A Python client for nREPL, the Clojure network REPL
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nrepl-python-client Travis CI status

Surprisingly, an nREPL client written in Python.

It pretty much works.

Requires Python 2.7 or 3.3. Will work with any nREPL >= 0.2.0 endpoint that uses the default bencode socket transport. Support for other transports should be straightforward, thanks to an unpythonic multimethod thing that nrepl.connect() uses.


Clone from here and use the source, or you can install from PyPI, e.g.:

$ easy_install nrepl-python-client

Or alternatively:

$ pip install nrepl-python-client


Two options, currently. First, explicit, synchronous send/receive of messages to an nREPL endpoint:

>>> import nrepl
>>> c = nrepl.connect("nrepl://localhost:58226")
>>> c.write({"op": "eval", "code": "(reduce + (range 20))"})
>>> c.read()
{u'session': u'7fb4b7a0-f9e5-4f5f-b506-eb2d0a6e21b1', u'ns': u'user', u'value': u'190'}
>>> c.read()
{u'status': [u'done'], u'session': u'7fb4b7a0-f9e5-4f5f-b506-eb2d0a6e21b1'}

WatchableConnection provides a facility vaguely similar to Clojure watches, where a function is called asynchronously when an nREPL response is received if a predicate or a set of pattern-matching criteria provided with that function matches the response. For example (from the tests), this code will asynchronously capture out (i.e. stdout) content from multiple sessions' responses:

c = nrepl.connect("nrepl://localhost:58226")
wc = nrepl.WatchableConnection(c)
outs = {}
def add_resp (session, msg):
    out = msg.get("out", None)
    if out: outs[session].append(out)

def watch_new_sessions (msg, wc, key):
    session = msg.get("new-session")
    outs[session] = []
    wc.watch("session" + session, {"session": session},
            lambda msg, wc, key: add_resp(session, msg))

wc.watch("sessions", {"new-session": None}, watch_new_sessions)
wc.send({"op": "clone"})
wc.send({"op": "clone"})
wc.send({"op": "eval", "session": outs.keys()[0],
         "code": '(println "hello" "%s")' % outs.keys()[0]})
wc.send({"op": "eval", "session": outs.keys()[1],
         "code": '(println "hello" "%s")' % outs.keys()[1]})
#>> {u'fee02643-c5c6-479d-9fb4-d1934cfdd29f': [u'hello fee02643-c5c6-479d-9fb4-d1934cfdd29f\n'],
     u'696130c8-0310-4bb2-a880-b810d2a198d0': [u'hello 696130c8-0310-4bb2-a880-b810d2a198d0\n']}

The watch criteria dicts (e.g. {"new-session": None}) are used to constrain which responses received by the WatchableConnection will be passed to the corresponding callbacks:

  • {"new-session": None} will match any response that has any value in the "new-session" slot
  • {"session": session} will match any response that has the value session in the "session" slot.

Sets may also be used as values in criteria dicts to match responses that contain any of the set's values in the slot that the set is fond in the criteria dict.

Finally, regular predicates may be passed to watch() to handle more complex filtering.

The callbacks provided to watch() must accept three arguments: the matched incoming message, the instance of WatchableConnection, and the key under which the watch was registered.

Send help

  • Make this a more Proper Python Library. I've been away from Python for a loooooong time, and I don't know what the current best practices are around eggs, distribution, and so on. The library is on PyPI, but may not be following the latest best practices for all I know. Open an issue if you see a problem or some corner that could be made better.
  • Fix my busted Python. Like I said, the last time I did any serious Pythoning was in the 2.3 days or something (to use new-style classes, or not, that was the question, etc). If I goofed, open an issue with a fix.

Need Help?

Ping cemerick on freenode irc or twitter if you have questions or would like to contribute patches.


Copyright ©2013 Chas Emerick and other contributors

Distributed under the MIT License. Please see the LICENSE file at the top level of this repo.