diff --git a/test/unit/test_streaming_client_stream_decryptor.py b/test/unit/test_streaming_client_stream_decryptor.py
index c59ae4beb..69cd72c73 100644
--- a/test/unit/test_streaming_client_stream_decryptor.py
+++ b/test/unit/test_streaming_client_stream_decryptor.py
@@ -12,10 +12,8 @@
 # language governing permissions and limitations under the License.
 """Unit test suite for aws_encryption_sdk.streaming_client.StreamDecryptor"""
 import io
-import unittest
 
 import pytest
-import six
 from mock import MagicMock, call, patch, sentinel
 
 from aws_encryption_sdk.exceptions import CustomMaximumValueExceeded, NotSupportedError, SerializationError
@@ -29,8 +27,9 @@
 pytestmark = [pytest.mark.unit, pytest.mark.local]
 
 
-class TestStreamDecryptor(unittest.TestCase):
-    def setUp(self):
+class TestStreamDecryptor(object):
+    @pytest.fixture(autouse=True)
+    def apply_fixtures(self):
         self.mock_key_provider = MagicMock(__class__=MasterKeyProvider)
         self.mock_materials_manager = MagicMock(__class__=CryptoMaterialsManager)
         self.mock_materials_manager.decrypt_materials.return_value = MagicMock(
@@ -92,8 +91,8 @@ def setUp(self):
         # Set up decrypt patch
         self.mock_decrypt_patcher = patch("aws_encryption_sdk.streaming_client.decrypt")
         self.mock_decrypt = self.mock_decrypt_patcher.start()
-
-    def tearDown(self):
+        yield
+        # Run tearDown
         self.mock_deserialize_header_patcher.stop()
         self.mock_deserialize_header_auth_patcher.stop()
         self.mock_validate_header_patcher.stop()
@@ -186,12 +185,11 @@ def test_read_header_frame_too_large(self, mock_derive_datakey):
         test_decryptor.key_provider = self.mock_key_provider
         test_decryptor.source_stream = ct_stream
         test_decryptor._stream_length = len(VALUES["data_128"])
-        with six.assertRaisesRegex(
-            self,
-            CustomMaximumValueExceeded,
-            "Frame Size in header found larger than custom value: {found} > {custom}".format(found=1024, custom=10),
-        ):
+        with pytest.raises(CustomMaximumValueExceeded) as excinfo:
             test_decryptor._read_header()
+        excinfo.match(
+            "Frame Size in header found larger than custom value: {found} > {custom}".format(found=1024, custom=10)
+        )
 
     @patch("aws_encryption_sdk.streaming_client.Verifier")
     @patch("aws_encryption_sdk.streaming_client.DecryptionMaterialsRequest")
@@ -220,14 +218,13 @@ def test_prep_non_framed_content_length_too_large(self):
         mock_data_key = MagicMock()
         test_decryptor.data_key = mock_data_key
 
-        with six.assertRaisesRegex(
-            self,
-            CustomMaximumValueExceeded,
+        with pytest.raises(CustomMaximumValueExceeded) as excinfo:
+            test_decryptor._prep_non_framed()
+        excinfo.match(
             "Non-framed message content length found larger than custom value: {found} > {custom}".format(
                 found=len(VALUES["data_128"]), custom=len(VALUES["data_128"]) // 2
-            ),
-        ):
-            test_decryptor._prep_non_framed()
+            )
+        )
 
     def test_prep_non_framed(self):
         test_decryptor = StreamDecryptor(key_provider=self.mock_key_provider, source=self.mock_input_stream)
@@ -288,10 +285,9 @@ def test_read_bytes_from_non_framed_message_body_too_small(self):
         test_decryptor = StreamDecryptor(key_provider=self.mock_key_provider, source=ct_stream)
         test_decryptor.body_length = len(VALUES["data_128"] * 2)
         test_decryptor._header = self.mock_header
-        with six.assertRaisesRegex(
-            self, SerializationError, "Total message body contents less than specified in body description"
-        ):
+        with pytest.raises(SerializationError) as excinfo:
             test_decryptor._read_bytes_from_non_framed_body(1)
+        excinfo.match("Total message body contents less than specified in body description")
 
     def test_read_bytes_from_non_framed_no_verifier(self):
         ct_stream = io.BytesIO(VALUES["data_128"])
@@ -497,8 +493,9 @@ def test_read_bytes_from_framed_body_bad_sequence_number(self):
         frame_data.final_frame = False
         frame_data.ciphertext = b"asdfzxcv"
         self.mock_deserialize_frame.return_value = (frame_data, False)
-        with six.assertRaisesRegex(self, SerializationError, "Malformed message: frames out of order"):
+        with pytest.raises(SerializationError) as excinfo:
             test_decryptor._read_bytes_from_framed_body(4)
+        excinfo.match("Malformed message: frames out of order")
 
     @patch("aws_encryption_sdk.streaming_client.StreamDecryptor._read_bytes_from_non_framed_body")
     @patch("aws_encryption_sdk.streaming_client.StreamDecryptor._read_bytes_from_framed_body")
@@ -549,8 +546,9 @@ def test_read_bytes_unknown(self, mock_read_frame, mock_read_block):
         test_decryptor = StreamDecryptor(key_provider=self.mock_key_provider, source=ct_stream)
         test_decryptor._header = MagicMock()
         test_decryptor._header.content_type = None
-        with six.assertRaisesRegex(self, NotSupportedError, "Unsupported content type"):
+        with pytest.raises(NotSupportedError) as excinfo:
             test_decryptor._read_bytes(5)
+        excinfo.match("Unsupported content type")
 
     @patch("aws_encryption_sdk.streaming_client._EncryptionStream.close")
     def test_close(self, mock_close):
@@ -565,5 +563,6 @@ def test_close(self, mock_close):
     def test_close_no_footer(self, mock_close):
         self.mock_header.content_type = ContentType.FRAMED_DATA
         test_decryptor = StreamDecryptor(key_provider=self.mock_key_provider, source=self.mock_input_stream)
-        with six.assertRaisesRegex(self, SerializationError, "Footer not read"):
+        with pytest.raises(SerializationError) as excinfo:
             test_decryptor.close()
+        excinfo.match("Footer not read")