Fix tracing decorator with late configuration

CHANGELOG.md

@@ -7,14 +7,19 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased](https://github.com/open-telemetry/opentelemetry-python/compare/v1.12.0rc2-0.32b0...HEAD)
 
+
+- Fix tracing decorator with late configuration
+  ([#2754](https://github.com/open-telemetry/opentelemetry-python/pull/2754))
+
+
 ## [1.12.0rc2-0.32b0](https://github.com/open-telemetry/opentelemetry-python/releases/tag/v1.12.0rc2-0.32b0) - 2022-07-04
 
 
 
 - Fix instrument name and unit regexes
   ([#2796](https://github.com/open-telemetry/opentelemetry-python/pull/2796))
 - Add optional sessions parameter to all Exporters leveraging requests.Session
-  ([#2783](https://github.com/open-telemetry/opentelemetry-python/pull/2783))
+  ([#2783](https://github.com/open-telemetry/opentelemetry-python/pull/2783)
 - Add min/max fields to Histogram
   ([#2759](https://github.com/open-telemetry/opentelemetry-python/pull/2759))
 - `opentelemetry-exporter-otlp-proto-http` Add support for OTLP/HTTP log exporter

opentelemetry-api/src/opentelemetry/trace/__init__.py

@@ -434,8 +434,10 @@ def _tracer(self) -> Tracer:
     def start_span(self, *args, **kwargs) -> Span:  # type: ignore
         return self._tracer.start_span(*args, **kwargs)  # type: ignore
 
-    def start_as_current_span(self, *args, **kwargs) -> Span:  # type: ignore
-        return self._tracer.start_as_current_span(*args, **kwargs)  # type: ignore
+    @contextmanager  # type: ignore
+    def start_as_current_span(self, *args, **kwargs) -> Iterator[Span]:  # type: ignore
+        with self._tracer.start_as_current_span(*args, **kwargs) as span:  # type: ignore
+            yield span
 
 
 class NoOpTracer(Tracer):

opentelemetry-api/tests/trace/test_proxy.py

@@ -15,10 +15,15 @@
 # pylint: disable=W0212,W0222,W0221
 import typing
 import unittest
+from contextlib import contextmanager
 
 from opentelemetry import trace
 from opentelemetry.test.globals_test import TraceGlobalsTest
-from opentelemetry.trace.span import INVALID_SPAN_CONTEXT, NonRecordingSpan
+from opentelemetry.trace.span import (
+    INVALID_SPAN_CONTEXT,
+    NonRecordingSpan,
+    Span,
+)
 
 
 class TestProvider(trace.NoOpTracerProvider):
@@ -35,6 +40,11 @@ class TestTracer(trace.NoOpTracer):
     def start_span(self, *args, **kwargs):
         return TestSpan(INVALID_SPAN_CONTEXT)
 
+    @contextmanager
+    def start_as_current_span(self, *args, **kwargs):  # type: ignore
+        with trace.use_span(self.start_span(*args, **kwargs)) as span:  # type: ignore
+            yield span
+
 
 class TestSpan(NonRecordingSpan):
     pass
@@ -73,3 +83,21 @@ def test_proxy_tracer(self):
         # creates real spans
         with tracer.start_span("") as span:
             self.assertIsInstance(span, TestSpan)
+
+    def test_late_config(self):
+        # get a tracer and instrument a function as we would at the
+        # root of a module
+        tracer = trace.get_tracer("test")
+
+        @tracer.start_as_current_span("span")
+        def my_function() -> Span:
+            return trace.get_current_span()
+
+        # call function before configuring tracing provider, should
+        # return INVALID_SPAN from the NoOpTracer
+        self.assertEqual(my_function(), trace.INVALID_SPAN)
+
+        # configure tracing provider
+        trace.set_tracer_provider(TestProvider())
+        # call function again, we should now be getting a TestSpan
+        self.assertIsInstance(my_function(), TestSpan)