Set status for ended spans

Fixes #292

Author: ocelotl

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

@@ -66,7 +66,7 @@
 import typing
 from contextlib import contextmanager
 
-from opentelemetry.trace.status import Status
+from opentelemetry.trace.status import Status, UnknownStatus
 from opentelemetry.util import loader, types
 
 # TODO: quarantine
@@ -124,10 +124,12 @@ class SpanKind(enum.Enum):
     https://github.com/open-telemetry/opentelemetry-specification/pull/226.
     """
 
-    #: Default value. Indicates that the span is used internally in the application.
+    #: Default value. Indicates that the span is used internally in the
+    # application.
     INTERNAL = 0
 
-    #: Indicates that the span describes an operation that handles a remote request.
+    #: Indicates that the span describes an operation that handles a remote
+    # request.
     SERVER = 1
 
     #: Indicates that the span describes a request to some remote service.
@@ -144,9 +146,13 @@ class SpanKind(enum.Enum):
     CONSUMER = 4
 
 
-class Span:
+class Span(object):
     """A span represents a single operation within a trace."""
 
+    def __init__(self, auto_update_status=False):
+        # FIXME what is the standard to follow for documentation of attributes?
+        self._auto_update_status = auto_update_status
+
     def start(self, start_time: typing.Optional[int] = None) -> None:
         """Sets the current time as the span's start time.
 
@@ -237,8 +243,27 @@ def __exit__(
         exc_tb: typing.Optional[python_types.TracebackType],
     ) -> None:
         """Ends context manager and calls `end` on the `Span`."""
+
         self.end()
 
+        if (
+            exc_val is not None and
+            self.status is None and
+            self._auto_update_status
+        ):
+            if isinstance(exc_val, Status):
+                self.set_status(exc_val.status)
+
+            else:
+                self.set_status(UnknownStatus)
+
+            # return False
+        # return True
+
+        # FIXME https://docs.python.org/3.7/reference/datamodel.html?highlight=__exit__#object.__exit__
+        # says that a boolean value should be returned. Is the lack of the
+        # return of a boolean value here intentional?
+
 
 class TraceOptions(int):
     """A bitmask that represents options specific to the trace.