feature: Use boto3 DEFAULT_SESSION when no boto3 session specified. (aws#1545)

Author: nadiaya

src/sagemaker/session.py

@@ -123,7 +123,7 @@ def _initialize(self, boto_session, sagemaker_client, sagemaker_runtime_client):
         Creates or uses a boto_session, sagemaker_client and sagemaker_runtime_client.
         Sets the region_name.
         """
-        self.boto_session = boto_session or boto3.Session()
+        self.boto_session = boto_session or boto3.DEFAULT_SESSION or boto3.Session()
 
         self._region_name = self.boto_session.region_name
         if self._region_name is None:

tests/unit/test_session.py

@@ -52,6 +52,18 @@ def boto_session():
     return boto_mock
 
 
+@patch("boto3.DEFAULT_SESSION")
+def test_default_session(boto3_default_session):
+    sess = Session()
+    assert sess.boto_session is boto3_default_session
+
+
+@patch("boto3.Session")
+def test_new_session_created(boto3_session):
+    sess = Session()
+    assert sess.boto_session is boto3_session.return_value
+
+
 def test_process(boto_session):
     session = Session(boto_session)