ENH: Improve error reporting for wrong merge cols

doc/source/whatsnew/v1.2.0.rst

@@ -231,6 +231,7 @@ Other enhancements
 - :class:`Window` now supports all Scipy window types in ``win_type`` with flexible keyword argument support (:issue:`34556`)
 - :meth:`testing.assert_index_equal` now has a ``check_order`` parameter that allows indexes to be checked in an order-insensitive manner (:issue:`37478`)
 - :func:`read_csv` supports memory-mapping for compressed files (:issue:`37621`)
+- Improve error reporting for :meth:`DataFrame.merge()` when invalid merge column definitions were given (:issue:`16228`)
 
 .. _whatsnew_120.api_breaking.python:
 

pandas/core/reshape/merge.py

@@ -1203,11 +1203,9 @@ def _validate_specification(self):
             if self.left_index and self.right_index:
                 self.left_on, self.right_on = (), ()
             elif self.left_index:
-                if self.right_on is None:
-                    raise MergeError("Must pass right_on or right_index=True")
+                raise MergeError("Must pass right_on or right_index=True")
             elif self.right_index:
-                if self.left_on is None:
-                    raise MergeError("Must pass left_on or left_index=True")
+                raise MergeError("Must pass left_on or left_index=True")
             else:
                 # use the common columns
                 common_cols = self.left.columns.intersection(self.right.columns)
@@ -1228,8 +1226,19 @@ def _validate_specification(self):
                     'Can only pass argument "on" OR "left_on" '
                     'and "right_on", not a combination of both.'
                 )
+            if self.left_index or self.right_index:
+                raise MergeError(
+                    'Can only pass argument "on" OR "left_index" '
+                    'and "right_index", not a combination of both.'
+                )
             self.left_on = self.right_on = self.on
         elif self.left_on is not None:
+            if self.left_index:
+                raise MergeError(
+                    'Can only pass argument "left_on" OR "left_index" not both.'
+                )
+            if not self.right_index and self.right_on is None:
+                raise MergeError('Must pass "right_on" OR "right_index".')
             n = len(self.left_on)
             if self.right_index:
                 if len(self.left_on) != self.right.index.nlevels:
@@ -1239,6 +1248,12 @@ def _validate_specification(self):
                     )
                 self.right_on = [None] * n
         elif self.right_on is not None:
+            if self.right_index:
+                raise MergeError(
+                    'Can only pass argument "right_on" OR "right_index" not both.'
+                )
+            if not self.left_index and self.left_on is None:
+                raise MergeError('Must pass "left_on" OR "left_index".')
             n = len(self.right_on)
             if self.left_index:
                 if len(self.right_on) != self.left.index.nlevels:

pandas/tests/reshape/merge/test_merge.py

@@ -2283,3 +2283,57 @@ def test_merge_join_categorical_multiindex():
     expected = expected.drop(["Cat", "Int"], axis=1)
     result = a.join(b, on=["Cat1", "Int1"])
     tm.assert_frame_equal(expected, result)
+
+
+@pytest.mark.parametrize("func", ["merge", "merge_asof"])
+@pytest.mark.parametrize(
+    ("kwargs", "err_msg"),
+    [
+        ({"left_on": "a", "left_index": True}, ["left_on", "left_index"]),
+        ({"right_on": "a", "right_index": True}, ["right_on", "right_index"]),
+    ],
+)
+def test_merge_join_cols_error_reporting_duplicates(func, kwargs, err_msg):
+    # GH: 16228
+    left = DataFrame({"a": [1, 2], "b": [3, 4]})
+    right = DataFrame({"a": [1, 1], "c": [5, 6]})
+    msg = rf'Can only pass argument "{err_msg[0]}" OR "{err_msg[1]}" not both\.'
+    with pytest.raises(MergeError, match=msg):
+        getattr(pd, func)(left, right, **kwargs)
+
+
+@pytest.mark.parametrize("func", ["merge", "merge_asof"])
+@pytest.mark.parametrize(
+    ("kwargs", "err_msg"),
+    [
+        ({"left_on": "a"}, ["right_on", "right_index"]),
+        ({"right_on": "a"}, ["left_on", "left_index"]),
+    ],
+)
+def test_merge_join_cols_error_reporting_missing(func, kwargs, err_msg):
+    # GH: 16228
+    left = DataFrame({"a": [1, 2], "b": [3, 4]})
+    right = DataFrame({"a": [1, 1], "c": [5, 6]})
+    msg = rf'Must pass "{err_msg[0]}" OR "{err_msg[1]}"\.'
+    with pytest.raises(MergeError, match=msg):
+        getattr(pd, func)(left, right, **kwargs)
+
+
+@pytest.mark.parametrize("func", ["merge", "merge_asof"])
+@pytest.mark.parametrize(
+    "kwargs",
+    [
+        {"right_index": True},
+        {"left_index": True},
+    ],
+)
+def test_merge_join_cols_error_reporting_on_and_index(func, kwargs):
+    # GH: 16228
+    left = DataFrame({"a": [1, 2], "b": [3, 4]})
+    right = DataFrame({"a": [1, 1], "c": [5, 6]})
+    msg = (
+        r'Can only pass argument "on" OR "left_index" '
+        r'and "right_index", not a combination of both\.'
+    )
+    with pytest.raises(MergeError, match=msg):
+        getattr(pd, func)(left, right, on="a", **kwargs)