Python code to support TCP/IP communication using asyncio.

User Guide

The basic classes are:

  • Client: a TCP/IP client. One use case is CSCs that communicate via TCP/IP with low-level controllers. Providing a connect_callback makes it easy to report the connection state as an event, and also allows you to detect and handle a failed connection.

  • OneClientReadLoopServer: an abstract TCP/IP server that only accepts a single client connection at a time (rejecting extra attempts to connect) and runs a read loop while connected. One common use case is mock servers in CSC packages. Override the read_and_dispatch method in a subclass to use it.

  • OneClientServer: the concrete parent class of OneClientReadLoopServer; a server with no read loop. Most users will find OneClientReadLoopServer more convenient.

  • BaseOneClientServerTestCase: a base class for unit tests of servers.

A simple example of using Client and OneClientReadLoopServer together is provided in tests/ This example is written as a unit test in order to avoid bit rot.

The server classes are designed so to be inherited from, in order to add application-specific behavior. An example of inheriting from OneClientReadLoopServer is MockDomeController in ts_atdome. One can also inherit from Client; see CommandTelemetryClient in ts_hexrotcomm for an example.

Strings and JSON

In the Python standard TCP/IP library, all reading and writing is done using bytes. To write a str you must encode it to bytes and, usually, append a terminator, then write the resulting bytes. To read a str you usually readuntil a terminator, decode the read bytes to a str and, usually, strip the terminator.

However, Client and the server classes offer convenience methods to do this for you, as well as methods to read and write json-encoded data (via the common base class BaseClientOrServer):

The encoding and terminator constructor arguments allow you specify the desired encoding and terminator for these methods. The default encoding and terminator will work in many cases.

Binary Structs

To read and write a binary struct, define a ctypes.Structure and use BaseClientOrServer.read_into and BaseClientOrServer.write_from. For example (where client_or_server is an instance of Client or OneClientServer):

import ctypes

class TrivialStruct(ctypes.Structure):
    _fields_ = [
        ("a_double", ctypes.c_double),
        ("three_ints", ctypes.c_int64 * 3),

# Reading
data = TrivialStruct()
    await client_or_server.read_into(data)
except (asyncio.IncompleteReadError, ConnectionError):
    # Connection is closed
    ...handle the closed connection

# Writing
data = TrivialStruct(a_double=1.2e3, three_ints=(-1, 2, 3))
await client_or_server.write_from(data)

Details of using asyncio.StreamReader and asyncio.StreamWriter

The asyncio documentation is good, but I found a few things surprising. Client and OneClientServer are designed to hide some of these details, but this information may still be helpful.

In the examples below reader is an asyncio.StreamReader and writer is an asyncio.StreamWriter.

  • When you write data, be sure to call drain, else the data may not actually be written:

    await writer.drain()
  • When you read data, check for a closed connection by catching (asyncio.IncompleteReadError, ConnectionError). For example:

        data = reader.readuntil(tcpip.DEFAULT_TERMINATOR) # or or reader.readline
        # If data is empty then the connection is closed and reader.at_eof() will be True.
        # If there was unread data on the reader, then a partial result will be returned.
        # reader.at_eof() will be true if the result is partial.
        # There are other read methods that handle EOF differently.
        # reader.readexactly and tcpip.read_into raise asyncio.IncompleteReadError
        # if the connection is closed before enough data is available.
    except (asyncio.IncompleteReadError, ConnectionError):
        # Connection is closed.
        # Do something to tell your application not to write any more data,
        # such as cancelling the write loop...
        # Close the writer
        await lsst.ts.tcpip.close_stream_writer(writer)
        # Deal with lack of data, e.g. by raising an exception,
        # or returning None, or...

    A few read methods do not raise asyncio.IncompleteReadError. See asyncio.StreamReader documentation for details.

  • You can only reliably detect a closed connection in the stream reader; writing to stream writer after the other end has disconnected does not raise an exception.

  • When a reader is closed, reader.at_eof() is not false right away. It will go false if you try to read data, or if you simply wait long enough.

  • When you are done with a TCP/IP stream, close the stream writer (you cannot close a stream reader). To close a stream writer, call close_stream_writer:

    from lsst.ts import tcpip
    await tcpip.close_stream_writer(writer)

    This convenience function calls asyncio.StreamWriter.close and asyncio.StreamWriter.wait_closed. It also catches and ignores ConnectionError, which is raised if the writer is already closed.

    Warning: asyncio.StreamWriter.wait_closed may raise asyncio.CancelledError if the writer is being closed. close_stream_writer does not catch and ignore that exception, because I felt that was too risky.

Python API reference

lsst.ts.tcpip Package



Close an asyncio.StreamWriter and wait for it to finish closing.

read_into(reader, struct)

Read binary data from a stream reader into a ctypes.Structure.

write_from(writer, *structs)

Write binary data from one or more ctypes.Structures to a stream writer.


BaseClientOrServer(*, log[, ...])

Abstract base class for a TCP/IP client or server.


Base class for unit tests of subclasses of OneClienServer.

Client(host, port, log[, connect_callback, ...])

A TCP/IP socket client.

OneClientReadLoopServer(*, port[, host, ...])

A OneClientServer that reads and processes data in a loop.

OneClientServer(host, port, log[, ...])

A TCP/IP socket server that serves a single client.

TestOneClientReadLoopServer(*, port[, host, ...])

A simple implementation of OneClientReadLoopServer for unit tests.


lsst.ts.tcpip is developed at You can find Jira issues for this module at labels=ts_tcpip.

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