Part 2: Netrefs and Exceptions

In Part 1: Introduction to Classic RPyC, we have seen how to use rpyc classic connection to do almost anything remotely.

So far everything seemed normal. Now it’s time to get our hands dirty and understand more what happens under the hood!


Start a classic server using:

python bin/

And connect your client:

>>> import rpyc
>>> conn = rpyc.classic.connect("localhost")


We know that we can use conn.modules.sys to access the sys module the server… But what kind of magical object is that thing anyway?

>>> type(conn.modules.sys)
<netref class 'builtins.module'>
>>> type(conn.modules.sys.path)
<netref class 'builtins.list'>
>>> type(conn.modules.os.path.abspath)
<netref class 'builtins.function'>

Voila, netrefs (network references, also known as transparent object proxies) are special objects that delegate everything done on them locally to the corresponding remote objects. Netrefs may not be real lists of functions or modules, but they “do their best” to look and feel like the objects they point to… in fact, they even fool python’s introspection mechanisms!

>>> isinstance(conn.modules.sys.path, list)

>>> import inspect
>>> inspect.isbuiltin(conn.modules.os.listdir)
>>> inspect.isfunction(conn.modules.os.path.abspath)
>>> inspect.ismethod(conn.modules.os.path.abspath)
>>> inspect.ismethod(conn.modules.sys.stdout.write)

Cool, eh?

We all know that the best way to understand something is to smash it, slice it up and spill the contents into the world! So let’s do that:

>>> dir(conn.modules.sys.path)
['____conn__', '____id_pack__', '__add__', '__class__', '__contains__', '__delattr__',
'__delitem__', '__delslice__', '__doc__', '__eq__', '__ge__', '__getattribute__',
'__getitem__', '__getslice__', '__gt__', '__hash__', '__iadd__', '__imul__',
'__init__', '__iter__', '__le__', '__len__', '__lt__', '__mul__', '__ne__', '__new__',
'__reduce__', '__reduce_ex__', '__repr__', '__reversed__', '__rmul__', '__setattr__',
'__setitem__', '__setslice__', '__str__', 'append', 'count', 'extend', 'index', 'insert',
'pop', 'remove', 'reverse', 'sort']

In addition to some expected methods and properties, you will have noticed ____conn__ and ____id_pack__. These properties store over which connection the object should be resolved and an identifier that allows the server to lookup the object from a dictionary.


Let’s continue on this exhilarating path of destruction. After all, things are not always bright, and problems must be dealt with. When a client makes a request that fails (an exception is raised on the server side), the exception propagates transparently to the client. Have a look at this snippet:

>>> conn.modules.sys.path[300]         # there are only 12 elements in the list...
======= Remote traceback =======
Traceback (most recent call last):
  File "D:\projects\rpyc\core\", line 164, in _dispatch_request
    res = self._handlers[handler](self, *args)
  File "D:\projects\rpyc\core\", line 321, in _handle_callattr
    return attr(*args, **dict(kwargs))
IndexError: list index out of range

======= Local exception ========
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "D:\projects\rpyc\core\", line 86, in method
    return self.____sync_req__(consts.HANDLE_CALLATTR, name, args, kwargs)
  File "D:\projects\rpyc\core\", line 53, in ____sync_req__
    return self.____conn__.sync_request(handler, self.____id_pack__, *args)
  File "D:\projects\rpyc\core\", line 224, in sync_request
  File "D:\projects\rpyc\core\", line 196, in serve
    self._serve(msg, seq, args)
  File "D:\projects\rpyc\core\", line 189, in _serve
    self._dispatch_exception(seq, args)
  File "D:\projects\rpyc\core\", line 182, in _dispatch_exception
    raise obj
IndexError: list index out of range

As you can see, we get two tracebacks: the remote one, showing what went wrong on the server, and a local one, showing what we did to cause it.

Custom Exception Handling Example

The server example:

import rpyc
import urllib.error
from rpyc.utils.server import OneShotServer

class HelloService(rpyc.Service):
    def exposed_foobar(self, remote_str):
        raise urllib.error.URLError("test")

if __name__ == "__main__":
    server = OneShotServer(
        protocol_config={'import_custom_exceptions': True}

The client example:

import rpyc
import urllib.error
rpyc.core.vinegar._generic_exceptions_cache["urllib.error.URLError"] = urllib.error.URLError

if __name__ == "__main__":
    conn = rpyc.connect("localhost", 12345)
    except urllib.error.URLError:
        print('caught a URLError')

Continue to Part 3: Services and New Style RPyC