Cap'n Proto serialization/RPC system - Python bindings
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More thorough docs are available at


pycapnp's distribution has no requirements beyond a C++11 compatible compiler. GCC 4.8+ or Clang 3.3+ should work fine.

pycapnp has additional development dependencies, including cython and py.test. See requirements.txt for them all.

Building and installation

Install with pip install pycapnp. You can set the CC environment variable to control which compiler is used, ie CC=gcc-4.8 pip install pycapnp.

Or you can clone the repo like so:

git clone
pip install --install-option '--force-cython' ./pycapnp

Note: for OSX, if using clang from Xcode 5, you may need to set CFLAGS like so:

CFLAGS='-stdlib=libc++' pip install pycapnp

If you wish to install using the latest upstream C++ Cap'n Proto:

pip install --install-option "--libcapnp-url" --install-option "" --install-option "--force-bundled-libcapnp" .

Python Versions

Python 2.6/2.7 are supported as well as Python 3.2+. PyPy 2.1+ is also supported.

One oddity to note is that Text type fields will be treated as byte strings under Python 2, and unicode strings under Python 3. Data fields will always be treated as byte strings.


This project uses git-flow. Essentially, just make sure you do your changes in the develop branch. You can run the tests by installing pytest with pip install pytest, and then run py.test from the test directory.

Binary Packages

Building a dumb binary distribution:

python bdist_dumb

Building a Python wheel distributiion:

python bdist_wheel

If it fails with an error like clang: error: no such file or directory: 'capnp/lib/capnp.cpp', then you need to cythonize fist. This can be done with:

python build --force-cython


There is some basic documentation here.

The examples directory has one example that shows off pycapnp quite nicely. Here it is, reproduced:

from __future__ import print_function
import os
import capnp

import addressbook_capnp

def writeAddressBook(file):
    addresses = addressbook_capnp.AddressBook.new_message()
    people = addresses.init('people', 2)

    alice = people[0] = 123 = 'Alice' = ''
    alicePhones = alice.init('phones', 1)
    alicePhones[0].number = "555-1212"
    alicePhones[0].type = 'mobile' = "MIT"

    bob = people[1] = 456 = 'Bob' = ''
    bobPhones = bob.init('phones', 2)
    bobPhones[0].number = "555-4567"
    bobPhones[0].type = 'home'
    bobPhones[1].number = "555-7654"
    bobPhones[1].type = 'work'
    bob.employment.unemployed = None


def printAddressBook(file):
    addresses =

    for person in addresses.people:
        print(, ':',
        for phone in person.phones:
            print(phone.type, ':', phone.number)

        which = person.employment.which()

        if which == 'unemployed':
        elif which == 'employer':
            print('employer:', person.employment.employer)
        elif which == 'school':
            print('student at:',
        elif which == 'selfEmployed':
            print('self employed')

if __name__ == '__main__':
    f = open('example', 'w')

    f = open('example', 'r')

Also, pycapnp has gained RPC features that include pipelining and a promise style API. Refer to the calculator example in the examples directory for a much better demonstration:

import capnp
import socket

import test_capability_capnp

class Server(test_capability_capnp.TestInterface.Server):

    def __init__(self, val=1):
        self.val = val

    def foo(self, i, j, **kwargs):
        return str(i * 5 + self.val)

def server(write_end):
    server = capnp.TwoPartyServer(write_end, bootstrap=Server(100))

def client(read_end):
    client = capnp.TwoPartyClient(read_end)

    cap = client.bootstrap()
    cap = cap.cast_as(test_capability_capnp.TestInterface)

    remote =
    response = remote.wait()

    assert response.x == '125'

if __name__ == '__main__':
    read_end, write_end = socket.socketpair(socket.AF_UNIX)
    # This is a toy example using socketpair.
    # In real situations, you can use any socket.


Common Problems

If you get an error on installation like:

gcc-4.8: error: capnp/capnp.c: No such file or directory

gcc-4.8: fatal error: no input files

Then you have too old a version of setuptools. Run pip install -U setuptools then try again.

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