Make cell_methods-derived bounds optional in getvar

This is particularly useful for ice data where we can get the time
bounds for averaging intervals. However, because we were attaching the
bounds as an xarray Dataset, we could't serialise a variable straight
back to disk, which was catching people out. Instead, we can make the
bounds optional, only if you want them. In this case, return a full
xarray Dataset, rather than just a DataArray.

Closes #284.

cosima_cookbook/querying.py

@@ -284,6 +284,7 @@ def getvar(
     frequency=None,
     attrs=None,
     attrs_unique=None,
+    return_dataset=False,
     **kwargs,
 ):
     """For a given experiment, return an xarray DataArray containing the
@@ -310,6 +311,10 @@ def getvar(
             must be unique on the returned variables. Defaults to
             {'cell_methods': 'time: mean'} and should not generally be
             changed.
+    return_dataset - if True, return xarray.Dataset, containing the
+                     requested variable, along with its time_bounds,
+                     if present.  Otherwise (default), return
+                     xarray.DataArray containing only the variable
 
     Note that if start_time and/or end_time are used, the time range
     of the resulting dataset may not be bounded exactly on those
@@ -339,8 +344,11 @@ def getvar(
         attrs_unique,
     )
 
-    # we know at least one variable was returned
-    variables = _bounds_vars_for_variable(*ncfiles[0])
+    variables = [variable]
+    if return_dataset:
+        # we know at least one variable was returned, so we can index ncfiles
+        # ask for the extra variables associated with cell_methods, etc.
+        variables += _bounds_vars_for_variable(*ncfiles[0])
 
     # chunking -- use first row/file and assume it's the same across the whole dataset
     xr_kwargs = {"chunks": _parse_chunks(ncfiles[0].NCVar)}
@@ -365,7 +373,11 @@ def _preprocess(d):
         **xr_kwargs,
     )
 
-    da = ds[variable]
+    if return_dataset:
+        da = ds
+    else:
+        # if we want a dataarray, we'll strip off the extra info
+        da = ds[variable]
 
     # Check the chunks given were actually in the data
     chunks = xr_kwargs.get("chunks", None)
@@ -376,9 +388,6 @@ def _preprocess(d):
                 f"chunking along dimensions {missing_chunk_dims} is not possible. Available dimensions for chunking are {set(da.dims)}"
             )
 
-    for attr in variables[1:]:
-        da.attrs[attr] = ds[attr]
-
     da.attrs["ncfiles"] = ncfiles
 
     # Get experiment metadata, delete extraneous fields and add
@@ -402,7 +411,7 @@ def _preprocess(d):
 def _bounds_vars_for_variable(ncfile, ncvar):
     """Return a list of names for a variable and its bounds"""
 
-    variables = [ncvar.varname]
+    variables = []
 
     if "cell_methods" not in ncvar.attrs:
         # no cell methods, so no need to look for bounds

test/test_querying.py

@@ -593,13 +593,15 @@ def test_disambiguation_by_frequency(session):
 
 
 def test_time_bounds_on_dataarray(session):
-    var_salt = cc.querying.getvar("querying", "salt", session, decode_times=False)
+    var_salt = cc.querying.getvar(
+        "querying", "salt", session, decode_times=False, return_dataset=True
+    )
 
     # we should have added time_bounds into the DataArray's attributes
-    assert "time_bounds" in var_salt.attrs
+    assert "time_bounds" in var_salt
 
     # and time_bounds should itself be a DataArray
-    assert isinstance(var_salt.attrs["time_bounds"], xr.DataArray)
+    assert isinstance(var_salt["time_bounds"], xr.DataArray)
 
 
 def test_query_with_attrs(session):