BUG: raise when doing arithmetic on DataFrame and list of iterables

doc/source/whatsnew/v1.2.0.rst

@@ -371,6 +371,7 @@ Numeric
 - Bug in :meth:`DataFrame.diff` with ``datetime64`` dtypes including ``NaT`` values failing to fill ``NaT`` results correctly (:issue:`32441`)
 - Bug in :class:`DataFrame` arithmetic ops incorrectly accepting keyword arguments (:issue:`36843`)
 - Bug in :class:`IntervalArray` comparisons with :class:`Series` not returning :class:`Series` (:issue:`36908`)
+- Bug in :class:`DataFrame` allowing arithmetic operations with list of array-likes with undefined results. Behavior changed to raising ``ValueError`` (:issue:`36702`)
 
 Conversion
 ^^^^^^^^^^

pandas/core/ops/__init__.py

@@ -13,7 +13,7 @@
 from pandas._typing import Level
 from pandas.util._decorators import Appender
 
-from pandas.core.dtypes.common import is_list_like
+from pandas.core.dtypes.common import is_array_like, is_list_like
 from pandas.core.dtypes.generic import ABCDataFrame, ABCIndexClass, ABCSeries
 from pandas.core.dtypes.missing import isna
 
@@ -311,6 +311,11 @@ def to_series(right):
             )
 
     elif is_list_like(right) and not isinstance(right, (ABCSeries, ABCDataFrame)):
+        # GH 36702. Raise when attempting arithmetic with list of array-like.
+        if any(is_array_like(el) for el in right):
+            raise ValueError(
+                f"Unable to coerce list of {type(right[0])} to Series/DataFrame"
+            )
         # GH17901
         right = to_series(right)
 

pandas/tests/frame/test_arithmetic.py

@@ -1577,3 +1577,17 @@ def test_arith_reindex_with_duplicates():
     result = df1 + df2
     expected = pd.DataFrame([[np.nan, 0, 0]], columns=["first", "second", "second"])
     tm.assert_frame_equal(result, expected)
+
+
+@pytest.mark.parametrize("arg1", [pd.DataFrame({"x": [1, 2], "y": [1, 2]})])
+@pytest.mark.parametrize(
+    "arg2", [[pd.Series([1, 1])], [pd.Series([1, 1]), pd.Series([1, 1])]]
+)
+def test_arith_list_of_arraylike_raise(arg1, arg2):
+    # GH 36702. Raise when trying to add list of array-like to DataFrame
+
+    msg = f"Unable to coerce list of {type(arg2[0])} to Series/DataFrame"
+    with pytest.raises(ValueError, match=msg):
+        arg1 + arg2
+    with pytest.raises(ValueError, match=msg):
+        arg2 + arg1