BUG: Join with a list of a single element behaves as a join with a single element (pandas-dev#57676)

Author: Dacops

doc/source/whatsnew/v3.0.0.rst

@@ -334,6 +334,7 @@ Bug fixes
 - Fixed bug in :meth:`.DataFrameGroupBy.median` where nat values gave an incorrect result. (:issue:`57926`)
 - Fixed bug in :meth:`DataFrame.cumsum` which was raising ``IndexError`` if dtype is ``timedelta64[ns]`` (:issue:`57956`)
 - Fixed bug in :meth:`DataFrame.join` inconsistently setting result index name (:issue:`55815`)
+- Fixed bug in :meth:`DataFrame.join` join with a list of a single element should behave as a join with a single element (:issue:`57676`)
 - Fixed bug in :meth:`DataFrame.to_string` that raised ``StopIteration`` with nested DataFrames. (:issue:`16098`)
 - Fixed bug in :meth:`DataFrame.transform` that was returning the wrong order unless the index was monotonically increasing. (:issue:`57069`)
 - Fixed bug in :meth:`DataFrame.update` bool dtype being converted to object (:issue:`55509`)

pandas/core/frame.py

@@ -10587,6 +10587,9 @@ def join(
         from pandas.core.reshape.concat import concat
         from pandas.core.reshape.merge import merge
 
+        if isinstance(other, list) and len(other) == 1:
+            other = other[0]
+
         if isinstance(other, Series):
             if other.name is None:
                 raise ValueError("Other Series must have a name")

pandas/tests/frame/methods/test_join.py

@@ -107,8 +107,6 @@ def test_suffix_on_list_join():
 
     # check proper errors are raised
     msg = "Suffixes not supported when joining multiple DataFrames"
-    with pytest.raises(ValueError, match=msg):
-        first.join([second], lsuffix="y")
     with pytest.raises(ValueError, match=msg):
         first.join([second, third], rsuffix="x")
     with pytest.raises(ValueError, match=msg):
@@ -562,3 +560,21 @@ def test_frame_join_tzaware(self):
 
         tm.assert_index_equal(result.index, expected)
         assert result.index.tz.zone == "US/Central"
+
+    def test_join_list_with_single_element(self):
+        test1 = DataFrame(
+            {"cat": pd.Categorical(["a", "v", "d"])},
+            index=Index(["a", "b", "c"], name="y"),
+        )
+        test2 = DataFrame(
+            {"foo": np.arange(6)},
+            index=MultiIndex.from_tuples(
+                [(0, "a"), (0, "b"), (0, "c"), (1, "a"), (1, "b"), (1, "c")],
+                names=("x", "y"),
+            ),
+        )
+
+        result = test2.join([test1])
+        expected = test2.join(test1)
+
+        tm.assert_frame_equal(result, expected)