chore: update InvalidIntoExprError to mention nw.lit (#1393)

* update intoexpr error to mention literal

* shorter lines

* update test

---------

Co-authored-by: Marco Gorelli <[email protected]>

narwhals/exceptions.py

@@ -50,8 +50,13 @@ def __init__(self, message: str) -> None:
     def from_invalid_type(cls, invalid_type: type) -> InvalidIntoExprError:
         message = (
             f"Expected an object which can be converted into an expression, got {invalid_type}\n\n"
-            "Hint: if you were trying to select a column which does not have a string column name, then "
-            "you should explicitly use `nw.col`.\nFor example, `df.select(nw.col(0))` if you have a column "
-            "named `0`."
+            "Hint:\n"
+            "- if you were trying to select a column which does not have a string\n"
+            "  column name, then you should explicitly use `nw.col`.\n"
+            "  For example, `df.select(nw.col(0))` if you have a column named `0`.\n"
+            "- if you were trying to create a new literal column, then you \n"
+            "  should explicitly use `nw.lit`.\n"
+            "  For example, `df.select(nw.lit(0))` if you want to create a new\n"
+            "  column with literal value `0`."
         )
         return InvalidIntoExprError(message)

tests/frame/select_test.py

@@ -40,7 +40,7 @@ def test_non_string_select() -> None:
 
 def test_int_select_pandas() -> None:
     df = nw.from_native(pd.DataFrame({0: [1, 2], "b": [3, 4]}))
-    with pytest.raises(InvalidIntoExprError, match="\n\nHint: if you were trying"):
+    with pytest.raises(InvalidIntoExprError, match="\n\nHint:\n- if you were trying"):
         nw.to_native(df.select(0))  # type: ignore[arg-type]