Synchronize input arg parsing between DataFrame and LazyFrame with_co…

…lumns

py-polars/polars/internals/dataframe/frame.py

@@ -5571,8 +5571,6 @@ def with_columns(
         └─────┴──────┴───────┴──────┴───────┘
 
         """
-        if exprs is not None and not isinstance(exprs, Sequence):
-            exprs = [exprs]
         return (
             self.lazy().with_columns(exprs, **named_exprs).collect(no_optimization=True)
         )

py-polars/polars/internals/lazyframe/frame.py

@@ -2474,11 +2474,15 @@ def with_columns(
         elif exprs is None and not named_exprs:
             raise ValueError("Expected at least one of 'exprs' or **named_exprs")
 
-        exprs = (
-            []
-            if exprs is None
-            else ([exprs] if isinstance(exprs, pli.Expr) else list(exprs))
-        )
+        if exprs is None:
+            exprs = []
+        elif isinstance(exprs, pli.Expr):
+            exprs = [exprs]
+        elif isinstance(exprs, pli.Series):
+            exprs = [pli.lit(exprs)]
+        else:
+            exprs = list(exprs)
+
         exprs.extend(
             pli.expr_to_lit_or_expr(expr).alias(name)
             for name, expr in named_exprs.items()

py-polars/tests/unit/test_lazy.py

@@ -1000,6 +1000,14 @@ def test_with_column_renamed(fruits_cars: pl.DataFrame) -> None:
     assert res.columns[0] == "C"
 
 
+def test_with_columns_single_series() -> None:
+    df = pl.DataFrame({"a": [1, 2]})
+    result = df.lazy().with_columns(pl.Series("b", [3, 4])).collect()
+
+    expected = pl.DataFrame({"a": [1, 2], "b": [3, 4]})
+    assert_frame_equal(result, expected)
+
+
 def test_reverse() -> None:
     out = pl.DataFrame({"a": [1, 2], "b": [3, 4]}).lazy().reverse()
     expected = pl.DataFrame({"a": [2, 1], "b": [4, 3]})