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JNA libpython bindings to the tech ecosystem.

Clojars Project travis integration

  • Bridge between JVM objects and Python objects easily; use Python in your Java and use some Java in your Python.
  • Python objects are linked to the JVM GC such that when they are no longer reachable from the JVM their references are released. Scope based resource contexts are also available.
  • Finding the python libraries is done dynamically allowing one system to run on multiple versions of python.
  • REPL oriented design means fast, smooth, iterative development.
  • Carin Meier has written excellent posts on plotting and advanced text generation. She also has some great examples.


We aim to integrate Python into Clojure at a deep level. This means that we want to be able to load/use python modules almost as if they were Clojure namespaces. We also want to be able to use Clojure to extend Python objects. I gave a talk at Clojure Conj 2019 that outlines more of what is going on.

This code is a concrete example that generates an embedding for faces:

(ns facial-rec.face-feature
  (:require [libpython-clj.require :refer [require-python]]
            [libpython-clj.python :refer [py. py.. py.-] :as py]
            [tech.v2.datatype :as dtype]))

(require-python 'mxnet
                '(mxnet ndarray module io model))
(require-python 'cv2)
(require-python '[numpy :as np])

(defn load-model
  [& {:keys [model-path checkpoint]
      :or {model-path "models/recognition/model"
           checkpoint 0}}]
  (let [[sym arg-params aux-params] (mxnet.model/load_checkpoint model-path checkpoint)
        all-layers (py. sym get_internals)
        target-layer (py/get-item all-layers "fc1_output")
        model (mxnet.module/Module :symbol target-layer
                                   :context (mxnet/cpu)
                                   :label_names nil)]
    (py. model bind :data_shapes [["data" [1 3 112 112]]])
    (py. model set_params arg-params aux-params)

(defonce model (load-model))

(defn face->feature
    (if-let [new-img (cv2/imread img-path)]
      (let [new-img (cv2/cvtColor new-img cv2/COLOR_BGR2RGB)
            new-img (np/transpose new-img [2 0 1])
            input-blob (np/expand_dims new-img :axis 0)
            data (mxnet.ndarray/array input-blob)
            batch ( :data [data])]
        (py. model forward batch :is_train false)
        (-> (py. model get_outputs)
            (py. asnumpy)
            (#(dtype/make-container :java-array :float32 %))))
      (throw (Exception. (format "Failed to load img: %s" img-path))))))


user> (require '[libpython-clj.require :refer [require-python]])
...logging info....
user> (require-python '[numpy :as np])
user> (def test-ary (np/array [[1 2][3 4]]))
user> test-ary
[[1 2]
 [3 4]]

We have a document on all the features but beginning usage is pretty simple. Import your modules, use the things from Clojure. We have put effort into making sure things like sequences and ranges transfer between the two languages.


One very complimentary aspect of Python with respect to Clojure is it's integration with cutting edge native libraries. Our support isn't perfect so some understanding of the mechanism is important to diagnose errors and issues.

Current, we launch the python3 executable and print out various different bits of configuration as json. We parse the json and use the output to attempt to find the shared library so for example if we are loading python 3.6 we look for on Linux or libpython3.6m.dylib on the Mac.

This pathway has allowed us support Conda albeit with some work. For examples using Conda, check out the facial rec repository above or look into how we build our test docker containers.

Further Information

New To Clojure

New to Clojure or the JVM? Try remixing the nextjournal entry and playing around there. For more resources on learning and getting more comfortable with Clojure, we have an introductory document.


How to run project tasks

This project has java sources in additon to regular clojure sources. The build alias have the javac built into the tasks, however for running tests or repl you would need to run that as a seperate step if you need to recompile the java classes. Example:

  • clojure -A:javac
  • clojure -A:test

Also note, that if you have tools installed you may substitue the clj command for clojure below:

  • clj -A:test

Compile Java classes

$ clojure -A:javac


$ clojure -A:repl
nREPL server started on port 56785 on host localhost - nrepl://localhost:56785


$  clojure -A:test

Package jar

$ clojure -A:jar

Clean target directory

$ clojure -A:clean

Local install

To install jar to local .m2 :

$ clojure -A:install

Deploy to clojars

Put your credentials to settings.xml (or uncomment login and password prompt in dev/src/build.clj).

$ clojure -A:deploy

This command will sign jar before deploy, using your gpg key. (see dev/src/build.clj for signing options)


Copyright © 2019 Chris Nuernberger

This program and the accompanying materials are made available under the terms of the Eclipse Public License 2.0 which is available at

Can you improve this documentation? These fine people already did:
Chris Nuernberger, Carin Meier, Arsene Rei & skullgoblet1089
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