[Dataframes] Call ray.init() on ray.dataframe import (#1626)

* ray.init on dataframe import

* wrapping ray.init in a try/except

* removing ray.init calls from test code

* resolving flake8

python/ray/dataframe/__init__.py

@@ -1,6 +1,7 @@
 from __future__ import absolute_import
 from __future__ import division
 from __future__ import print_function
+import threading
 
 DEFAULT_NPARTITIONS = 10
 
@@ -26,3 +27,10 @@ __all__ = [
     "DataFrame", "from_pandas", "to_pandas", "Series", "read_csv",
     "read_parquet"
 ]
+
+try:
+    if threading.current_thread().name == "MainThread":
+        import ray
+        ray.init()
+except AssertionError:
+    pass

python/ray/dataframe/test/test_dataframe.py

@@ -5,7 +5,6 @@ from __future__ import print_function
 import pytest
 import numpy as np
 import pandas as pd
-import ray
 import ray.dataframe as rdf
 
 
@@ -145,7 +144,6 @@ def create_test_dataframe():
 
 
 def test_int_dataframe():
-    ray.init()
 
     pandas_df = pd.DataFrame({'col1': [0, 1, 2, 3],
                               'col2': [4, 5, 6, 7],

python/ray/dataframe/test/test_io.py

@@ -5,7 +5,6 @@ from __future__ import print_function
 import pytest
 import numpy as np
 import pandas as pd
-import ray
 import ray.dataframe as rdf
 import ray.dataframe.io as io
 import os
@@ -58,7 +57,6 @@ def teardown_csv_file():
 
 
 def test_from_parquet_small():
-    ray.init()
 
     setup_parquet_file(SMALL_ROW_SIZE)
 

python/ray/dataframe/test/test_series.py

@@ -4,9 +4,6 @@ from __future__ import print_function
 
 import pytest
 import ray.dataframe as rdf
-import ray
-
-ray.init()
 
 
 @pytest.fixture