fix: Convert assertion into warning (#474)

* fix: Convert assertion into warning

Fix #472

* fix: Add missing import

Author: untitaker

sentry_sdk/tracing.py

@@ -5,7 +5,7 @@
 from datetime import datetime
 
 import sentry_sdk
-from sentry_sdk.utils import capture_internal_exceptions
+from sentry_sdk.utils import capture_internal_exceptions, logger
 from sentry_sdk._compat import PY2
 from sentry_sdk._types import MYPY
 
@@ -272,9 +272,9 @@ def finish(self, hub=None):
             # resolved to a concrete decision. If `sampled` is `None`, it's
             # likely that somebody used `with sentry_sdk.Hub.start_span(..)` on a
             # non-transaction span and later decided to make it a transaction.
-            assert (
-                self.sampled is not None
-            ), "Need to set transaction when entering span!"
+            if self.sampled is None:
+                logger.warning("Discarding transaction Span without sampling decision")
+
             return None
 
         return hub.capture_event(