Add ray.is_initialized() function. (#2818)

* Add ray.is_initialized() function.

* Add assert.

python/ray/__init__.py

@@ -50,7 +50,8 @@ from ray.local_scheduler import ObjectID, _config  # noqa: E402
 from ray.profiling import profile  # noqa: E402
 from ray.worker import (error_info, init, connect, disconnect, get, put, wait,
                         remote, get_gpu_ids, get_resource_ids, get_webui_url,
-                        register_custom_serializer, shutdown)  # noqa: E402
+                        register_custom_serializer, shutdown,
+                        is_initialized)  # noqa: E402
 from ray.worker import (SCRIPT_MODE, WORKER_MODE, LOCAL_MODE,
                         PYTHON_MODE)  # noqa: E402
 from ray.worker import global_state  # noqa: E402
@@ -67,9 +68,10 @@ __version__ = "0.5.2"
 __all__ = [
     "error_info", "init", "connect", "disconnect", "get", "put", "wait",
     "remote", "profile", "actor", "method", "get_gpu_ids", "get_resource_ids",
-    "get_webui_url", "register_custom_serializer", "shutdown", "SCRIPT_MODE",
-    "WORKER_MODE", "LOCAL_MODE", "PYTHON_MODE", "global_state", "ObjectID",
-    "_config", "__version__", "internal"
+    "get_webui_url", "register_custom_serializer", "shutdown",
+    "is_initialized", "SCRIPT_MODE", "WORKER_MODE", "LOCAL_MODE",
+    "PYTHON_MODE", "global_state", "ObjectID", "_config", "__version__",
+    "internal"
 ]
 
 import ctypes  # noqa: E402

python/ray/worker.py

@@ -1987,6 +1987,15 @@ def print_error_messages_raylet(worker):
         pass
 
 
+def is_initialized():
+    """Check if ray.init has been called yet.
+
+    Returns:
+        True if ray.init has already been called and false otherwise.
+    """
+    return ray.worker.global_worker.connected
+
+
 def print_error_messages(worker):
     """Print error messages in the background on the driver.