update prep_stream_data unit tests

Author: mattsb42-aws

test/unit/test_utils.py

@@ -23,6 +23,7 @@
 import aws_encryption_sdk.internal.utils
 from aws_encryption_sdk.exceptions import InvalidDataKeyError, SerializationError, UnknownIdentityError
 from aws_encryption_sdk.internal.defaults import MAX_FRAME_SIZE, MESSAGE_ID_LENGTH
+from aws_encryption_sdk.internal.utils.streams import InsistentReaderBytesIO
 from aws_encryption_sdk.structures import DataKey, EncryptedDataKey, MasterKeyInfo, RawDataKey
 
 from .test_values import VALUES
@@ -31,16 +32,19 @@
 
 
 def test_prep_stream_data_passthrough():
-    test = aws_encryption_sdk.internal.utils.prep_stream_data(sentinel.not_a_string_or_bytes)
+    test = aws_encryption_sdk.internal.utils.prep_stream_data(io.BytesIO(b"some data"))
 
-    assert test is sentinel.not_a_string_or_bytes
+    assert isinstance(test, InsistentReaderBytesIO)
 
 
 @pytest.mark.parametrize("source", (u"some unicode data ловие", b"\x00\x01\x02"))
 def test_prep_stream_data_wrap(source):
     test = aws_encryption_sdk.internal.utils.prep_stream_data(source)
 
+    # Check the wrapped stream
     assert isinstance(test, io.BytesIO)
+    # Check the wrapping stream
+    assert isinstance(test, InsistentReaderBytesIO)
 
 
 class TestUtils(unittest.TestCase):