perf(span-processor): skip span_formatter when debug logging is off

langfuse/_client/span_processor.py

@@ -12,6 +12,7 @@
 """
 
 import base64
+import logging
 import os
 import threading
 from typing import Callable, Dict, List, Optional, cast
@@ -218,9 +219,13 @@ def on_end(self, span: ReadableSpan) -> None:
                 )
                 return
 
-            langfuse_logger.debug(
-                f"Trace: Processing span name='{span._name}' | Full details:\n{span_formatter(span)}"
-            )
+            # span_formatter serializes the full span; skip it unless DEBUG is on
+            if langfuse_logger.isEnabledFor(logging.DEBUG):
+                langfuse_logger.debug(
+                    "Trace: Processing span name='%s' | Full details:\n%s",
+                    span.name,
+                    span_formatter(span),
+                )
 
             super().on_end(span)
         finally:

tests/unit/test_span_processor.py

@@ -1,8 +1,12 @@
+import logging
 from typing import Sequence
+from unittest.mock import patch
 
-from opentelemetry.sdk.trace import ReadableSpan
+import pytest
+from opentelemetry.sdk.trace import ReadableSpan, TracerProvider
 from opentelemetry.sdk.trace.export import SpanExporter, SpanExportResult
 
+import langfuse._client.span_processor as span_processor_module
 from langfuse._client.environment_variables import (
     LANGFUSE_FLUSH_AT,
     LANGFUSE_FLUSH_INTERVAL,
@@ -54,3 +58,41 @@ def test_span_processor_uses_env_flush_settings_when_constructor_omits_them(
         assert processor._batch_processor._schedule_delay_millis == 3250
     finally:
         processor.shutdown()
+
+
+@pytest.fixture
+def tracer_with_processor():
+    processor = LangfuseSpanProcessor(
+        public_key="pk-test",
+        secret_key="sk-test",
+        base_url="http://localhost:3000",
+        span_exporter=NoOpSpanExporter(),
+    )
+    provider = TracerProvider()
+    provider.add_span_processor(processor)
+    yield provider.get_tracer("test-instrumentor")
+    processor.shutdown()
+
+
+@pytest.mark.parametrize(
+    ("level", "expected_formatter_calls"),
+    [(logging.WARNING, 0), (logging.DEBUG, 1)],
+)
+def test_on_end_formats_span_only_when_debug_enabled(
+    caplog, tracer_with_processor, level, expected_formatter_calls
+):
+    caplog.set_level(level, logger="langfuse")
+
+    with patch.object(
+        span_processor_module, "span_formatter", return_value="{}"
+    ) as span_formatter:
+        # gen_ai.* attribute makes the span pass the default export filter
+        with tracer_with_processor.start_as_current_span(
+            "llm-call", attributes={"gen_ai.system": "test"}
+        ):
+            pass
+
+    assert span_formatter.call_count == expected_formatter_calls
+    assert ("Processing span name='llm-call'" in caplog.text) == bool(
+        expected_formatter_calls
+    )