The python backend

The python backend provides a flexible programming environment, allowing users to route, generate and manipulate channel events using the Python 3 scripting language.

Every instance has its own interpreter, which can be loaded with multiple Python modules. These modules may contain member functions accepting a single float parameter, which can then be used as target channels. For each incoming event, the handler function is called. Channels in the global scope may be assigned a default handler function.

Python modules may also register socket objects (and an associated callback function) with the MIDIMonster core, which will then alert the module when there is data ready to be read.

To interact with the MIDIMonster core, import the midimonster module from within your module.

The midimonster module provides the following functions:

Function Usage example Description
output(string, float) midimonster.output("foo", 0.75) Output a value event to a channel on this instance
inputvalue(string) midimonster.inputvalue("foo") Get the last input value on a channel of this instance
outputvalue(string) midimonster.outputvalue("bar") Get the last output value on a channel of this instance
current() print(midimonster.current()) Returns the name of the input channel whose handler function is currently running or None if the interpreter was called from another context
timestamp() print(midimonster.timestamp()) Get the internal core timestamp (in milliseconds)
interval(function, long) midimonster.interval(toggle, 100) Register a function to be called periodically. Interval is specified in milliseconds (accurate to 10msec). Calling interval with the same function again updates the interval. Specifying the interval as 0 cancels the interval
manage(function, socket) midimonster.manage(handler, socket) Register a (connected/listening) socket to the MIDIMonster core. Calls function(socket) when the socket is ready to read. Calling this method with None as the function argument unregisters the socket. A socket may only have one associated handler
cleanup_handler(function) midimonster.cleanup_handler(save_all) Register a function to be called when the instance is destroyed (on MIDIMonster shutdown). One cleanup handler can be registered per instance. Calling this function when the instance already has a cleanup handler registered replaces the handler, returning the old one.

When a channel handler executes, calling midimonster.inputvalue() for that exact channel returns the previous value, while the argument to the handler is the current value. The stored value is updated after the handler finishes executing.

Example Python module:

import socket
import midimonster

# Simple channel handler
def in1(value):
    midimonster.output("out1", 1 - value)

# Socket data handler
def socket_handler(sock):
    # This should get some more error handling
    data = sock.recv(1024)
    print("Received %d bytes from socket: %s" % (len(data), data))
    if(len(data) == 0):
        # Unmanage the socket if it has been closed
        midimonster.manage(None, sock)
        sock.close()

# Interval handler
def ping():
    print(midimonster.timestamp())

def save_positions():
    # Store some data to disk

# Register an interval
midimonster.interval(ping, 1000)
# Create and register a client socket (add error handling as you like)
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.connect(("localhost", 8990))
midimonster.manage(socket_handler, s)
midimonster.cleanup_handler(save_positions)

Input values range between 0.0 and 1.0, output values are clamped to the same range.

Note that registered sockets that have been closed (socket.recv() returned 0 bytes) need to be unregistered from the MIDIMonster core, otherwise the core socket multiplexing mechanism will report an error and shut down the MIDIMonster.

Global configuration

The python backend does not take any global configuration.

Instance configuration

Option Example value Default value Description
module my_handlers.py none (Path to) Python module source file, relative to configuration file location
default-handler mu_handlers.default none Function to be called as handler for all top-level channels (not belonging to a module)

A single instance may have multiple module options specified. This will make all handlers available within their module namespaces (see the section on channel specification).

Channel specification

Channel names may be any valid Python function name. To call handler functions in a module, specify the channel as the functions qualified path (by prefixing it with the module name and a dot).

Example mappings:

py1.my_handlers.in1 < py1.foo
py1.out1 > py2.module.handler

Known bugs / problems

Output values will not trigger corresponding input event handlers unless the channel is mapped back in the MIDIMonster configuration. This is intentional.

Output events generated from cleanup handlers called during shutdown will not be routed, as the core routing facility has already shut down at this point. There are no plans to change this behaviour.

Importing a Python module named midimonster is probably a bad idea and thus unsupported.

The MIDIMonster is, at its core, single-threaded. Do not try to use Python’s threading module with the MIDIMonster.

Note that executing Python code blocks the MIDIMonster core. It is not a good idea to call functions that take a long time to complete (such as time.sleep()) within your Python modules.