feat: Access transaction in current scope

Specially when trying to add spans to automatically instrumented
transactions, users need access to the current transaction.

This gives direct access no matter how deep the code is in the
transaction/span tree.

sentry_sdk/scope.py

@@ -134,14 +134,30 @@ def fingerprint(self, value):
         """When set this overrides the default fingerprint."""
         self._fingerprint = value
 
-    @_attr_setter
+    @property
+    def transaction(self):
+        # type: () -> Optional[Span]
+        """Return the transaction (root span) in the scope."""
+        # XXX: update return type to Optional[Transaction]
+        try:
+            return self._span._span_recorder.spans[0]
+        except (AttributeError, IndexError):
+            return None
+
+    @transaction.setter
     def transaction(self, value):
         # type: (Optional[str]) -> None
         """When set this forces a specific transaction name to be set."""
+        # XXX: the docstring above is misleading. The implementation of
+        # apply_to_event prefers an existing value of event.transaction over
+        # anything set in the scope.
+        # XXX: note that with the introduction of the Scope.transaction getter,
+        # there is a semantic and type mismatch between getter and setter. The
+        # getter returns a transaction, the setter sets a transaction name.
+        # Without breaking version compatibility, we could make the setter set a
+        # transaction name or transaction (self._span) depending on the type of
+        # the value argument.
         self._transaction = value
-        span = self._span
-        if span:
-            span.transaction = value
 
     @_attr_setter
     def user(self, value):

tests/test_tracing.py

@@ -3,7 +3,7 @@
 
 import pytest
 
-from sentry_sdk import Hub, capture_message
+from sentry_sdk import Hub, capture_message, start_span
 from sentry_sdk.tracing import Span
 
 
@@ -180,3 +180,17 @@ def before_send(event, hint):
         pass
 
     assert len(events) == 1
+
+
+def test_get_transaction_from_scope(sentry_init, capture_events):
+    sentry_init(traces_sample_rate=1.0)
+    events = capture_events()
+
+    with start_span(transaction="/"):
+        with start_span(op="child-span"):
+            with start_span(op="child-child-span"):
+                scope = Hub.current.scope
+                assert scope.span.op == "child-child-span"
+                assert scope.transaction.transaction == "/"
+
+    assert len(events) == 1