fix dangerous default arguments (#4006)

Co-authored-by: Keewis <keewis@posteo.de>

Author: pnijhara

xarray/tests/test_backends.py

@@ -87,6 +87,7 @@
     dask_version = "10.0"
 
 ON_WINDOWS = sys.platform == "win32"
+default_value = object()
 
 
 def open_example_dataset(name, *args, **kwargs):
@@ -3630,11 +3631,21 @@ def create_tmp_geotiff(
     ny=3,
     nz=3,
     transform=None,
-    transform_args=[5000, 80000, 1000, 2000.0],
-    crs={"units": "m", "no_defs": True, "ellps": "WGS84", "proj": "utm", "zone": 18},
+    transform_args=default_value,
+    crs=default_value,
     open_kwargs=None,
     additional_attrs=None,
 ):
+    if transform_args is default_value:
+        transform_args = [5000, 80000, 1000, 2000.0]
+    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
     from rasterio.transform import from_origin

xarray/tests/test_conventions.py

@@ -363,8 +363,12 @@ def create_store(self):
 
     @contextlib.contextmanager
     def roundtrip(
-        self, data, save_kwargs={}, open_kwargs={}, allow_cleanup_failure=False
+        self, data, save_kwargs=None, open_kwargs=None, allow_cleanup_failure=False
     ):
+        if save_kwargs is None:
+            save_kwargs = {}
+        if open_kwargs is None:
+            open_kwargs = {}
         store = CFEncodedInMemoryStore()
         data.dump_to_store(store, **save_kwargs)
         yield open_dataset(store, **open_kwargs)