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Chapter 21 - tcp_mojifinder.py crashes on empty search result #61
Description
Issue
If index.search(query) in tcp_mojifinder.py returns empty result, it crashes TCP because asyncio.StreamWriter.writelines(data) raises AssertionError: Data should not be empty error if data is an empty iterable.
How to reproduce
Start the TCP server
% ./tcp_mojifinder.py
Building index.
Serving on ('127.0.0.1', 2323). Hit CTRL-C to stop.
...Connect to the TCP server and send a search query which returns no results, e.g. missing
% nc localhost 2323
?> missing
% The TCP server terminal will spit out an error stack trace like
% ./tcp_mojifinder.py
Building index.
Serving on ('127.0.0.1', 2323). Hit CTRL-C to stop.
From ('127.0.0.1', 63253): 'missing'
Unhandled exception in client_connected_cb
transport: <_SelectorSocketTransport fd=7 read=polling write=<idle, bufsize=0>>
Traceback (most recent call last):
File "/Users/tim/projects/python/example-code-2e/21-async/mojifinder/./tcp_mojifinder.py", line 33, in finder
results = await search(query, index, writer) # <13>
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/Users/tim/projects/python/example-code-2e/21-async/mojifinder/./tcp_mojifinder.py", line 48, in search
writer.writelines(lines) # <4>
~~~~~~~~~~~~~~~~~^^^^^^^
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
self._transport.writelines(data)
~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^
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
self._write_ready()
~~~~~~~~~~~~~~~~~^^
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
assert self._buffer, 'Data should not be empty'
^^^^^^^^^^^^
AssertionError: Data should not be empty
Activity
Chapter 21 - fix tcp_mojifinder.py crashes on empty search result
marktech0813 commentedon Nov 5, 2025
hi, can I handle this issues now?
Fix--tcp_mojifinder.py crashes on empty search result fluentpython#61
marktech0813 commentedon Nov 5, 2025
Please check PR.