Correctly detect user-set parent_span=None

Author: sentrivana

sentry_sdk/scope.py

@@ -33,7 +33,7 @@
     normalize_incoming_data,
     PropagationContext,
 )
-from sentry_sdk.traces import StreamedSpan, NoOpStreamedSpan
+from sentry_sdk.traces import _DEFAULT_PARENT_SPAN, StreamedSpan, NoOpStreamedSpan
 from sentry_sdk.tracing import (
     BAGGAGE_HEADER_NAME,
     SENTRY_TRACE_HEADER_NAME,
@@ -1177,9 +1177,9 @@ def start_span(
     def start_streamed_span(
         self,
         name: str,
-        attributes: "Optional[Attributes]" = None,
-        parent_span: "Optional[StreamedSpan]" = None,
-        active: bool = True,
+        attributes: "Optional[Attributes]",
+        parent_span: "Optional[StreamedSpan]",
+        active: bool,
     ) -> "StreamedSpan":
         # TODO: rename to start_span once we drop the old API
         if isinstance(parent_span, NoOpStreamedSpan):
@@ -1189,7 +1189,9 @@ def start_streamed_span(
                 "currently active span instead."
             )
 
-        if parent_span is None or isinstance(parent_span, NoOpStreamedSpan):
+        if parent_span is _DEFAULT_PARENT_SPAN or isinstance(
+            parent_span, NoOpStreamedSpan
+        ):
             parent_span = self.span  # type: ignore
 
         # If no eligible parent_span was provided and there is no currently

sentry_sdk/traces.py

@@ -59,18 +59,24 @@ def __str__(self) -> str:
 }
 
 
+# Sentinel value for an unset parent_span to be able to distinguish it from
+# a None set by the user
+_DEFAULT_PARENT_SPAN = object()
+
+
 def start_span(
     name: str,
     attributes: "Optional[Attributes]" = None,
-    parent_span: "Optional[StreamedSpan]" = None,
+    parent_span: "Optional[StreamedSpan]" = _DEFAULT_PARENT_SPAN,
     active: bool = True,
 ) -> "StreamedSpan":
     """
     Start a span.
 
     The span's parent, unless provided explicitly via the `parent_span` argument,
     will be the current active span, if any. If there is none, this span will
-    become the root of a new span tree.
+    become the root of a new span tree. If you explicitly want this span to be
+    top-level without a parent, set `parent_span=None`.
 
     `start_span()` can either be used as context manager or you can use the span
     object it returns and explicitly end it via `span.end()`. The following is
@@ -102,7 +108,8 @@ def start_span(
 
     :param parent_span: A span instance that the new span should consider its
         parent. If not provided, the parent will be set to the currently active
-        span, if any.
+        span, if any. If set to `None`, this span will become a new root-level
+        span.
     :type parent_span: "Optional[StreamedSpan]"
 
     :param active: Controls whether spans started while this span is running