test: cover Jupyter MIME output paths

src/ipython_webapp/jupyter/app.py

@@ -29,8 +29,8 @@ def serialize_event(event: KernelMessage | InputRequest) -> dict[str, object]:
             "password": event.password,
         }
 
-    # Jupyter keeps binary MIME payloads in message buffers. Base64 makes
-    # those buffers safe to carry in the same NDJSON stream as text events.
+    # Some Jupyter messages keep binary payloads in message buffers. Base64
+    # makes those buffers safe to carry beside JSON MIME data in the stream.
     return {
         "type": "kernel_message",
         "msg_type": event.msg_type,

tests/test_jupyter_webapp.py

@@ -3,16 +3,30 @@ import unittest
 
 import httpx
 
+from ipython_shell.jupyter.transport import KernelMessage
 from ipython_webapp.jupyter.app import (
     InputReply,
     app,
     reply_to_input,
     run_code,
+    serialize_event,
     shell_app,
 )
 
 
 class JupyterWebAppTests(unittest.IsolatedAsyncioTestCase):
+    def test_serialize_event_encodes_binary_buffers(self):
+        event = KernelMessage(
+            msg_type="display_data",
+            parent_id="call-1",
+            content={"data": {"application/octet-stream": "present"}},
+            buffers=[b"\x00\xff"],
+        )
+
+        serialized = serialize_event(event)
+
+        self.assertEqual(serialized["buffers"], ["AP8="])
+
     async def test_run_endpoint_streams_json_events_with_mime_data(self):
         transport = httpx.ASGITransport(app=app)
         async with httpx.AsyncClient(

tests/test_transport.py

@@ -67,6 +67,27 @@ class AsyncTransportTests(unittest.IsolatedAsyncioTestCase):
         )
         self.assertEqual(error.content["ename"], "ValueError")
 
+    async def test_transport_preserves_matplotlib_mime_output(self):
+        transport = JupyterTransport()
+        await transport.start()
+        try:
+            call_id = await transport.execute(
+                "import matplotlib.pyplot as plt\n"
+                "plt.plot([1, 2, 3], [4, 5, 6])"
+            )
+            messages = [
+                message async for message in transport.messages_for(call_id)
+            ]
+        finally:
+            await transport.shutdown()
+
+        display = next(
+            message
+            for message in messages
+            if message.msg_type == "display_data"
+        )
+        self.assertIn("image/png", display.content["data"])
+
     async def test_transport_routes_input_reply_without_parent_stdin(self):
         transport = JupyterTransport()
         await transport.start()