feat(task): modify default behaviour for task compression argument.

The expected behaviour of the compression argument was to overwrite
compression args if a dict with those args was provided, to disable
compression if something which evaluates to False was provided, and use
the default parameters for the container otherwise. The behaviour
previously would instead disable compression if no args were provided.

draco/core/task.py

@@ -259,10 +259,11 @@ class SingleTask(MPILoggedTask, pipeline.BasicContMixin):
     compression : dict or bool, optional
         Set compression options for each dataset. Provided as a dict with the dataset
         names as keys and values for `chunks`, `compression`, and `compression_opts`.
-        If set to `False`, chunks and compression will be disabled for all datasets.
-        Otherwise, the default parameters set in the dataset spec are used.
-        Note that this will modify these parameters on the container itself, such that
-        if it is written out again downstream in the pipeline these will be used.
+        If set to `False` (or anything that evaluates to `False`, such as an empty dict),
+        chunks and compression will be disabled for all datasets. Otherwise, the default
+        parameters set in the dataset spec are used. Note that this will modify these
+        parameters on the container itself, such that if it is written out again downstream
+        in the pipeline these will be used.
     output_root : string
         Pipeline settable parameter giving the first part of the output path.
         Deprecated in favour of `output_name`.
@@ -291,7 +292,7 @@ class SingleTask(MPILoggedTask, pipeline.BasicContMixin):
     output_name = config.Property(default="{output_root}{tag}.h5", proptype=str)
     output_format = config.file_format()
     compression = config.Property(
-        default={}, proptype=lambda x: x if isinstance(x, dict) else bool(x)
+        default=True, proptype=lambda x: x if isinstance(x, dict) else bool(x)
     )
 
     nan_check = config.Property(default=True, proptype=bool)
@@ -445,11 +446,13 @@ def walk_dset_tree(grp, root=""):
             return datasets
 
         if not self.compression:
+            # Disable compression
             for ds in walk_dset_tree(output):
                 output._data._storage_root[ds].chunks = None
                 output._data._storage_root[ds].compression = None
                 output._data._storage_root[ds].compression_opts = None
-        else:
+        elif isinstance(self.compression, dict):
+            # We want to overwrite some compression settings
             datasets = walk_dset_tree(output)
             for ds in self.compression:
                 if ds in datasets:
@@ -459,7 +462,10 @@ def walk_dset_tree(grp, root=""):
                         )
                         setattr(output._data._storage_root[ds], key, val)
                     # shorthand for bitshuffle
-                    if output[ds].compression in ("bitshuffle", fileformats.H5FILTER):
+                    if output[ds].compression in (
+                        "bitshuffle",
+                        fileformats.H5FILTER,
+                    ):
                         output[ds].compression = fileformats.H5FILTER
                         if output[ds].compression_opts is None:
                             output._data._storage_root[ds].compression_opts = (