Add tests for expanding

Author: mroeschke

streamz/dataframe/tests/test_dataframes.py

@@ -709,6 +709,33 @@ def test_windowing_n(func, n, getter):
     assert_eq(L[-1], func(getter(df).iloc[len(df) - n:] + 10))
 
 
+@pytest.mark.parametrize('func', [
+    lambda x: x.sum(),
+    lambda x: x.mean(),
+    lambda x: x.count(),
+    lambda x: x.size,
+    lambda x: x.var(ddof=1),
+    lambda x: x.std(ddof=1),
+    lambda x: x.var(ddof=0),
+])
+@pytest.mark.parametrize('getter', [
+    lambda df: df,
+    lambda df: df.x,
+])
+def test_expanding(func, getter):
+    df = pd.DataFrame({'x': [1.], 'y': [2.]})
+    sdf = DataFrame(example=df)
+
+    L = func(getter(sdf).expanding()).stream.gather().sink_to_list()
+
+    for i in range(5):
+        sdf.emit(df)
+
+    result = pd.concat(L, axis=1).T
+    expected = func(getter(pd.concat([df] * 5)).expanding())
+    assert_eq(result, expected)
+
+
 @pytest.mark.parametrize('func', [
     lambda x: x.sum(),
     lambda x: x.mean(),