("calendar", None) 514 )

~/miniconda3/envs/xarray/lib/python3.9/site-packages/xarray-0.0.0-py3.9.egg/xarray/coding/times.py in encode_cf_datetime(dates, units, calendar) 448 units = infer_datetime_units(dates) 449 else: --> 450 units = _cleanup_netcdf_time_units(units) 451 452 if calendar is None:

~/miniconda3/envs/xarray/lib/python3.9/site-packages/xarray-0.0.0-py3.9.egg/xarray/coding/times.py in _cleanup_netcdf_time_units(units) 399 400 def _cleanup_netcdf_time_units(units): --> 401 delta, ref_date = _unpack_netcdf_time_units(units) 402 try: 403 units = "{} since {}".format(delta, format_timestamp(ref_date))

~/miniconda3/envs/xarray/lib/python3.9/site-packages/xarray-0.0.0-py3.9.egg/xarray/coding/times.py in _unpack_netcdf_time_units(units) 130 131 delta_units, ref_date = [s.strip() for s in matches.groups()] --> 132 ref_date = _ensure_padded_year(ref_date) 133 134 return delta_units, ref_date

~/miniconda3/envs/xarray/lib/python3.9/site-packages/xarray-0.0.0-py3.9.egg/xarray/coding/times.py in _ensure_padded_year(ref_date) 105 # appropriately 106 matches_start_digits = re.match(r"(\d+)(.*)", ref_date) --> 107 ref_year, everything_else = [s for s in matches_start_digits.groups()] 108 ref_date_padded = "{:04d}{}".format(int(ref_year), everything_else) 109

AttributeError: 'NoneType' object has no attribute 'groups' ``` Anything else we need to know?: Are there detail specifications for the valid units string? Setting it to 1/3/2000 surprised me (my locale: LANG=en_CA.UTF-8). Environment: Latest code from github: xarray version 0.0.0

Output of <tt>xr.show_versions()</tt>

INSTALLED VERSIONS ------------------ commit: None python: 3.9.2 | packaged by conda-forge | (default, Feb 21 2021, 05:02:46) [GCC 9.3.0] python-bits: 64 OS: Linux OS-release: 5.10.11-200.fc33.x86_64 machine: x86_64 processor: x86_64 byteorder: little LC_ALL: None LANG: en_CA.UTF-8 LOCALE: en_CA.UTF-8 libhdf5: 1.10.6 libnetcdf: 4.8.0 xarray: 0.0.0 pandas: 1.2.4 numpy: 1.20.2 scipy: None netCDF4: 1.5.6 pydap: None h5netcdf: None h5py: None Nio: None zarr: 2.7.1 cftime: 1.4.1 nc_time_axis: None PseudoNetCDF: None rasterio: None cfgrib: None iris: None bottleneck: None dask: 2021.04.0 distributed: 2021.04.0 matplotlib: None cartopy: None seaborn: None numbagg: None pint: None setuptools: 49.6.0.post20210108 pip: 21.0.1 conda: None pytest: None IPython: 7.22.0 sphinx: None