Set default value as object() instead of None

xarray/tests/test_backends.py

@@ -86,6 +86,7 @@
     dask_version = "10.0"
 
 ON_WINDOWS = sys.platform == "win32"
+default_value = object()
 
 
 def open_example_dataset(name, *args, **kwargs):
@@ -3604,14 +3605,14 @@ def create_tmp_geotiff(
     ny=3,
     nz=3,
     transform=None,
-    transform_args=None,
-    crs=None,
+    transform_args=default_value,
+    crs=default_value,
     open_kwargs=None,
     additional_attrs=None,
 ):
-    if transform_args is None:
+    if transform_args is default_value:
         transform_args = [5000, 80000, 1000, 2000.0]
-    if crs is None:
+    if crs is default_value:
         crs = {"units": "m", "no_defs": True, "ellps": "WGS84", "proj": "utm", "zone": 18}
     # yields a temporary geotiff file and a corresponding expected DataArray
     import rasterio