Add runtime_context to get some runtime fields in worker (#4065)

python/ray/runtime_context.py

@@ -0,0 +1,34 @@
+from __future__ import absolute_import
+from __future__ import division
+from __future__ import print_function
+
+import ray.worker
+
+
+class RuntimeContext(object):
+    """A class used for getting runtime context."""
+
+    def __init__(self, worker=None):
+        self.worker = worker
+
+    @property
+    def current_driver_id(self):
+        """Get current driver ID for this worker or driver.
+
+        Returns:
+            If called by a driver, this returns the driver ID. If called in
+                a task, return the driver ID of the associated driver.
+        """
+        assert self.worker is not None
+        return self.worker.task_driver_id
+
+
+_runtime_context = None
+
+
+def _get_runtime_context():
+    global _runtime_context
+    if _runtime_context is None:
+        _runtime_context = RuntimeContext(ray.worker.get_global_worker())
+
+    return _runtime_context