René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Chapter 21 - tcp_mojifinder.py crashes on empty search result","articleBody":"## Issue\n\nIf [`index.search(query)`](https://github.com/fluentpython/example-code-2e/blob/master/21-async/mojifinder/tcp_mojifinder.py#L45)  in `tcp_mojifinder.py` returns empty result, it crashes TCP because [`asyncio.StreamWriter.writelines(data)`](https://docs.python.org/3/library/asyncio-stream.html#asyncio.StreamWriter.writelines) raises `AssertionError: Data should not be empty` error if `data` is an empty iterable.\n\n\n### How to reproduce\n\n\nStart the TCP server\n```shell\n% ./tcp_mojifinder.py \nBuilding index.\nServing on ('127.0.0.1', 2323). Hit CTRL-C to stop.\n...\n```\n\nConnect to the TCP server and send a search query which returns no results, e.g. `missing`\n```shell\n% nc localhost 2323\n?\u003e missing\n% \n```\n\nThe TCP server terminal will spit out an error stack trace like\n```shell\n% ./tcp_mojifinder.py \nBuilding index.\nServing on ('127.0.0.1', 2323). Hit CTRL-C to stop.\n From ('127.0.0.1', 63253): 'missing'\nUnhandled exception in client_connected_cb\ntransport: \u003c_SelectorSocketTransport fd=7 read=polling write=\u003cidle, bufsize=0\u003e\u003e\nTraceback (most recent call last):\n  File \"/Users/tim/projects/python/example-code-2e/21-async/mojifinder/./tcp_mojifinder.py\", line 33, in finder\n    results = await search(query, index, writer)  # \u003c13\u003e\n              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^\n  File \"/Users/tim/projects/python/example-code-2e/21-async/mojifinder/./tcp_mojifinder.py\", line 48, in search\n    writer.writelines(lines)  # \u003c4\u003e\n    ~~~~~~~~~~~~~~~~~^^^^^^^\n  File \"/opt/homebrew/Cellar/python@3.13/3.13.5/Frameworks/Python.framework/Versions/3.13/lib/python3.13/asyncio/streams.py\", line 343, in writelines\n    self._transport.writelines(data)\n    ~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^\n  File \"/opt/homebrew/Cellar/python@3.13/3.13.5/Frameworks/Python.framework/Versions/3.13/lib/python3.13/asyncio/selector_events.py\", line 1174, in writelines\n    self._write_ready()\n    ~~~~~~~~~~~~~~~~~^^\n  File \"/opt/homebrew/Cellar/python@3.13/3.13.5/Frameworks/Python.framework/Versions/3.13/lib/python3.13/asyncio/selector_events.py\", line 1090, in _write_sendmsg\n    assert self._buffer, 'Data should not be empty'\n           ^^^^^^^^^^^^\nAssertionError: Data should not be empty\n```","author":{"url":"https://github.com/timheiko","@type":"Person","name":"timheiko"},"datePublished":"2025-08-08T16:06:09.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":2},"url":"https://github.com/61/example-code-2e/issues/61"}