Python implementation of AT Protocol’s XRPC + Lexicon. lexrpc includes a simple XRPC client, server, and Flask web server integration. All three include full Lexicon support for validating inputs, outputs, and parameters against their schemas.

Install from PyPI with pip install lexrpc or pip install lexrpc[flask].

License: This project is placed in the public domain.


The lexrpc client let you call methods dynamically by their NSIDs. To make a call, first instantiate a Client object with the server address and method lexicons, then use method NSIDs to make calls, passing input as a dict and parameters as kwargs:

from lexrpc import Client

lexicons = [...]
client = Client('https://xrpc.example.com', lexicons)
output = client.com.example.my_query({'foo': 'bar'}, param_a=5)

Note that - characters in method NSIDs are converted to _s, eg the call above is for the method com.example.my-query.


To implement an XRPC server, use the Server class. It validates parameters, inputs, and outputs. Use the method decorator to register method handlers and call to call them, whether from your web framework or anywhere else.

from lexrpc import Server

lexicons = [...]
server = Server(lexicons)

def my_query_hander(input, **params):
    output = {'foo': input['foo'], 'b': params['param_a'] + 1}
    return output

# Extract nsid and decode query parameters from an HTTP request,
# call the method, return the output in an HTTP response
nsid = request.path.removeprefix('/xrpc/')
input = request.json()
params = server.decode_params(nsid, request.query_params())
output = server.call(nsid, input, **params)

Flask server

To serve XRPC methods in a Flask web app, first install the lexrpc package with the flask extra, eg pip install lexrpc[flask]. Then, instantiate a Server and register method handlers as described above. Finally, attach the server to your Flask app with flask_server.init_flask.

from flask import Flask
from lexrpc.flask_server import init_flask

# instantiate a Server like above
server = ...

app = Flask('my-server')
init_flask(server, app)

This configures the Flask app to serve the methods registered with the lexrpc server as per the spec. Each method is served at the path /xrpc/[NSID], procedures via POSTs and queries via GETs. Parameters are decoded from query parameters, input is taken from the JSON HTTP request body, and output is returned in the JSON HTTP response body. The Content-Type response header is set to application/json.


Release instructions

Here’s how to package, test, and ship a new release.

  1. Run the unit tests.

    source local/bin/activate.csh
    python3 -m unittest discover
  2. Bump the version number in pyproject.toml and docs/conf.py. git grep the old version number to make sure it only appears in the changelog. Change the current changelog entry in README.md for this new version from unreleased to the current date.

  3. Build the docs. If you added any new modules, add them to the appropriate file(s) in docs/source/. Then run ./docs/build.sh. Check that the generated HTML looks fine by opening docs/_build/html/index.html and looking around.

  4. git commit -am 'release vX.Y'

  5. Upload to test.pypi.org for testing.

    python3 -m build
    setenv ver X.Y
    twine upload -r pypitest dist/lexrpc-$ver*
  6. Install from test.pypi.org.

    cd /tmp
    python3 -m venv local
    source local/bin/activate.csh
    pip3 uninstall lexrpc # make sure we force pip to use the uploaded version
    pip3 install --upgrade pip
    pip3 install -i https://test.pypi.org/simple --extra-index-url https://pypi.org/simple lexrpc==$ver
  7. Smoke test that the code trivially loads and runs.

    source local/bin/activate.csh
    # run test code below

    Test code to paste into the interpreter:

    from lexrpc import Server
    server = Server([{
        'lexicon': 1,
        'id': 'io.example.ping',
        'defs': {
            'main': {
                'type': 'query',
                'description': 'Ping the server',
                'parameters': {'message': { 'type': 'string' }},
                'output': {
                    'encoding': 'application/json',
                    'schema': {
                        'type': 'object',
                        'required': ['message'],
                        'properties': {'message': { 'type': 'string' }},
    def ping(input, message=''):
        return {'message': message}
    print(server.call('io.example.ping', {}, message='hello world'))
  8. Tag the release in git. In the tag message editor, delete the generated comments at bottom, leave the first line blank (to omit the release “title” in github), put ### Notable changes on the second line, then copy and paste this version’s changelog contents below it.

    git tag -a v$ver --cleanup=verbatim
    git push && git push --tags
  9. Click here to draft a new release on GitHub. Enter vX.Y in the Tag version box. Leave Release title empty. Copy ### Notable changes and the changelog contents into the description text box.

  10. Upload to pypi.org!

    twine upload dist/lexrpc-$ver.tar.gz dist/lexrpc-$ver-py3-none-any.whl
  11. Wait for the docs to build on Read the Docs, then check that they look ok.

  12. On the Versions page, check that the new version is active, If it’s not, activate it in the Activate a Version section.


0.2 - 2023-03-13

Bluesky’s Lexicon design and schema handling is still actively changing, so this is an interim release. It generally supports the current lexicon design, but not full schema validation yet. I’m not yet trying to fast follow the changes too closely; as they settle down and stabilize, I’ll put more effort into matching and fully implementing them. Stay tuned!

Breaking changes:

0.1 - 2022-12-13

Initial release!

Tested interoperability with the lexicon, xprc, and xrpc-server packages in bluesky-social/atproto. Lexicon and XRPC themselves are still very early and under active development; caveat hacker!