fix: add utf8_literal function to create UTF8 literal expressions in tests

Author: kosiew

python/tests/test_functions.py

@@ -23,7 +23,7 @@
 
 from datafusion import SessionContext, column
 from datafusion import functions as f
-from datafusion import literal
+from datafusion import literal, Expr
 
 np.seterr(invalid="ignore")
 
@@ -905,10 +905,15 @@ def test_temporal_functions(df):
     )
 
 
+def utf8_literal(value: str) -> Expr:
+    """Creates a new expression representing a UTF8 literal value."""
+    return literal(pa.scalar(value, type=pa.string()))
+
+
 def test_arrow_cast(df):
     df = df.select(
-        f.arrow_cast(column("a"), literal("Float64")).alias("a_as_float"),
-        f.arrow_cast(column("a"), literal("Int32")).alias("a_as_int"),
+        f.arrow_cast(column("a"), utf8_literal("Float64")).alias("a_as_float"),
+        f.arrow_cast(column("a"), utf8_literal("Int32")).alias("a_as_int"),
     )
     result = df.collect()
     assert len(result) == 1