fix(tracing): Keep original function signature when decorated (getsentry#3178)

Our trace decorator was leading to a change of signature of the decorated function.

Author: sentrivana

sentry_sdk/tracing_utils.py

@@ -645,6 +645,11 @@ async def func_with_tracing(*args, **kwargs):
             ):
                 return await func(*args, **kwargs)
 
+        try:
+            func_with_tracing.__signature__ = inspect.signature(func)  # type: ignore[attr-defined]
+        except Exception:
+            pass
+
     # Synchronous case
     else:
 
@@ -668,6 +673,11 @@ def func_with_tracing(*args, **kwargs):
             ):
                 return func(*args, **kwargs)
 
+        try:
+            func_with_tracing.__signature__ = inspect.signature(func)  # type: ignore[attr-defined]
+        except Exception:
+            pass
+
     return func_with_tracing
 
 

tests/tracing/test_decorator.py

@@ -1,7 +1,9 @@
+import inspect
 from unittest import mock
 
 import pytest
 
+from sentry_sdk.tracing import trace
 from sentry_sdk.tracing_utils import start_child_span_decorator
 from sentry_sdk.utils import logger
 from tests.conftest import patch_start_tracing_child
@@ -76,3 +78,38 @@ async def test_trace_decorator_async_no_trx():
                 "test_decorator.my_async_example_function",
             )
             assert result2 == "return_of_async_function"
+
+
+def test_functions_to_trace_signature_unchanged_sync(sentry_init):
+    sentry_init(
+        traces_sample_rate=1.0,
+    )
+
+    def _some_function(a, b, c):
+        pass
+
+    @trace
+    def _some_function_traced(a, b, c):
+        pass
+
+    assert inspect.getcallargs(_some_function, 1, 2, 3) == inspect.getcallargs(
+        _some_function_traced, 1, 2, 3
+    )
+
+
+@pytest.mark.asyncio
+async def test_functions_to_trace_signature_unchanged_async(sentry_init):
+    sentry_init(
+        traces_sample_rate=1.0,
+    )
+
+    async def _some_function(a, b, c):
+        pass
+
+    @trace
+    async def _some_function_traced(a, b, c):
+        pass
+
+    assert inspect.getcallargs(_some_function, 1, 2, 3) == inspect.getcallargs(
+        _some_function_traced, 1, 2, 3
+    )