Change pyspark imports to optional

ibis/pyspark/tests/test_basic.py

@@ -1,18 +1,18 @@
 import pandas as pd
 import pandas.util.testing as tm
-import pyspark.sql.functions as F
 import pytest
-from pyspark.sql import SparkSession
-from pyspark.sql.window import Window
 
 import ibis
 
 pytestmark = pytest.mark.pyspark
-pytest.importorskip('pyspark')
 
 
 @pytest.fixture(scope='session')
 def client():
+    pytest.importorskip('pyspark')
+    from pyspark.sql import SparkSession
+    import pyspark.sql.functions as F
+
     session = SparkSession.builder.getOrCreate()
     client = ibis.pyspark.connect(session)
     df = client._session.range(0, 10)
@@ -72,6 +72,8 @@ def test_aggregation_col(client):
 
 
 def test_aggregation(client):
+    import pyspark.sql.functions as F
+
     table = client.table('table1')
     result = table.aggregate(table['id'].max()).compile()
     expected = table.compile().agg(F.max('id'))
@@ -80,6 +82,8 @@ def test_aggregation(client):
 
 
 def test_groupby(client):
+    import pyspark.sql.functions as F
+
     table = client.table('table1')
     result = table.groupby('id').aggregate(table['id'].max()).compile()
     expected = table.compile().groupby('id').agg(F.max('id'))
@@ -88,6 +92,9 @@ def test_groupby(client):
 
 
 def test_window(client):
+    import pyspark.sql.functions as F
+    from pyspark.sql.window import Window
+
     table = client.table('table1')
     w = ibis.window()
     result = (