py-js: python3 objects for max

Simple (and extensible) python3 externals for MaxMSP.


WARNING this is pre-alpha software.

If you are interested to try this out, please note that the current implementation only works on MacOS right now, and requires a compiler to be installed on your system (xcode or the commandline tools via xcode-select --install and that the default build script uses your existing homebrew installed python (currently 3.9.2) and assumes you have already pip installed cython (more detailed installation steps below if required)

Git clone the py-js source 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 automatically create a py package in your $HOME/Max 8/Packages directory:

$ ./

Open up any of the patchers in the generated package and also look at the .maxhelp patcher to understand how py and the pyjs objects work.

Have fun!


This is a project which provides two 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 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 
        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 


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 a more ambitious framework for using python3 in max.

There are two implementation variations:

  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 py'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 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/jsextension object, which can make it portable and usable in standalones.

(Note that only the two first methods work reliably right now. With the latter requiring some slight manual post-build tweaks to get the standalone working with python. Embedding the python interpreter in the external itself is still in progress and not implemented.

Deployment Scenario py pyjs
Link to sys python 1 1
Embed in package 1 1
Embed in external 0 0

Key Features

The more mature py external has the following c-level methods:

category method param(s) in/out can change ns
core import module in yes
core eval expression out no
core exec statement in yes
core execfile file in yes
extra assign var, data in yes
extra call var(s), data out no
extra code expr or stmt out? yes
extra anything expr or stmt out? yes
extra pipe var, funcs out no
editor read file n/a no
editor load file n/a no
interobj scan n/a no
interobj send name, msg, .. n/a no
meta count n/a no

The more recently developed pyjs external implements the following c-level methods:

category method param(s) in/out can change ns
core import module in yes
core eval expression out no
core exec statement in yes
core execfile file in yes
extra code expr or stmt out? yes
in-code eval_to_json expression out no

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 when appropriate.

  • 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 an 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 for free. 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.


  • As mentioned earlier, the package and standalone deployment variations are still not yet working.

  • The py and pyjs objects are currently marked as experimental 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 bugin python which is being worked on and hopefully may be fixed in future versions.

  • As an example of the above, Numpy, the popular python numerical analysis package, falls in the above category. Indeed, it actually crashes Max if imported in a new patch after first use in a prior patch. To address this special case, the module is provided as an object in the api module (and this prevents a crash if used again). As above, just restart Max and use it in one patch normally. After closing the first patch, restart Max to use it again in a new patch. (New patch is taken to mean new document.)

  • core features 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 easy to port to windows. I'm personally not in a position to do so since I don't have another Max license left for Windows and I'm on Macs and Linux.

The following is required:


Not sure if full xcode is required, perhaps only the command line tools are sufficient

$ xcode-select --install

otherwise download xcode from the app store.

py external source and maxsdk

The py external is developed as a max package with a source folder which contains the max-sdk as a subfolder, which is conveniently available as a git submodule.

First git clone the py repo:

$ git clone

Then cd into the newly cloned source directory and run the following to get the max-sdk

$ git submodule init
$ git submodule update 


Python is used to develop py, and should be installed on a mac. However, py is tested again Homebrew python on a MacOS Mojave 10.14.6.

The latest python3 can be easily installed can be installed as follows if you already have brew other click on the link above and install it before this step.

$ brew install python

see: for further info if you are interested.

cython (optional)

Cython is used for wrapping the max api. One could de-couple the cython generated c code from the external and it would work fine since the latter is developed directly using the python c-api, but you would lose the nice feature of calling the max api from python scripts running inside py objects.

Install cython as follows:

pip install cython

Build it

In the root of the package:

make -C source/py build



or in the py/sources/py directory

make build

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

Alternative Builds

Embed Python in the Package (Now working with Standalones)

NOTE: not working in standalones (use external method)

In the root of the py-js directory:

make -C source/py bin-homebrew-pkg

or in py-js/source/py

make 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 test the py and pyjs objects.

Recent changes in Max have allowed for this to work in standalones. Just create your standlone application from a patcher which which includes the py and pyjs objects. Once it is built in say patch $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.

Embed Python in the External itself

WARNING: this currently 'partially' works. Strangely, it works for one exernal and not the other! Not sure why...

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

To use your system homebrew python to do this:

make homebrew-ext

Another implementation variation builds both externals using a minimal static python build. This has provden reproducibly successful (see py-js/source/py/targets/static-ext after building a static-python build. A more robust implementation will following the ogoing cleanup.

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 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 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).

Style it

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

$ brew install clang-format

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

Prior Art and Thanks

Every now and then when I am developing a patch in Max, I yearn for some simple python function or the other, like the any and all builtins for example, and I then spend more time than I want researching a Max workaround.

Thinking that there must be a max external out there, I looked around and found the following:

  • Thomas Grill's py/pyext – Python scripting objects for Pure Data and Max which looked very promising but then I read that the 'available Max port is not actively maintained.' I also noted that it's written in C++ and needs an additional c++ flext layer to compile. But I was further dissuaded from diving in as it supported only python 2 which seemed difficult to swallow considering it is no longer supported. Ironically, this project has become more active recently, so the above may no longer apply.

  • 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 16 months as of this writing. Again c++ and using pybind11 which I'm not familiar with.

  • 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.

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

So I 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 won'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.