# HG changeset patch
# User Aurélien Campéas <aurelien.campeas@pythonian.fr>
# Date 1718971171 -7200
#      Fri Jun 21 13:59:31 2024 +0200
# Node ID 27d9a6dfc334e7f5cb4424bbc2e39cae1fd9db0a
# Parent  f4bf4aaf68e94bed5d5a9b1f1f5dcbf66f335fe5
test/util/diff: exhibit issue with diff computing involving nans

diff --git a/test/test_util.py b/test/test_util.py
--- a/test/test_util.py
+++ b/test/test_util.py
@@ -277,6 +277,62 @@
 """, ds2s1)
 
 
+def test_diff_nans():
+    s1 = pd.Series(
+        [1, np.nan, 3],
+        index=pd.date_range(datetime(2024, 1, 1), freq='h', periods=3)
+    )
+    s2 = pd.Series(
+        [1, np.nan, 3, 4],
+        index=pd.date_range(datetime(2024, 1, 1), freq='h', periods=4)
+    )
+    d = diff(s1, s2)
+    # looks good
+    assert_df("""
+2024-01-01 03:00:00    4.0
+""", d)
+
+    s3 = pd.Series(
+        [1, np.nan, np.nan, 4],
+        index=pd.date_range(datetime(2024, 1, 1), freq='h', periods=4)
+    )
+    d = diff(s2, s3)
+    # still good
+    assert_df("""
+2024-01-01 02:00:00   NaN
+""", d)
+
+    n1 = pd.Series(
+        [np.nan],
+        index=pd.date_range(datetime(2024, 1, 1), freq='h', periods=1)
+    )
+    n2 = pd.Series(
+        [np.nan, np.nan, 3],
+        index=pd.date_range(datetime(2024, 1, 1), freq='h', periods=3)
+    )
+    d = diff(n1, n2)
+    # oops !
+    assert_df("""
+2024-01-01 00:00:00    NaN
+2024-01-01 01:00:00    NaN
+2024-01-01 02:00:00    3.0
+""", d)
+
+
+    n1 = pd.Series(
+        [np.nan, 2, 3],
+        index=pd.date_range(datetime(2024, 1, 1), freq='h', periods=3)
+    )
+    n1 = n1.drop(n1.index[1])  # erase the '2' point
+    n2 = pd.Series(
+        [np.nan, np.nan, 3],
+        index=pd.date_range(datetime(2024, 1, 1), freq='h', periods=3)
+    )
+    d = diff(n1, n2)
+    # oops !
+    assert len(d) == 0
+
+
 def test_diff_duplicated():
     # with a duplicated row (left)
     s1 = pd.Series(