diff --git a/singer/messages.py b/singer/messages.py
index 3848801..e53c48d 100644
--- a/singer/messages.py
+++ b/singer/messages.py
@@ -222,22 +222,22 @@ def format_message(message):
     return json.dumps(message.asdict(), use_decimal=True)
 
 
-def write_message(message):
-    sys.stdout.write(format_message(message) + '\n')
-    sys.stdout.flush()
+def write_message(message, out=sys.stdout):
+    out.write(format_message(message) + '\n')
+    out.flush()
 
 
-def write_record(stream_name, record, stream_alias=None, time_extracted=None):
+def write_record(stream_name, record, stream_alias=None, time_extracted=None, out=sys.stdout):
     """Write a single record for the given stream.
 
     write_record("users", {"id": 2, "email": "mike@stitchdata.com"})
     """
     write_message(RecordMessage(stream=(stream_alias or stream_name),
                                 record=record,
-                                time_extracted=time_extracted))
+                                time_extracted=time_extracted), out=out)
 
 
-def write_records(stream_name, records):
+def write_records(stream_name, records, out=sys.stdout):
     """Write a list of records for the given stream.
 
     chris = {"id": 1, "email": "chris@stitchdata.com"}
@@ -245,10 +245,11 @@ def write_records(stream_name, records):
     write_records("users", [chris, mike])
     """
     for record in records:
-        write_record(stream_name, record)
+        write_record(stream_name, record, out=out)
 
 
-def write_schema(stream_name, schema, key_properties, bookmark_properties=None, stream_alias=None):
+def write_schema(stream_name, schema, key_properties, bookmark_properties=None, stream_alias=None,
+                out=sys.stdout):
     """Write a schema message.
 
     stream = 'test'
@@ -266,22 +267,22 @@ def write_schema(stream_name, schema, key_properties, bookmark_properties=None,
             stream=(stream_alias or stream_name),
             schema=schema,
             key_properties=key_properties,
-            bookmark_properties=bookmark_properties))
+            bookmark_properties=bookmark_properties), out=out)
 
 
-def write_state(value):
+def write_state(value, out=sys.stdout):
     """Write a state message.
 
     write_state({'last_updated_at': '2017-02-14T09:21:00'})
     """
-    write_message(StateMessage(value=value))
+    write_message(StateMessage(value=value), out=out)
 
 
-def write_version(stream_name, version):
+def write_version(stream_name, version, out=sys.stdout):
     """Write an activate version message.
 
     stream = 'test'
     version = int(time.time())
     write_version(stream, version)
     """
-    write_message(ActivateVersionMessage(stream_name, version))
+    write_message(ActivateVersionMessage(stream_name, version), out=out)