Raise exception if pyarrow is imported before ray. (#1283)

* Raise exception if pyarrow is imported before ray.

* Pip install pyarrow when building doc so we don't have to mock it.

* Raise ImportError instead of Exception.

doc/requirements-doc.txt

@@ -5,6 +5,7 @@ funcsigs
 mock
 numpy
 opencv-python
+pyarrow
 psutil
 recommonmark
 redis

doc/source/conf.py

@@ -32,8 +32,6 @@ MOCK_MODULES = ["gym",
                 "tensorflow.python",
                 "tensorflow.python.client",
                 "tensorflow.python.util",
-                "pyarrow",
-                "pyarrow.plasma",
                 "smart_open",
                 "ray.local_scheduler",
                 "ray.plasma",

python/ray/__init__.py

@@ -4,6 +4,12 @@ from __future__ import print_function
 
 import os
 import sys
+
+if "pyarrow" in sys.modules:
+    raise ImportError("Ray must be imported before pyarrow because Ray "
+                      "requires a specific version of pyarrow (which is "
+                      "packaged along with Ray).")
+
 # Add the directory containing pyarrow to the Python path so that we find the
 # pyarrow version packaged with ray and not a pre-existing pyarrow.
 pyarrow_path = os.path.join(os.path.abspath(os.path.dirname(__file__)),