Tips & Tricks

Best practices

Avoid circular communication topologies.

When you can have a circular communication pattern in your system (A–>B–>C–>A) this can cause some problems:

  • when reusing a proxy it causes a deadlock because the proxy is already being used for an active remote call. See the deadlock example.
  • with the multiplex servertype, the server itself may also block for all other remote calls because the handling of the first is not yet completed.

Avoid circularity, or use oneway method calls on at least one of the links in the chain. Another possible way out of a lock situation is to set COMMTIMEOUT so that after a certain period in a locking situation the caller aborts with a TimeoutError, effectively breaking the deadlock.

‘After X simultaneous proxy connections, Pyro seems to freeze!’ Fix: Release your proxies when you can.

A connected proxy that is unused takes up resources on the server. In the case of the threadpool server type, it locks up a single thread. If you have too many connected proxies at the same time, the server may run out of threads and won’t be able to accept new connections.

You can use the THREADPOOL_SIZE config item to increase the maximum number of threads that Pyro will use. Or use the multiplex server instead, which doesn’t have this limitation. Another option is to set COMMTIMEOUT to a certain value on your server, which will free up unused connections after the given time. But your client code may now crash with a TimeoutError or ConnectionClosedError when it tries to use a proxy that worked earlier. You can use Pyro’s autoreconnect feature to work around this but it makes the code more complex.

It is however advised to close (release) proxies that your program no longer needs, to free resources both in the client and in the server. Don’t worry about reconnecting, Pyro does that automatically for you once the proxy is used again. You can use explicit _pyroRelease calls or use the proxy from within a context manager. It’s not a good idea to release it after every single remote method call though, because then the cost of reconnecting the socket can be bad for performance.

Avoid large binary blobs over the wire.

Pyro is not designed to efficiently transfer large amounts of binary data over the network. Try to find another protocol that better suits this requirement. Read Binary data transfer / file transfer for some more details about this. How to deal with Numpy data (large or small) is explained here Pyro and Numpy.

Note that Pyro has a 2 gigabyte message size limitation at this time.

Minimize object graphs that travel over the wire.

Pyro will serialize the whole object graph you’re passing, even when only a tiny fraction of it is used on the receiving end. Be aware of this: it may be necessary to define special lightweight objects for your Pyro interfaces that hold the data you need, rather than passing a huge object structure. It’s good design practice as well to have an “external API” that is different from your internal code, and tuned for minimal communication overhead or complexity.

Consider using basic data types instead of custom classes.

Because Pyro serializes the objects you’re passing, it needs to know how to serialize custom types. While you can teach Pyro about these (see Changing the way your custom classes are (de)serialized) it may sometimes be easier to just use a builtin datatype instead. For instance if you have a custom class whose state essentially is a set of numbers, consider then that it may be easier to just transfer a set or a list of those numbers rather than an instance of your custom class. It depends on your class and data of course, and whether the receiving code expects just the list of numbers or really needs an instance of your custom class.


If you configure it (see Overview of Config Items) Pyro will write a bit of debug information, errors, and notifications to a log file. It uses Python’s standard logging module for this (See ). Once enabled, your own program code could use Pyro’s logging setup as well. But if you want to configure your own logging, make sure you do that before any Pyro imports. Then Pyro will skip its own autoconfig.

A little example to enable logging by setting the required environment variables from the shell:

$ export PYRO_LOGFILE=pyro.log
$ python

Another way is by modifiying os.environ from within your code itself, before any import of Pyro4 is done:

import os
os.environ["PYRO_LOGFILE"] = "pyro.log"
os.environ["PYRO_LOGLEVEL"] = "DEBUG"

import Pyro4
# do stuff...

Finally, it is possible to initialize the logging by means of the standard Python logging module only, but then you still have to tell Pyro4 what log level it should use (or it won’t log anything):

import logging
logging.basicConfig()  # or your own sophisticated setup
# ... set level of other logger names as desired ...

import Pyro4
# do stuff...

The various logger names are similar to the module that uses the logger, so for instance logging done by code in Pyro4.core will use a logger category name of Pyro4.core. Look at the top of the source code of the various modules from Pyro to see what the exact names are.

Multiple network interfaces

This is a difficult subject but here are a few short notes about it. At this time, Pyro doesn’t support running on multiple network interfaces at the same time. You can bind a deamon on INADDR_ANY ( though, including the name server. But weird things happen with the URIs of objects published through these servers, because they will point to and your clients won’t be able to connect to the actual objects.

The name server however contains a little trick. The broadcast responder can also be bound on and it will in fact try to determine the correct ip address of the interface that a client needs to use to contact the name server on. So while you cannot run Pyro daemons on (to respond to requests from all possible interfaces), sometimes it is possible to run only the name server on The success ratio of all this depends heavily on your network setup.

Same major Python version required when using pickle, cloudpickle, dill or marshal

When Pyro is configured to use pickle, cloudpickle, dill or marshal as its serialization format, it is required to have the same major Python versions on your clients and your servers. Otherwise the different parties cannot decipher each others serialized data. This means you cannot let Python 2.x talk to Python 3.x with Pyro when using these serializers. However it should be fine to have Python 3.3 talk to Python 3.4 for instance. It may still be required to specify the pickle or dill protocol version though, because that needs to be the same on both ends as well. For instance, Python 3.4 introduced version 4 of the pickle protocol and as such won’t be able to talk to Python 3.3 which is stuck on version 3 pickle protocol. You’ll have to tell the Python 3.4 side to step down to protocol 3. There is a config item for that. The same will apply for dill protocol versions. If you are using cloudpickle, you can just set the pickle protocol version (as pickle is used under the hood).

The implementation independent serialization protocols serpent and json don’t have these limitations.

Wire protocol version

Here is a little tip to find out what wire protocol version a given Pyro server is using. This could be useful if you are getting ProtocolError: invalid data or unsupported protocol version or something like that. It also works with Pyro 3.x.


This is a way to figure out the protocol version number a given Pyro server is using: by reading the first 6 bytes from the server socket connection. The Pyro daemon will respond with a 4-byte string “PYRO” followed by a 2-byte number that is the protocol version used:

$ nc <pyroservername> <pyroserverport> | od -N 6 -t x1c
0000000  50  59  52  4f  00  05
          P   Y   R   O  \0 005

This one is talking protocol version 00 05 (5). This low number means it is a Pyro 3.x server. When you try it on a Pyro 4 server:

$ nc <pyroservername> <pyroserverport> | od -N 6 -t x1c
0000000  50  59  52  4f  00  2c
          P   Y   R   O  \0   ,

This one is talking protocol version 00 2c (44). For Pyro4 the protocol version started at 40 for the first release and is now at 46 for the current release at the time of writing.


To find out the protocol version that your client code is using, you can use this:

$ python -c "import Pyro4.constants as c; print(c.PROTOCOL_VERSION)"

Asynchronous (‘future’) normal function calls

Pyro provides an async proxy to call remote methods asynchronously, see Asynchronous (‘future’) remote calls & call chains. For normal Python code, Python provides a similar mechanism in the form of the Pyro4.futures.Future class (also available as Pyro4.Future). With a syntax that is slightly different from normal method calls, it provides the same asynchronous function calls as the async proxy has. Note that Python itself has a similar thing in the standard library since version 3.2, see . However Pyro’s Future object is available on older Python versions too. It works slightly differently and perhaps a little bit easier as well.

You create a Future object for a callable that you want to execute in the background, and receive its results somewhere in the future:

def add(x,y):
    return x+y

futurecall = Pyro4.Future(add)
result = futurecall(4,5)
# do some other stuff... then access the value
summation = result.value

Actually calling the Future object returns control immediately and results in a Pyro4.futures.FutureResult object. This is the exact same class as with the async proxy. The most important attributes are value, ready and the wait method. See Asynchronous (‘future’) remote calls & call chains for more details.

You can also chain multiple calls, so that the whole call chain is executed sequentially in the background. You can do this directly on the Future object, with the Pyro4.futures.Future.then() method. It has the same signature as the then method from the FutureResult class:

futurecall = Pyro4.Future(something) \
    .then(somethingelse, 44) \
    .then(lastthing, optionalargument="something")

There’s also a Pyro4.futures.Future.iferror() method that allows you to register a callback to be invoked when an exception occurs. This method also exists on the FutureResult class. See the futures example for more details and example code.

You can delay the execution of the future for a number of seconds via the Pyro4.futures.Future.delay() method, and you can cancel it altogether via the Pyro4.futures.Future.cancel() method (which only works if the future hasn’t been evaluated yet).

DNS setup

Pyro depends on a working DNS configuration, at least for your local hostname (i.e. ‘pinging’ your local hostname should work). If your local hostname doesn’t resolve to an IP address, you’ll have to fix this. This can usually be done by adding an entry to the hosts file. For OpenSUSE, you can also use Yast to fix it (go to Network Settings, enable “Assign hostname to loopback IP”).

If Pyro detects a problem with the dns setup it will log a WARNING in the logfile (if logging is enabled), something like: weird DNS setup: your-computer-hostname resolves to localhost (127.x.x.x)

Pyro behind a NAT router/firewall

You can run Pyro behind a NAT router/firewall. Assume the external hostname is ‘’ and the external port is 5555. Also assume the internal host is ‘server1.lan’ and the internal port is 9999. You’ll need to have a NAT rule that maps to server1.lan:9999. You’ll need to start your Pyro daemon, where you specify the nathost and natport arguments, so that Pyro knows it needs to ‘publish’ URIs containing that external location instead of just using the internal addresses:

# running on server1.lan
d = Pyro4.Daemon(port=9999, nathost="", natport=5555)
uri = d.register(Something, "thing")
print(uri)     # ""

As you see, the URI now contains the external address.

Pyro4.core.Daemon.uriFor() by default returns URIs with a NAT address in it (if nathost and natport were used). You can override this by setting nat=False:

# d = Pyro4.Daemon(...)
print(d.uriFor("thing"))                # ""
print(d.uriFor("thing", nat=False))     # "PYRO:thing@localhost:36124"
uri2 = d.uriFor(uri.object, nat=False)  # get non-natted uri

The Name server can also be started behind a NAT: it has a couple of command line options that allow you to specify a nathost and natport for it. See Starting the Name Server.


The broadcast responder always returns the internal address, never the external NAT address. Also, the name server itself won’t translate any URIs that are registered with it. So if you want it to publish URIs with ‘external’ locations in them, you have to tell the Daemon that registers these URIs to use the correct nathost and natport as well.


In some situations the NAT simply is configured to pass through any port one-to-one to another host behind the NAT router/firewall. Pyro facilitates this by allowing you to set the natport to 0, in which case Pyro will replace it by the internal port number.

‘Failed to locate the nameserver’ or ‘Connection refused’ error, what now?

Usually when you get an error like “failed to locate the name server” or “connection refused” it is because there is a configuration problem in your network setup, such as a firewall blocking certain network connections. Sometimes it can be because you configured Pyro wrong. A checklist to follow to diagnose your issue can be as follows:

  • is the name server on a network interface that is visible on the network? If it’s on localhost, then it’s definitely not! (check the URI)
  • is the Pyro object’s daemon on a network interface that is visible on the network? If it’s on localhost, then it’s definitely not! (check the URI)
  • with what URI is the Pyro object registered in the Name server? See previous item.
  • can you ping the server from your client machine?
  • can you telnet to the given host+port from your client machine?
  • dealing with IPV4 versus IPV6: do both client and server use the same protocol?
  • is the server’s ip address as shown one of an externally reachable network interface?
  • do you have your server behind a NAT router? See Pyro behind a NAT router/firewall.
  • do you have a firewall or packetfilter running that prevents the connection?
  • do you have the same Pyro versions on both server and client?
  • what does the pyro logfiles tell you (enable it via the config items on both the server and the client, including the name server. See Logging.
  • (if not using the default:) do you have a compatible serializer configuration?
  • (if not using the default:) do you have a symmetric hmac key configuration?
  • can you obtain a few bytes from the wire using netcat, see Wire protocol version.

Binary data transfer / file transfer

Pyro is not meant to transfer large amounts of binary data (images, sound files, video clips): the protocol is not designed nor optimized for these kinds of data. The occasional transmission of such data is fine (Flame: Foreign Location Automatic Module Exposer even provides a convenience method for that, if you like: Pyro4.utils.flame.Flame.sendfile()) but if you’re dealing with a lot of them or with big files, it is usually better to use something else to do the actual data transfer (file share+file copy, ftp, http, scp, rsync).

Also, Pyro has a 2 gigabyte message size limitation at this time (if your Python implementation and system memory even allow the process to reach this size). You can avoid this problem if you use the remote iterator feature (return chunks via an iterator or generator function and consume them on demand in your client).


Serpent and binary data: If you do transfer binary data using the serpent serializer, you have to be aware of the following. The wire protocol is text based so serpent has to encode any binary data. It uses base-64 to do that. This means on the receiving side, instead of the raw bytes, you get a little dictionary like this instead: {'data': 'aXJtZW4gZGUgam9uZw==', 'encoding': 'base64'} Your client code needs to be aware of this and to get the original binary data back, it has to base-64 decode the data element by itself. This is perhaps done the easiest by using the serpent.tobytes helper function from the serpent library, which will convert the result to actual bytes if needed (and leave it untouched if it is already in bytes form)

The following table is an indication of the relative speeds when dealing with large amounts of binary data. It lists the results of the hugetransfer example, using python 3.5, over a 1000 Mbps LAN connection:

serializer str mb/sec bytes mb/sec bytearray mb/sec bytearray w/iterator
pickle 77.8 79.6 69.9 35.0
marshal 71.0 73.0 73.0 37.8
serpent 25.0 14.1 13.5 13.5
json 31.5 not supported not supported not supported

The json serializer only works with strings, it can’t serialize binary data at all. The serpent serializer can, but read the note above about why it’s quite inefficent there. Marshal and pickle are relatively efficient, speed-wise. But beware, when using pickle, there’s quite a difference in dealing with various types:

pickle datatype differences

Python 2.x: efficient; directly encoded as a byte sequence, because that’s what it is. Python 3.x: inefficient; encoded in UTF-8 on the wire, because it is a unicode string.
Python 2.x: same as str (Python 2.7) Python 3.x: efficient; directly encoded as a byte sequence.
Inefficient; encoded as UTF-8 on the wire (pickle does this in both Python 2.x and 3.x)
array("B") (array of unsigned ints of size 1)
Python 2.x: very inefficient; every element is encoded as a separate token+value. Python 3.x: efficient; uses machine type encoding on the wire (a byte sequence).
numpy arrays
usually cannot be transferred directly, see Pyro and Numpy.

Alternative: avoid most of the serialization overhead by (ab)using annotations

Pyro allows you to add custom annotation chunks to the request and response messages (see Message annotations). Because these are binary chunks they will not be passed through the serializer at all. There is a 64Kb total annotation size limit on messages though, so you have to split up larger files. The filetransfer example contains fully working example code to see this in action. It combines this with the remote iterator capability of Pyro to easily get all chunks of the file. It has to split up the file in small chunks but is still quite a bit faster than transmitting bytes through regular response values. Also it is using only regular Pyro high level logic and no low level network or socket code.

Alternative: integrating raw socket transfer in a Pyro server

It is possible to get data transfer speeds that are close to the limit of your network adapter by doing the actual data transfer via low-level socket code and everything else via Pyro. This keeps the amount of low-level code to a minimum. Have a look at the filetransfer example again, to see a possible way of doing this. It creates a special Daemon subclass that uses Pyro for everything as usual, but for actual file transfer it sets up a dedicated temporary socket connection over which the file data is transmitted.

MSG_WAITALL socket option

Pyro will use the MSG_WAITALL socket option to receive large messages, if it decides that the feature is available and working correctly. This avoids having to use a slower function that needs a loop to get all data. On most systems that define the socket.MSG_WAITALL symbol, it works fine, except on Windows: even though the option is there, it doesn’t work reliably. Pyro thus won’t use it by default on Windows, and will use it by default on other systems. You should set the USE_MSG_WAITALL config item to False yourself, if you find that your system has an unreliable implementation of this socket option. Please let me know what system (os/python version) it is so we could teach Pyro to select the correct option automatically in a new version.

IPV6 support

Pyro4 supports IPv6 since version 4.18. You can use IPv6 addresses in the same places where you would normally have used IPv4 addresses. There’s one exception: the address notation in a Pyro URI. For a numeric IPv6 address in a Pyro URI, you have to enclose it in brackets. For example:


points at a Pyro object located on the IPv6 “::1” address (localhost). When Pyro displays a numeric IPv6 location from an URI it will also use the bracket notation. This bracket notation is only used in Pyro URIs, everywhere else you just type the IPv6 address without brackets.

To tell Pyro to prefer using IPv6 you can use the PREFER_IP_VERSION config item. It is set to 4 by default, for backward compatibility reasons. This means that unless you change it to 6 (or 0), Pyro will be using IPv4 addressing.

There is a new method to see what IP addressing is used: Pyro4.socketutil.getIpVersion(), and a few other methods in Pyro4.socketutil gained a new optional argument to tell it if it needs to deal with an ipv6 address rather than ipv4, but these are rarely used in client code.

Pyro and Numpy

Pyro doesn’t support Numpy out of the box. You’ll see certain errors occur when trying to use numpy objects (ndarrays, etcetera) with Pyro:

TypeError: array([1, 2, 3]) is not JSON serializable
TypeError: don't know how to serialize class <type 'numpy.ndarray'>
TypeError: don't know how to serialize class <class 'numpy.int64'>

These errors are caused by Numpy datatypes not being serializable by serpent or json serializers. There are several reasons these datatypes are not supported out of the box:

  1. numpy is a third party library and there are many, many others. It is not Pyro’s responsibility to understand all of them.
  2. numpy is often used in scenarios with large amounts of data. Sending these large arrays over the wire through Pyro is often not the best solution. It is not useful to provide transparent support for numpy types when you’ll be running into trouble often such as slow calls and large network overhead.
  3. Pyrolite (Pyrolite - client library for Java and .NET) would have to get numpy support as well and that is a lot of work (because every numpy type would require a mapping to the appropriate Java or .NET type)

If you understand this but still want to use numpy with Pyro, and pass numpy objects over the wire, you can do it! Choose one of the following options:

  1. Don’t use Numpy datatypes as arguments or return values. Convert them to standard Python datatypes before using them in Pyro. So instead of just na = numpy.array(...); return na;, use this instead: return na.tolist(). Or perhaps even return array.array('i', na) (serpent understands array.array just fine). Note that the elements of a numpy array usually are of a special numpy datatype as well (such as numpy.int32). If you don’t convert these individually as well, you will still get serialization errors. That is why something like list(na) doesn’t work: it seems to return a regular python list but the elements are still numpy datatypes. You have to use the full conversions as mentioned earlier. Note that you’ll have to do a bit more work to deal with multi-dimensional arrays: you have to convert the shape of the array separately.
  2. If possible don’t return the whole array. Redesign your API so that you might perhaps only return a single element from it, or a few, if that is all the client really needs.
  3. Tell Pyro to use pickle, cloudpickle or dill as serializer. These serializers can deal with numpy datatypes out of the box. However they have security implications. See Security. (If you choose to use them anyway, also be aware that you must tell your name server about it as well, see Using the name server with pickle, cloudpickle or dill serializers)

Pyro via HTTP and JSON

Pyro provides a HTTP gateway server that translates HTTP requests into Pyro calls. It responds with JSON messages. This allows clients (including web browsers) to use a simple http interface to call Pyro objects. Pyro’s JSON serialization format is used so the gateway simply passes the JSON response messages back to the caller. It also provides a simple web page that shows how stuff works.

Starting the gateway:

You can launch the HTTP gateway server via the command line tool. This will create a web server using Python’s wsgiref server module. Because the gateway is written as a wsgi app, you can also stick it into a wsgi server of your own choice. Import pyro_app from Pyro4.utils.httpgateway to do that (that’s the app you need to use).

synopsys: python -m Pyro4.utils.httpgateway [options] (or simply: pyro4-httpgateway [options])

A short explanation of the available options can be printed with the help option:

-h, –help

Print a short help message and exit.

Most other options should be self explanatory; you can set the listening host and portname etc. An important option is the exposed names regex option: this controls what objects are accessible from the http gateway interface. It defaults to something that won’t just expose every internal object in your system. If you want to toy a bit with the examples provided in the gateway’s web page, you’ll have to change the option to something like: r'Pyro\.|test\.' so that those objects are exposed. This regex is the same as used when listing objects from the name server, so you can use the nsc tool to check it (with the listmatching command).

Using the gateway:

You request the url http://localhost:8080/pyro/<<objectname>>/<<method>> to invoke a method on the object with the given name (yes, every call goes through a naming server lookup). Parameters are passed via a regular query string parameter list (in case of a GET request) or via form post parameters (in case of a POST request). The response is a JSON document. In case of an exception, a JSON encoded exception object is returned. You can easily call this from your web page scripts using XMLHttpRequest or something like JQuery’s $.ajax(). Have a look at the page source of the gateway’s web page to see how this could be done. Note that you have to comply with the browser’s same-origin policy: if you want to allow your own scripts to access the gateway, you’ll have to make sure they are loaded from the same website.

The http gateway server is stateless at the moment. This means every call you do will end be processed by a new Pyro proxy in the gateway server. This is not impacting your client code though, because every call that it does is also just a stateless http call. It only impacts performance: doing large amounts of calls through the http gateway will perform much slower as the same calls processed by a native Pyro proxy (which you can instruct to operate in batch mode as well). However because Pyro is quite efficient, a call through the gateway is still processed in just a few milliseconds, naming lookup and json serialization all included.

Special http request headers:

  • X-Pyro-Options: add this header to the request to set certain pyro options for the call. Possible values (comma-separated):
    • oneway: force the Pyro call to be a oneway call and return immediately. The gateway server still returns a 200 OK http response as usual, but the response data is empty. This option is to override the semantics for non-oneway method calls if you so desire.
  • X-Pyro-Gateway-Key: add this header to the request to set the http gateway key. You can also set it on the request with a $key=.... querystring parameter.

Special Http response headers:

  • X-Pyro-Correlation-Id: contains the correlation id Guid that was used for this request/response.

Http response status codes:

  • 200 OK: all went well, response is the Pyro response message in JSON serialized format
  • 403 Forbidden: you’re trying to access an object that is not exposed by configuration
  • 404 Not Found: you’re requesting a non existing object
  • 500 Internal server error: something went wrong during request processing, response is serialized exception object (if available)

Client information on the current_context, correlation id

Pyro provides a thread-local object with some information about the current Pyro method call, such as the client that’s performing the call. It is available as Pyro4.current_context (shortcut to Pyro4.core.current_context). When accessed in a Pyro server it contains various attributes:


(Pyro4.socketutil.SocketConnection) this is the socket connection with the client that’s doing the request. You can check the source to see what this is all about, but perhaps the single most useful attribute exposed here is sock, which is the socket connection. So the client’s IP address can for instance be obtained via Pyro4.current_context.client.sock.getpeername()[0] . However, since for oneway calls the socket connection will likely be closed already, this is not 100% reliable. Therefore Pyro stores the result of the getpeername call in a separate attribute on the context: client_sock_addr (see below)


(tuple) the socket address of the client doing the call. It is a tuple of the client host address and the port.


(int) request sequence number


(int) message flags, see Pyro4.message.Message


(int) numerical id of the serializer used for this communication, see Pyro4.message.Message .


(dict) message annotations, key is a 4-letter string and the value is a byte sequence. Used to send and receive annotations with Pyro requests. See Message annotations for more information about that.


(dict) message annotations, key is a 4-letter string and the value is a byte sequence. Used in client code, the annotations returned by a Pyro server are available here. See Message annotations for more information about that.


(uuid.UUID, optional) correlation id of the current request / response. If you set this (in your client code) before calling a method on a Pyro proxy, Pyro will transfer the correlation id to the server context. If the server on their behalf invokes another Pyro method, the same correlation id will be passed along. This way it is possible to relate all remote method calls that originate from a single call. To make this work you’ll have to set this to a new uuid.UUID in your client code right before you call a Pyro method. Note that it is required that the correlation id is of type uuid.UUID. Note that the HTTP gateway (see Pyro via HTTP and JSON) also creates a correlation id for every request, and will return it via the X-Pyro-Correlation-Id HTTP-header in the response. It will also accept this header optionally on a request in which case it will use the value from the header rather than generating a new id.

For an example of how this information can be retrieved, and how to set the correlation_id, see the callcontext example. See the usersession example to learn how you could use it to build user-bound resource access without concurrency problems.

Message annotations

Pyro’s wire protocol allows for a very flexible messaging format by means of annotations. Annotations are extra information chunks that are added to the pyro messages traveling over the network. Pyro internally uses a couple of chunks to exchange extra data between a proxy and a daemon: correlation ids (annotation CORR) and hmac signatures (annotation HMAC). These chunk types are reserved and you should not touch them. All other annotation types are free to use in your own code (and will be ignored by Pyro itself). There’s no limit on the number of annotations you can add to a message, but each individual annotation cannot be larger than 64 Kb.

An annotation is a low level datastructure (to optimize the generation of network messages): a chunk identifier string of exactly 4 characters (such as “CODE”), and its value, a byte sequence. If you want to put specific data structures into an annotation chunk value, you have to encode them to a byte sequence yourself (of course, you could utilize a Pyro serializer for this). When processing a custom annotation, you have to decode it yourself as well. Communicating annotations with Pyro is done via a normal dictionary of chunk id -> data bytes. Pyro will take care of encoding this dictionary into the wire message and extracting it out of a response message.

Custom user annotations:

You can add your own annotations to messages. For server code, you do this by setting the response_annotations property of the Pyro4.current_context in your Pyro object, right before returning the regular response value. Pyro will add the annotations dict to the response message. In client code, you can set the annotations property of the Pyro4.current_context object right before the proxy method call. Pyro will then add that annotations dict to the request message.

The older method to to this (before Pyro 4.56) was to create a subclass of Proxy or Daemon and override the methods Pyro4.core.Proxy._pyroAnnotations() or Pyro4.core.Daemon.annotations() respectively. These methods should return the custom annotations dict that should be added to request/response messages. This is still possible to not break older code.

Reacting on annotations:

In your server code, in the Daemon, you can use the Pyro4.current_context to access the annotations of the last message that was received. In your client code, you can do that as well, but you should look at the response_annotations of this context object instead. If you’re using large annotation chunks, it is advised to clear these fields after use. See Client information on the current_context, correlation id.

The older method to do this (before Pyro 4.56) for client code was to create a proxy subclass and override the method Pyro4.core.Proxy._pyroResponseAnnotations(). Pyro calls this method with the dictionary of any annotations received in a response message from the daemon, and the message type identifier of the response message. This still works to not break older code.

For an example of how you can work with custom message annotations, see the callcontext example.

Connection handshake

When a proxy is first connecting to a Pyro daemon, it exchanges a few messages to set up and validate the connection. This is called the connection handshake. Part of it is the daemon returning the object’s metadata (see Metadata from the daemon). You can hook into this mechanism and influence the data that is initially exchanged during the connection setup, and you can act on this data. You can disallow the connection based on this, for example.

You can set your own data on the proxy attribute Pyro4.core.Proxy._pyroHandshake. You can set any serializable object. Pyro will send this as the handshake message to the daemon when the proxy tries to connect. In the daemon, override the method Pyro4.core.Daemon.validateHandshake() to customize/validate the connection setup. This method receives the data from the proxy and you can either raise an exception if you don’t want to allow the connection, or return a result value if you are okay with the new connection. The result value again can be any serializable object. This result value will be received back in the Proxy where you can act on it if you subclass the proxy and override Pyro4.core.Proxy._pyroValidateHandshake().

For an example of how you can work with connections handshake validation, see the handshake example. It implements a (bad!) security mechanism that requires the client to supply a “secret” password to be able to connect to the daemon.

Efficient dispatchers or gateways that don’t de/reserialize messages

Imagine you’re designing a setup where a Pyro call is essentially dispatched or forwarded to another server. The dispatcher (sometimes also called gateway) does nothing else than deciding who the message is for, and then forwarding the Pyro call to the actual object that performs the operation.

This can be built easily with Pyro by ‘intercepting’ the call in a dispatcher object, and performing the remote method call again on the actual server object. There’s nothing wrong with this except for perhaps two things:

  1. Pyro will deserialize and reserialize the remote method call parameters on every hop, this can be quite inefficient if you’re dealing with many calls or large argument data structures.
  2. The dispatcher object is now dependent on the method call argument data types, because Pyro has to be able to de/reserialize them. This often means the dispatcher also needs to have access to the same source code files that define the argument data types, that the client and server use.

As long as the dispatcher itself doesn’t have to know what is even in the actual message, Pyro provides a way to avoid both issues mentioned above: use the Pyro4.core.SerializedBlob. If you use that as the (single) argument to a remote method call, Pyro will not deserialize the message payload until you ask for it by calling the deserialized() method on it. Which is something you only do in the actual server object, and not in the dispatcher. Because the message is then never de/reserialized in the dispatcher code, you avoid the serializer overhead, and also don’t have to include the source code for the serialized types in the dispatcher. It just deals with a blob of serialized bytes.

An example that shows how this mechanism can be used, can be found as blob-dispatch in the examples folder.