sockio

sockio

A python concurrency agnostic socket library.

Helpful when handling with instrumentation which work over TCP and implement simple REQ-REP communication protocols (example: SCPI).

So far implemented REQ-REP and streaming semantics with auto-reconnection facilites.

Base implementation written in asyncio with support for different concurrency models:

• asyncio
• classic blocking API
• future based API
• python 2 compatible blocking API (for those pour souls stuck with python 2)

Installation

From within your favorite python environment:

pip install sockio

Usage

asyncio

import asyncio
from sockio.aio import TCP

async def main():
    sock = TCP('acme.example.com', 5000)
    # Assuming a SCPI complient on the other end we can ask for:
    reply = await sock.write_readline(b'*IDN?\n')
    print(reply)

asyncio.run(main())

classic

from sockio.sio import TCP

sock = TCP('acme.example.com', 5000)
reply = sock.write_readline(b'*IDN?\n')
print(reply)

concurrent.futures

from sockio.sio import TCP

sock = TCP('acme.example.com', 5000, resolve_futures=False)
reply = sock.write_readline(b'*IDN?\n').result()
print(reply)

python 2 compatibility

from sockio.py2 import TCP

sock = TCP('acme.example.com', 5000)
reply = sock.write_readline(b'*IDN?\n').result()
print(reply)

Features

The main goal of a sockio TCP object is to facilitate communication with instruments which listen on a TCP socket.

The most frequent cases include instruments which expect a REQ/REP semantics with ASCII protocols like SCPI. In these cases most commands translate in small packets being exchanged between the host and the instrument. Care has been taken in this library to make sure we reduce latency as much as possible. This translates into the following defaults when creating a TCP object:

• TCP no delay is active. Can be disabled with TCP(..., no_delay=False). This prevents the kernel from applying Nagle's algorithm
• TCP ToS is set to LOWDELAY. This effectively prioritizes our packets if favor of other concurrent communications. Can be disabled with TCP(tos=IPTOS_NORMAL)

Price to pay

Before going in detail about the features, note that this abstraction comes with a price. Intentionally, when comparing with low level socket API, the following features are no longer available:

1. The cability of controlling the two ends of the socket independently (ex: close the write end)
2. While the low level socket.recv() returns empty string when EOF is reached, the TCP class raises ConnectionEOFError instead and closes both ends of the connection.
3. Clever low level operations like os.dup(), make socket non-blocking

REQ-REP semantics

Many instruments out there have a Request-Reply protocol. A sockio TCP provides write_read family of methods which simplify communication with these instruments. These methods are atomic which means different tasks or threads can safely work with the same socket object (although I would question myself why would I be doing that in my library/application).

Auto-reconnection

sock = TCP('acme.example.com', 5000)
reply = await sock.write_readline(b'*IDN?\n')
print(reply)

# ... kill the server connection somehow and bring it back to life again

# You can use the same socket object. It will reconnect automatically
# and work "transparently"
reply = await sock.write_readline(b'*IDN?\n')
print(reply)

The auto-reconnection facility is specially useful when, for example, you move equipement from one place to another, or you need to turn off the equipment during the night (planet Earth thanks you for saving energy!).

Timeout

The TCP constructor provides a connection_timeout that is used when the connection is open and timeout parameter that is taken into account when performing any data I/O operation (read, write, read_writeline, etc). By default, they are both None, meaning infinite timeout.

sock = TCP('acme.example.com', 5000, connection_timeout=0.1, timeout=1)

Additionally, you can override the object timeout on each data I/O method call by providing an alternative timeout parameter:

sock = TCP('acme.example.com', 5000, timeout=1)
# the next call will raise asyncio.TimeoutError if it takes more than 0.1s
reply = await sock.write_readline(b'*IDN?\n', timeout=0.1)
print(reply)

Custom EOL

In line based protocols, sometimes people decide \n is not a good EOL character. A sockio TCP can be customized with a different EOL character. Example:

sock = TCP('acme.example.com', 5000, eol=b'\r')

The EOL character can be overwritten in any of the readline methods. Example:

await sock.write_readline(b'*IDN?\n', eol=b'\r')

Connection event callbacks

You can be notified on connection_made, connection_lost and eof_received events by registering callbacks on the sockio TCP constructor

This is particularly useful if, for example, you want a specific procedure to be executed every time the socket is reconnected to make sure your configuration is right. Example:

async def connected():
    await sock.write(b'ACQU:TRIGGER HARDWARE\n')
    await sock.write(b'DISPLAY OFF\n')

sock = TCP('acme.example.com', 5000, on_connection_made=connected)

(see examples/req-rep/client.py)

Connection event callbacks are not available in python 2 compatibility module.

Streams

sockio TCPs are asynchronous iterable objects. This means that line streaming is as easy as:

sock = TCP('acme.example.com', 5000, eol=b'\r')

async for line in sock:
    print(line)

Streams are not available in python 2 compatibility module. Let me know if you need them by writing an issue. Also feel free to make a PR!

Missing features

• Connection retries
• trio event loop
• curio event loop

Join the party by bringing your own concurrency library with a PR!

I am looking in particular for implementations over trio and curio.