Tweak some coverage details of switch_context code

Author: nedbat

coverage/collector.py

@@ -256,10 +256,6 @@ def _start_tracer(self):
         if hasattr(tracer, 'should_start_context'):
             tracer.should_start_context = self.should_start_context
             tracer.switch_context = self.switch_context
-        elif self.should_start_context:
-            raise CoverageException(
-                "Can't support dynamic contexts with {}".format(self.tracer_name())
-            )
 
         fn = tracer.start()
         self.tracers.append(tracer)

coverage/control.py

@@ -561,8 +561,8 @@ def switch_context(self, new_context):
         .. versionadded:: 5.0
 
         """
-        if not self._started:
-            raise CoverageException(                    # pragma: only jython
+        if not self._started:                           # pragma: part started
+            raise CoverageException(
                 "Cannot switch context, coverage is not started"
                 )
         self._collector.switch_context(new_context)

metacov.ini

@@ -71,6 +71,7 @@ exclude_lines =
 partial_branches =
     pragma: part covered
     pragma: if failure
+    pragma: part started
     if env.TESTING:
     if .* env.JYTHON
     if .* env.IRONPYTHON