TST/REF: share shift tests (#44941)

Author: jbrockmendel

pandas/_testing/__init__.py

@@ -89,7 +89,10 @@
     assert_timedelta_array_equal,
     raise_assert_detail,
 )
-from pandas._testing.compat import get_dtype  # noqa:F401
+from pandas._testing.compat import (  # noqa:F401
+    get_dtype,
+    get_obj,
+)
 from pandas._testing.contexts import (  # noqa:F401
     RNGContext,
     decompress_file,

pandas/_testing/compat.py

@@ -11,3 +11,13 @@ def get_dtype(obj):
         return obj.dtypes.iat[0]
     else:
         return obj.dtype
+
+
+def get_obj(df: DataFrame, klass):
+    """
+    For sharing tests using frame_or_series, either return the DataFrame
+    unchanged or return it's first column as a Series.
+    """
+    if klass is DataFrame:
+        return df
+    return df._ixs(0, axis=1)