Add handling of cftime bounds

cf_xarray/helpers.py

@@ -23,6 +23,11 @@ def _guess_bounds_1d(da, dim):
         ADDED_INDEX = True
 
     diff = da.diff(dim)
+    # Here we would need some escape based on a check of whether we were
+    # looking at cftime or not. It's not clear to me whether the fix should be
+    # here or further upstream though (either the casting shouldn't happen or
+    # numpy should be able to handle operations with cftime or we have some odd
+    # use case and need a workaround here).
     lower = da - diff / 2
     upper = da + diff / 2
     bounds = xr.concat([lower, upper], dim="bounds")

cf_xarray/tests/test_accessor.py

@@ -878,6 +878,40 @@ def test_add_bounds_nd_variable() -> None:
         ds.cf.add_bounds("z").cf.add_bounds("x")
 
 
+def test_add_bounds_cftime() -> None:
+    ds = airds.copy(deep=False)
+    # Switch to cftime objects
+    time, time_units, time_calendar = xr.coding.times.encode_cf_datetime(ds["time"])
+    time_cftime = xr.coding.times.decode_cf_datetime(
+        time,
+        units=time_units,
+        calendar=time_calendar,
+        use_cftime=True,
+    )
+
+    ds["time"] = ("time", time_cftime)
+
+    da_time = ds["time"]
+    # Resorting to loop as something casts things to numpy types which causes explosions
+    time_diffs = np.array(
+        [da_time.values[i + 1] - da_time.values[i] for i in range(len(da_time) - 1)]
+    )
+
+    # `1:` indexing to mimic xarray's diff behaviour which drops the first value
+    lower = da_time.values[1:] - time_diffs / 2
+    lower = np.concatenate([[lower[0] - time_diffs[0]], lower])
+    upper = da_time.values[1:] + time_diffs / 2
+    upper = np.concatenate([[upper[0] - time_diffs[0]], upper])
+
+    lower = xr.DataArray(lower, dims=["time"], coords=da_time.coords)
+    upper = xr.DataArray(upper, dims=["time"], coords=da_time.coords)
+    expected = xr.concat([lower, upper], dim="bounds").transpose(..., "bounds")
+
+    ds.cf.add_bounds("time")
+    actual = ds.cf.add_bounds("time").time_bounds.reset_coords(drop=True)
+    xr.testing.assert_identical(actual, expected)
+
+
 def test_bounds() -> None:
     ds = airds.copy(deep=False).cf.add_bounds("lat")
 
@@ -947,7 +981,7 @@ def test_bounds_to_vertices() -> None:
     with pytest.raises(ValueError):
         dsv = dsb.cf.bounds_to_vertices("T")
 
-    # Words on datetime arrays to
+    # Works on datetime arrays too
     dsb = dsb.cf.add_bounds("time")
     dsv = dsb.cf.bounds_to_vertices()
     assert "time_bounds" in dsv

doc/contributing.rst

@@ -11,7 +11,26 @@
 Contributing
 ------------
 
-This section will be expanded later. For now it lists docstrings for a number of internal variables, classes and functions.
+This section will be expanded later. For now it tells you how to get setup to
+run the tests and lists docstrings for a number of internal variables, classes
+and functions.
+
+Running the tests
+~~~~~~~~~~~~~~~~~
+
+The simplest way is using conda/mamba (same as in the CI). Create yourself a
+conda/mamba environment (e.g. ``mamba create -f ci/environment.yml``).
+Activate your environment. Next install the local version of ``cf-xarray``,
+``python -m pip install --no-deps -e .``. Now you are ready to run the tests
+with ``pytest``.
+
+Pre-commit
+~~~~~~~~~~
+
+If you want the pre-commit hook too, after installing your environment as
+above, simply install pre-commit (``pip install pre-commit``) and then run
+``pre-commit install``. Now each time you commit you'll get the pre-commit
+checks for free.
 
 Variables
 ~~~~~~~~~