Update reference in test.

Add pandas test.

timely_beliefs/tests/test_belief_io.py

@@ -395,11 +395,11 @@ def test_agg_resampling_retains_metadata(resolution):
 
 
 def test_groupby_retains_metadata():
-    """ Test whether grouping by index level retains the metadata.
+    """Test whether grouping by index level retains the metadata.
 
     Succeeds with pandas==1.0.0
     Fails with pandas==1.1.0
-    Should be fixed with https://github.com/pandas-dev/pandas/pull/35688
+    Should be fixed with https://github.com/pandas-dev/pandas/pull/37461
     """
     df = example_df
     metadata = {md: getattr(example_df, md) for md in METADATA}
@@ -415,3 +415,13 @@ def assert_function(x):
     assert isinstance(df, tb.BeliefsDataFrame)
     for md in metadata:
         assert getattr(df, md) == metadata[md]
+
+
+def test_groupby_retains_attribute():
+    df = pd.DataFrame([[1, 2], [3, 4]], columns=["x", "y"])
+    df.a = "b"
+    assert df.a == "b"
+    df = df.groupby("x").apply(lambda x: x)
+    assert df.a == "b"
+    df = df.groupby("x").sum()
+    assert df.a == "b"