FEAT-modin-project#2013: More accurate handling of broken edge cases …

…in Pandas.

Signed-off-by: Itamar Turner-Trauring <itamar@itamarst.org>

modin/pandas/general.py

@@ -161,20 +161,27 @@ def merge_asof(
 
     ErrorMessage.default_to_pandas("`merge_asof`")
 
+    # As of Pandas 1.2 these should raise an error; before that it did
+    # something likely random:
+    if (
+        (on and (left_index or right_index))
+        or (left_on and left_index)
+        or (right_on and right_index)
+    ):
+        raise ValueError("Can't combine left/right_index with left/right_on or on.")
+
     # Pandas fallbacks for tricky cases:
     if (
-        # No idea how this works or why it does what it does:
+        # No idea how this works or why it does what it does; and in fact
+        # there's a Pandas bug suggesting it's wrong:
+        # https://github.com/pandas-dev/pandas/issues/33463
         (left_index and right_on is not None)
         # This is the case where by is a list of columns. If we're copying lots
         # of columns out of Pandas, maybe not worth trying our path, it's not
         # clear it's any better:
         or not isinstance(by, (str, type(None)))
         or not isinstance(left_by, (str, type(None)))
         or not isinstance(right_by, (str, type(None)))
-        # What does this even mean?! Pandas does _something_...
-        or (on and (left_index or right_index))
-        or (left_on and left_index)
-        or (right_on and right_index)
     ):
         if isinstance(right, DataFrame):
             right = to_pandas(right)

modin/pandas/test/test_general.py

@@ -319,6 +319,20 @@ def test_merge_asof_bad_arguments():
     with pytest.raises(ValueError):
         pd.merge_asof(modin_left, modin_right, on="a", right_on="can't do with by")
 
+    # Can't mix left_index with left_on or on, similarly for right.
+    with pytest.raises(ValueError):
+        pd.merge_asof(modin_left, modin_right, on="a", right_index=True)
+    with pytest.raises(ValueError):
+        pd.merge_asof(
+            modin_left, modin_right, left_on="a", right_on="a", right_index=True
+        )
+    with pytest.raises(ValueError):
+        pd.merge_asof(modin_left, modin_right, on="a", left_index=True)
+    with pytest.raises(ValueError):
+        pd.merge_asof(
+            modin_left, modin_right, left_on="a", right_on="a", left_index=True
+        )
+
     # Need both left and right
     with pytest.raises(Exception):  # Pandas bug, didn't validate inputs sufficiently
         pandas.merge_asof(pandas_left, pandas_right, left_on="a")