[SPARK-60684][CONNECT] Fast-path exact built-in string input

crates/pyspark-rs/src/session.rs

@@ -955,6 +955,14 @@ pub(crate) fn py_to_value(obj: &Bound<'_, PyAny>) -> PyResult<Value> {
         return Ok(Value::Null);
     }
 
+    // Exact built-in strings cannot implement numeric conversion protocols.
+    // Keep subclasses and strings that fail UTF-8 extraction on the generic path.
+    if let Ok(s) = obj.cast_exact::<pyo3::types::PyString>() {
+        if let Ok(s) = s.extract::<String>() {
+            return Ok(Value::String(s));
+        }
+    }
+
     // Check for bool (before int because bool is a subclass of int in Python)
     if let Ok(b) = obj.extract::<bool>() {
         return Ok(Value::Bool(b));

python/tests/test_dropin_offline.py

@@ -14,7 +14,7 @@
 import pytest
 
 from _mock_connect_server import message_field, mock_connect_session, struct_schema
-from pyspark.sql import SparkSession, types as T
+from pyspark.sql import Row, SparkSession, types as T
 
 
 # --------------------------------------------------------------------------- types
@@ -3085,3 +3085,33 @@ def allow_response_after_request():
     else:
         assert result == [("id", "bigint")]
     assert service.calls == 1
+
+
+# Exact built-in string conversion guards.
+
+
+@pytest.mark.parametrize("text", [
+    "", "1.25", "-42", "NaN", "a\x00b", "é雪😀", "e\u0301", "\uffff",
+])
+def test_exact_string_roundtrip(text):
+    actual = Row(value=text).value
+    assert type(actual) is str
+    assert actual == text
+
+
+@pytest.mark.parametrize("text", ["\ud800", "\udfff", "a\ud800b"])
+def test_exact_string_surrogate_error_preserves_generic_fallback(text):
+    with pytest.raises(TypeError):
+        Row(value=text)
+
+
+def test_exact_string_subclass_uses_generic_conversion():
+    # Pin existing subclass behavior; treating subclasses as text is a separate
+    # behavior change.
+    class IndexString(str):
+        def __index__(self):
+            return 17
+
+    actual = Row(value=IndexString("x")).value
+    assert type(actual) is int
+    assert actual == 17