Raising Environment Error on Single Node Clusters (#40)

* catching single node configurations

* test added

* environ.get over getenv

* updated error message

dask_databricks/databrickscluster.py

@@ -23,12 +23,19 @@ def __init__(
         loop: Optional[IOLoop] = None,
         asynchronous: bool = False,
     ):
-        self.spark_local_ip = os.getenv("SPARK_LOCAL_IP")
+        self.spark_local_ip = os.environ.get("SPARK_LOCAL_IP")
         if self.spark_local_ip is None:
             raise KeyError(
                 "Unable to find expected environment variable SPARK_LOCAL_IP. "
                 "Are you running this on a Databricks driver node?"
             )
+        if os.environ.get("MASTER") and "local[" in os.environ.get("MASTER"):
+            raise EnvironmentError(
+                "You appear to be trying to run a multi-node Dask cluster on a "
+                "single-node databricks cluster. Maybe you want "
+                "`dask.distributed.LocalCluster().get_client()` instead"
+
+            )
         try:
             name = spark.conf.get("spark.databricks.clusterUsageTags.clusterId")
         except AttributeError:

dask_databricks/tests/test_databricks.py

@@ -38,6 +38,14 @@ def test_databricks_cluster_raises_key_error_when_initialised_outside_of_databri
     with pytest.raises(KeyError):
         DatabricksCluster()
 
+def test_databricks_cluster_raises_environment_error_when_master_variable_implies_single_node(
+    monkeypatch,
+    set_spark_local_ip,
+    dask_cluster,
+):
+    monkeypatch.setenv("MASTER", "local[8]")
+    with pytest.raises(EnvironmentError):
+        DatabricksCluster()
 
 def test_databricks_cluster_create(set_spark_local_ip, dask_cluster):
     cluster = DatabricksCluster()