py-js: python3 objects for max

Simple (and extensible) python3 externals for MaxMSP

repo -

py-js test


This project provides a number of implementations of python3 externals for use in a live Max environment. If you are only interested in using python3 with Max in an offline capacity, check out the py2max project. Otherwise, read on!

The python3 externals are as follows (in order of relative maturity):

pymax-sdkcwell-featured, many packaging options + cython api
pyjsmax-sdkcjs-friendly -- written as a Max javascript-extension
mxpymax-sdkca translation of pdpython into Max
pymxmin-devkitc++concise, modern, using pybind11
jmxmax-sdkca planned jupyter client in Max

It is suggested to try out the py and pyjs objects first since they are the most mature and best documented of the collection. Please note that all of the externals currently only work on MacOS and that while many aspects of the core externals are quite functional and relatively stable, please consider this project as having a pre-alpha state and don't be surprised if Max seg-faults (especially if you try some of the more experimental features such as the cython wrapped api module).

Also note that the project is undergoing a slow restructuring effort, so a number of things might look funny and redundant.

With such caveats aside, installation is pretty straighforward:

  1. For compilation, make sure you have either Xcode or the command line tools installed via xcode-select --install in the terminal.

You should also have Homebrew python3 installed on your system (see below for more detailed installation instructions), but it is as simple as:brew install python

Note: that the default build script automatically reads your existing homebrew installed python version (currently 3.9.6 at the time of this writing.)

  1. Git clone the py-js repo and run the following in the cloned repo to get the required submodules:git submodule init git submodule update

Then run the following in the root directory of the py-js source (other installation options are detailed below) and make sure you understand that it will generated a py package in your $HOME/Max 8/Packages directory:./

Open up any of the patch files in the patcher directory of the generated max package, and also look at the .maxhelp patcher to understand how the py and the pyjs objects work.

Note that the default build creates a package with two externals which are linked to your system homebrew python3. This has the immediate benefit that you have access to your curated collection of python packages. The tradeoff is that these externals are dynamically linked with local dependencies and therefore not usable in standalones and relocatable Max packages.

Not to worry however, if have a need for portable relocatable python3 externals then read on!

Alternative Quickstart for Self-contained Python3 Externals

If you would like a couple of self-contained python3 externals which can be included in standalones you can download the pre-release OR even better, you can build your own with the following:cd py-js/sources/py python3 -m builder py_static --install && python3 -m builder static_ext

The above command automatically downloads python3 source from as well as dependencies from their respective sites, and then compiles a static version of python3 which is then used to compile the externals.

A little patience and you should find two externals in the py-js/externals folder: py.mxo and pyjs.mxo. Although they are somewhat different (see below for details), each external 'bundle' contains an embedded python3 interpreter with a zipped standard library in the Resources folder which also has a site-packages directory for your own code. The python interpreter in each external is statically compiled and self-contained without any non-system dependencies which makes it appropriate for use in 'relocatable' Max Packages and Standalones.

Now test whether these two externals work ok in a standalone: first build py-js/patchers/py_test_standalone.maxpat as a standalone application using Max then open it: you should see that the py external is working fine, yet the pyjs part of the test gives an error.

If you look inside the built standalone bundle,, you find that the py.mxo external has been copied into the folder but not pyjs.mxo. This is likely a bug in Max 8 but easily resolved. Fix it by manually copying the pyjs.mxo external into this folder and then copy the javascript and jsextensions folders from the root of the py-js project and place them into the folder. Now re-run the standalone app again and now the pyjs external should work. (Incidentally, If anyone knows of some scripting at the standalone build step to automate the manual fix above it woulld be greatl appreciated.)

Incidentally, a pre-built standalone that was built using exactly the same steps as above is in the releases section:

Note: it would normally be considered redundant to install two different python3 externals in your project. Nontheless, it's good to demonstrate that they can be used together if required.

Please read on for further details about what the externals can do.

Have fun!


Here's a summary of the features of the two core python3 max externals:

py external

    obj_count                    : number of active py objects
    registry                     : global registry to lookup object names

        py_repl                  : a basic single line repl for py
        py_repl_plus             : embeds a py object in a py_repl

py max external
        name                     : unique object name
        file                     : file to load into editor
        autoload                 : load file at start
        pythonpath               : add path to python sys.path
        debug                    : switch debug logging on/off

    methods (messages) 
            import <module>      : python import to object namespace
            eval <expression>    : python 'eval' semantics
            exec <statement>     : python 'exec' semantics
            execfile <path>      : python 'execfile' semantics
            assign <var> [arg]   : max-friendly msg assignments to py object namespace
            call <pyfunc> [arg]  : max-friendly python function calling
            pipe <arg> [pyfunc]  : process a py/max value via a pipe of py funcs
            code <expr|stmt>     : alternative way to eval or exec py code
            anything <expr|stmt> : anything version of the code method 

            sched <t> <fn> [arg] : defer a python function call by t millisecs

        code editor
            read <path>          : read text file into editor
            load <path>          : combo of read <path> -> execfile <path>
            run                  : run the current code in the editor
            scan                 : scan patcher and store names of child objects
            send <msg>           : send an arbitrary message to a named object

            count                : give a int count of current live py objects

        single inlet             : primary input (anything)

        left outlet              : primary output (anything)
        middle outlet            : bang on failure
        right outlet             : bang on success 
py-js test_py

pyjs external (experimental)

pyjs max external (jsextension)
        name                     : unique object name
        file                     : file to load in object namespace
        pythonpath               : add path to python sys.path
        debug                    : switch debug logging on/off
        core (messages)
            import <module>      : python import to object namespace
            eval <expression>    : python 'eval' semantics
            exec <stmnt>         : python 'exec' semantics
            execfile <path>      : python 'execfile' semantics
            code <expr|stmt>     : eval/exec/import python code (see above)

        in-code (non-message)
            eval_to_json <expr>  : python 'eval' returns json


py/js started out as an attempt (during a covid-19 lockdown) to develop a basic python3 external for maxmsp. It then evolved into an umbrella project for exploring a number of different ways of using python3 in max.

This overview will cover two of the most mature and best documented implementations:

  1. A py external which provides a more featureful two-way interface between max and python in a way that feels natural to both languages.
  2. A pyjs max external/jsextension providing a PyJS class and a minimal subset of the py external's features which work well with the max js object and javascript code (like returning json directly from evaluations of python expressions).

Both externals have access to builtin python modules and the whole universe of 3rd party modules, and further have the option of importing a builtin api module which uses cython to wrap selective portions of the max c-api. This allows regular python code to directly access the max-c-api and script Max objects.

The objective is to have 3 deployment variations:

  1. Linking the externals to your system python (homebrew, built from source, etc.) This has the benefit of re-using your existing python modules and is the default option.
  2. Embedding the python interpreter in a Max package: in this variation, a dedicated python distribution (zipped or otherwise) is placed in the support folder of the py/js package (or any other package) and is linked to the py external or pyjs extension (or both). This makes it size efficient and usable in standalones.
  3. The external itself as a container for the python interpreter: a custom python distribution (zipped or otherwise) is stored inside the external bundle itself, which can make it portable and usable in standalones.

As of this writing all three deployment scenarios are availabe, however it is worth looking more closely into the tradeoffs in each case. This topic is treated in more detail below (see Build Variations)

Deployment Scenariopypyjs
Link to sys python11
Embed in package11
Embed in external11

Key Features

The py external has the following c-level methods:

categorymethodparam(s)in/outcan change ns
extraassignvar, datainyes
extracallvar(s), dataoutno
extracodeexpr or stmtout?yes
extraanythingexpr or stmtout?yes
extrapipevar, funcsoutno
timeschedms, fun, argsoutno
interobjsendname, msg, ..n/ano

The pyjs external implements the following c-level methods:

categorymethodparam(s)in/outcan change ns
extracodeexpr or stmtout?yes

In both cases, the code method allows for import/exec/eval of python code, which can be said to make those 'fit-for-purpose' methods redundant. However, I have retained them since they are stricter in what they allow and further provide a helpful prefix in messages which indicates message intent.


py/js's core features have a one-to-one correspondance to python's very high layer as specified here. In the following, when we refer to object, we refer to instances of either the py or pyjs externals. A note of differences between the variations will be provided below.

  • Per-object namespaces. Each object has a unique name (which is provided automatically or can be set by the user), and responds to an import <module> message which loads the specified python module in its namespace (essentially a globals dictionary). Notably, namespaces can be different for each instance.
  • Eval Messages. Responds to an eval <expression> message in the left inlet which is evaluated in the context of the namespace. py objects output results to the left outlet, send a bang from the right outlet upon success or a bang from the middle outlet upon failure. pyjs objects just return an atomarray of the results.
  • Exec Messages. Responds to an exec <statement> message and an execfile <filepath> message which executes the statement or the file's code in the object's namespace. For py objects, this produces no output from the left outlet, sends a bang from the right outlet upon success or a bang from the middle outlet upon failure. For pyjs objects no output is given.


The extra category of methods makes the py or pyjs object play nice with the max/msp ecosystem:

Implemented for py objects at present:

  • Assign Messages. Responds to an assign <varname> [x1, x2, ..., xN] which is equivalent to <varname> = [x1, x2, ..., xN] in the python namespace. This is a way of creating variables in the object's python namespace using max message syntax. This produces no output from the left outlet, a bang from the right outlet upon success, or a bang from the middle outlet upon failure.
  • Call Messages. Responds to a call <func> arg1 arg2 ... argN kind of message where func is a python callable in the py object's namespace. This corresponds to the python callable(*args) syntax. This makes it easier to call python functions in a max-friendly way. If the callable does not have variable arguments, it will alternatively try to apply the arguments as a list i.e. call func(args). Future work will try make call correspond to a python generic function call: <callable> [arg1 arg2 ... arg_n] [key1=val1 key2=val2 ... keyN=valN]. This outputs results to the left outlet, a bang from the right outlet upon success, or a bang from the middle outlet upon failure.
  • Pipe message. Like a call in reverse, responds to a pipe <arg> <f1> <f2> ... <fN> message. In this sense, a value is piped through a chain of python functions in the objects namespace and returns the output to the left outlet, a bang from the right outlet upon success, or a bang from the middle outlet upon failure.

Implemented for both py and pyjs objects:

  • Code or Anything Messages. Responds to a code <expression || statement> or (anything) <expression || statement> message. Arbitrary python code (expression or statement) can be used here, because the whole message body is converted to a string, the complexity of the code is only limited by Max's parsing and excaping rules. (EXPERIMENTAL and evolving).

Implemented for pyjs objects only:

  • Evaluate to JSON. Can be used in javascript code only to automatically serialize the results of a python expression as a json string as follows: evaluate_to_json <expression> -> JSON.

Interobject Communication

Implemented for py objects only:

  • Scan Message. Responds to a scan message with arguments. This scans the parent patcher of the object and stores scripting names in the global registry.
  • Send Message. Responds to a send <object-name> <msg> <msg-body> message. Used to send typed messages to any named object. Evokes a scan for the patcher's objects if a registry of names is empty.

Editing Support

Implemented for py objects only.

  • Line REPL. The pyhas two bpatcher line repls, one of which embeds a py object and another which has an outlet to connect to one. The repls include a convenient menu with all of the py object's methods and also feature coll-based history via arrow-up/arrow-down recall of entries in a session. Of course, a coll can made to save all commands if required.
  • Experimental Remote Console. A new method (due to Ian Duncan) of sending code to the py node via udp has been implemented and allows for send-from-editor and send-from-interactive-console capabilities. The clients are still in their infancy, but this method looks promising since you get syntax highlighting, syntax checking, and other features. It assumes you want to treat your py nodes as remotely accessible server/interpreters-in-max.
  • Code Editor. Double-clicking the py object opens a code-editor. This is populated by a read message which reads a file into the editor and saves the filepath to an attribute. A load message also reads the file followed by execfile. Saving the text in the editor uses the attribute filepath and execs the saved text to the object's namespace.

For pyjs objects, code editing is already built into the js objects.


Implemented for both py and pyjs objects:

  • Exposing Max API to Python A portion of the max api in c74support/max-includes has been converted to a cython .pxd file called api_max.pxd. This makes it available for a cython implementation file, api.pyx which is converted to c-code during builds and embedded in the external. This code enables a custom python builtin module called api which can be imported by python scripts in py objects or via import messages to the object. This allows the subset of the max-api which has been wrapped in cython code to be called directly by python scripts or via messages in a patcher.


  • Packaging and deployment of python3 externals has improved considerably but is still a work-in-progress: basically needing further documentation, consolidation and cleanup. For example, there are currently two build systems which overlap: a bash/makefile build system and a new python based build system to handle more complex cases. Use the Homebrew variations in the bash/makefile build system for most build use cases and if you would like to build a self-contained static external then use python3 -m builder py_static --install && python3 -m builder static_ext in the py-js/sources/py directory. Clearly this is not optimal / user-friendly and needs work.
  • Despite their relative maturity, the py and pyjs objects are currently marked as pre-release pre-alpha and still need further unit/functional/integration testing and field testing of course!
  • As of this writing, the api module, does not (like apparently all 3rd party python c-extensions) unload properly between patches and requires a restart of Max to work after you close the first patch which uses it. Unfortunately, this is a known bug in python which is being worked on and may be fixed in future versions.
  • Numpy, the popular python numerical analysis package, falls in the above category. Indeed, it used to actually crash Max if imported in a new patch after first use in a prior patch. In python 3.9.x, it thankfully doesn't crash but gives the following error:[py __main__] import numpy: SystemError('Objects/structseq.c:401: bad argument to internal function')

This just means that you imported numpy, used it (hopefully without issue) then closed your patch and then, in the same Max session, re-opened it or created a new one and imported numpy again.

To fix it, just restart Max and use it normally in your patch. Treat each patch as a session and restart Max after each session. It's a pain, but unfortunately a limitation of current python c-extensions.

  • core features relying on pure python code are supposed to be the most stable, and should not crash under most circumstances, extra features are less stable since they are more experimental, etc..
  • The api module is the most experimental and evolving part of this project, and is completely optional. If you don't want to use it, don't import it.


Only tested on OS X at present. Should be relatively straightforward to port to windows (a pure python build script is being developed to make this easier).

The following is required:


Full xcode is not required, the freely available command line tools are sufficientxcode-select --install

otherwise download xcode from the app store.

py-js externals source and max-sdk

This project is developed as a max package with a source folder which contains the max-sdk as a subfolder. This is conveniently available as a git submodule.

First git clone the py-js repo:git clone

Then cd into the newly cloned source directory and run the following to get the max-sdkgit submodule init git submodule update

Homebrew Python3

Homebrew Python3 is required for the default non-portable build case. If it is not already installed see Homebrew for the install oneliner (provided here as well for reference):/bin/bash -c "$(curl -fsSL"

Once Homebrew is installed, the latest version of python3 can be easily installed as follows:brew install python

see: for further info if you are interested.


Cython is used for wrapping the max api. It is advised to install it in case you want to play around or extend tthe wrapped max api.

Install cython as follows:pip install cython

Build the Default Externals

In the root of the package:./

ormake -C source/py build

or in the py-js/sources/py directorymake build

This builds the default 'linked-to-system|homebrew python' version of py and (also python-in-javascript pyjs. Read further for alternative ways to build and install py and pyjs

Current Status of Builders

As of this writing this project has two separate build system which should be unified some day. You don't have to use both but it's understanding the differences.

  1. Homebrew python build system based on makefiles and bash scripts: re-uses your existing homebrew installation to build the externals with a number of variations. The default build uses this system.
  2. Python builder system: was developed to handle the complex case of downloading python (from and its dependencies for their respective sites in source form and then building minimal static python binaries with which to reliably compile python3 externals which are portable, relocatable, self-contained, small-in-size, and therefore usable in Max Packages and Standalones.

Other Build Variations

One of the objectives of this project is to cater to a number of build variations. As of this writing, the homebrew based variations (except for one strange case detailed below) work mostly ok. Externals built using the python-builder system work well and fulfil the portability requirements.

There is generally tradeoff of size vs. portability:

bin-homebrew-syshomebrewexternals300Knono [1]
bin-homebrew-exthomebrewexternals27.1MByesyes [2]

[1] an additional benefit is you can use all your system python packages

[2] not 100% working yet.

Embed Python in your Max Package (can work in Standalones)

In the root of the py-js directory:make -C source/py bin-homebrew-pkg

or in py-js/source/pymake bin-homebrew-pkg

This will create a py package in $HOME/Documents/Max 8/packages/py

Once this is done you can run some of the patchers to in the package test the py and pyjs objects.

NOTE: Recent changes in Max have allowed for this to work in standalones. Just create your standalone application from a patcher which which includes the py and pyjs objects. Once it is built into a <STANDALONE> then copy the whole aforementioned py package to <STANDALONE>/Contents/Resources/C74/packages and delete the redundant py.mxo in <STANDALONE>/Contents/Resources/C74/externals since it already exists in the just-copied package.

The most reliable method is to use the python builder package which is included in the project:cd py-js/sources/py python3 -m builder py_static --install && python3 -m builder static_ext

Embedding Python in the External itself (Alternative)

Here's another method using Homebrew / bash build system which currently works with a caveat: strangely, although it buids both py or pyjs successfuly, you have to pick one and can't use both at the same time for some strange reason. This is not an issue because the use of both is redundant.

This method places a minimized python distribution in the external py.mxo itself.

From the root of py-js, do this:cd source/py make homebrew-ext

The pymx variation

The pymx is a max wrapper of the python interpreter using the min-api c++ version of the Max api.

This needs:

  1. pybind11 which is used for wrapping the alternative min-api c++ version of the Max api to make it accessible to python.
  2. cmake is used in the min-api build system.

First, install cmake and pybind11 using homebrew:brew install cmakebrew install pybind11

then run the following script in the root of the project:./

Sidenote about building on a Mac

If you are developing the package in $HOME/Documents/Max 8/Packages/py and you have your icloud drive on for Documents, you will find that make or xcodebuild will reliably fail with 1 error during development, a codesigning error that is due to icloud sync creating detritus in the dev folder. This can be mostly ignored (unless your only focus is codesigning the external).

The solution is to move the external project folder to a non iCloud drive folder (such as $HOME/Downloads for example) and then run "xattr -cr ." in the project directory to remove the detritus (ironically which Apple's system is itself creating) and then it should succeed (provided you have your Info.plist and bundle id correctly specified).

I've tried this several times and and it works (for "sign to run locally" case and for the "Development" case).

Code Style

The coding style for this project can be applied automatically during the build process with clang-format. On OS X, you can easily install this using brew:brew install clang-format

The style used in this project is specified in the .clang-format file.

Prior Art and Thanks

I was motivated to start this project because I found myself recurrently wanting to use some python libraries or functions in Max.

Looking around for a python max external I found the following:

  • Thomas Grill's py/pyext – Python scripting objects for Pure Data and Max is the most mature Max/Python implementation and when I was starting this project, it seemed very promising but then I read that the 'available Max port is not actively maintained.' I also noted that it was written in C++ and that it needed an additional c++ flext layer to compile. I was further dissuaded from diving in as it supported, at the time, only python 2 which seemed difficult to swallow considering Python2 is basically not developed anymore. Ironically, this project has become more active recently, and I finally was persuaded to go back and try to compile it and finally got it running. I found it to be extremely technically impressive work, but it had probably suffered from the burden of having to maintain several moving dependencies (puredata, max, python, flext, c++). The complexity probably put off some possible contributors which have made the maintenance of the project easier for Thomas. In any case, it's an awesome project and it would be great if this project could somehow help py/ext in some way or the other.
  • max-py -- Embedding Python 2 / 3 in MaxMSP with pybind11. This looks like a reasonable effort, but only 9 commits and no further commits for 2 years as of this writing.
  • nt.python_for_max -- Basic implementation of python in max using a fork of Graham Wakefield's old c++ interface. Hasn't really been touched in 3 years.
  • net.loadbang.jython -- A jython implementation for Max which uses the MXJ java interface. It's looks like a solid effort using Jython 2.7 but the last commit was in 2015.

Around the time of the beginning of my first covid-19 lockdown, I stumbled upon Iain Duncan's Scheme for Max project, and I was quite inspired by his efforts and approach to embed a scheme implementation into a Max external.

So it was decided, during a period with less distractions than usual, to try to make a minimal python3 external, learn the max sdk, the python c-api, and how to write more than a few lines of c that didn't crash.

It's been an education and I have come to understand precisely a quote I remember somewhere about the c language: that it's "like a scalpel". I painfully now understand this to mean that in skilled hands it can do wonders, otherwise you almost always end up killing the patient.

Thanks to Luigi Castelli for his help on Max/Msp questions, to Stefan Behnel for his help with Cython questions, and to Iain Duncan for providing the initial inspiration and for saving me time with some great implementation ideas.

GitHub - shakfu/py-js at
Python3 externals for Max / MSP. Contribute to shakfu/py-js development by creating an account on GitHub.