Add CHELSA-W5E5 tile compression. (#102)

* Compress tile files with nccopy.

* Document changes in whatsnew.

doc/whatsnew.rst

@@ -33,7 +33,7 @@ New features
   (aliased ``'cw5e5'``) as a new temperature and precipitation data source in
   :func:`hyoga.open.atmosphere` (:issue:`86`, :pull:`87`), aggregated on 30x30
   degree tiles (:issue:`88`, :pull:`92`), concatenated yearly (:issue:`99`,
-  :pull:`100`).
+  :pull:`100`), and compressed with netCDF-4 (:issue:`101`, :pull:`102`).
 
 .. _CHELSA-W5E5: https://chelsa-climate.org/chelsa-w5e5-v1-0-daily-climate-data-at-1km-resolution/
 
@@ -46,7 +46,7 @@ Internal changes
 ~~~~~~~~~~~~~~~~
 
 - Add aggregators in :mod:`hyoga.open.aggregator` (:issue:`86`, :issue:`88`,
-  :issue:`99`, :pull:`87`, :pull:`92`, :pull:`100`).
+  :issue:`99`, :issue:`101`, :pull:`87`, :pull:`92`, :pull:`100`, :pull:`102`).
 
 .. _v0.3.1:
 

hyoga/open/aggregator.py

@@ -9,6 +9,8 @@
 """
 
 import os.path
+import subprocess
+import tempfile
 import xarray as xr
 import hyoga.open.downloader
 
@@ -171,9 +173,17 @@ def aggregate(self, inputs, output, recipe='avg'):
                 tile = tile.groupby('time.month')
                 tile = getattr(tile, recipe)(keep_attrs=True)
 
-                # store output as netcdf and return path
+                # store as uncompressed netcdf
                 print(f"aggregating {tilepath} ...")
                 tile.to_netcdf(tilepath)
 
+                # nccopy compression beats xarray by far
+                print(f"compressing {tilepath} ...")
+                dirname, basename = os.path.split(tilepath)
+                with tempfile.NamedTemporaryFile(
+                        dir=dirname, prefix=basename+'.') as tmp:
+                    subprocess.run(['nccopy', '-sd6', tilepath, tmp.name])
+                    os.replace(tmp.name, tilepath)
+
         # return multi-tile output pattern
         return output