API in a nutshell

execnet ad-hoc instantiates local and remote Python interpreters. Each interpreter is accessible through a Gateway which manages code and data communication. Channels allow to exchange data between the local and the remote end. Groups help to manage creation and termination of sub interpreters.

Gateways: bootstrapping Python interpreters

All Gateways are instantiated via a call to makegateway() passing it a gateway specification or URL.

Here is an example which instantiates a simple Python subprocess:

>>> gateway = execnet.makegateway()

gateways allow to remote execute code and exchange data bidirectionally.

examples for valid gateway specifications

• ssh=wyvern//python=python3.3//chdir=mycache specifies a Python3.3 interpreter on the host wyvern. The remote process will have mycache as its current working directory.
• ssh=-p 5000 myhost makes execnet pass “-p 5000 myhost” arguments to the underlying ssh client binary, effectively specifying a custom port.
• vagrant_ssh=default makes execnet connect to a Vagrant VM named default via SSH through Vagrant’s vagrant ssh command. It supports the same additional parameters as regular SSH connections.
• popen//python=python2.6//nice=20 specification of a python subprocess using the python2.6 executable which must be discoverable through the system PATH; running with the lowest CPU priority (“nice” level). By default current dir will be the current dir of the instantiator.
• popen//dont_write_bytecode uses the same executable as the current Python, and also passes the -B flag on startup, which tells Python not write .pyc or .pyo files. Note that this only works under CPython 2.6 and newer.
• popen//env:NAME=value specifies a subprocess that uses the same interpreter as the one it is initiated from and additionally remotely sets an environment variable NAME to value.
• popen//execmodel=eventlet specifies a subprocess that uses the same interpreter as the one it is initiated from but will run the other side using eventlet for handling IO and dispatching threads.
• socket=192.168.1.4:8888 specifies a Python Socket server process that listens on 192.168.1.4:8888

New in version 1.5.

• vagarant_ssh opens a python interpreter via the vagarant ssh command

remote_exec: execute source code remotely

All gateways offer a simple method to execute source code in the instantiated subprocess-interpreter:

It is allowed to pass a module object as source code in which case it’s source code will be obtained and get sent for remote execution. remote_exec returns a channel object whose symmetric counterpart channel is available to the remotely executing source.

Gateway.reconfigure([py2str_as_py3str=True, py3str_as_py2str=False])

reconfigures the string-coercion behaviour of the gateway

Channels: exchanging data with remote code

A channel object allows to send and receive data between two asynchronously running programs.

Grouped Gateways and robust termination

All created gateway instances are part of a group. If you call execnet.makegateway it actually is forwarded to the execnet.default_group. Group objects are container objects (see group examples) and manage the final termination procedure:

This method is implicitely called for each gateway group at process-exit, using a small timeout. This is fine for interactive sessions or random scripts which you rather like to error out than hang. If you start many processes then you often want to call group.terminate() yourself and specify a larger or not timeout.

threading models: gevent, eventlet, thread

New in version 1.2: (status: experimental!)

execnet supports “thread”, “eventlet” and “gevent” as thread models on each of the two sides. You need to decide which model to use before you create any gateways:

# content of threadmodel.py
import execnet
# locally use "eventlet", remotely use "thread" model
execnet.set_execmodel("eventlet", "thread")
gw = execnet.makegateway()
print (gw)
print (gw.remote_status())
print (gw.remote_exec("channel.send(1)").receive())

You need to have eventlet installed in your environment and then you can execute this little test file:

$ python threadmodel.py
<Gateway id='gw0' receive-live, eventlet model, 0 active channels>
<RInfo 'numchannels=0, numexecuting=0, execmodel=thread'>
1

Note

With python3 you can (as of December 2013) only use the thread model because neither eventlet-0.14.0 nor gevent-1.0 support Python3. When they start to support Python3, execnet will probably just work because it is itself Python3 compatible.

How to execute in the main thread

When the remote side of a gateway uses the ‘thread’ model, execution will preferably run in the main thread. This allows GUI loops or other code to behave correctly. If you, however, start multiple executions concurrently, they will run in non-main threads.

remote_status: get low-level execution info

All gateways offer a simple method to obtain some status information from the remote side.

Calling this method tells you e.g. how many execution tasks are queued, how many are executing and how many channels are active.

rsync: synchronise filesystem with remote

execnet implements a simple efficient rsyncing protocol. Here is a basic example for using RSync:

rsync = execnet.RSync('/tmp/source')
gw = execnet.makegateway()
rsync.add_target(gw, '/tmp/dest')
rsync.send()

And here is API info about the RSync class.

Debugging execnet

By setting the environment variable EXECNET_DEBUG you can configure a tracing mechanism:

EXECNET_DEBUG=1:  write per-process trace-files to execnet-debug-PID EXECNET_DEUBG=2:  perform tracing to stderr (popen-gateway slaves will send this to their instantiator)

cross-interpreter serialization of python objects

New in version 1.1.

Execnet exposes a function pair which you can safely use to store and load values from different Python interpreters (e.g. Python2 and Python3, PyPy and Jython). Here is a basic example:

>>> import execnet
>>> dump = execnet.dumps([1,2,3])
>>> execnet.loads(dump)
[1,2,3]

For more examples see Dumping and loading values across interpreter versions.