Node conventions

A node moves a message from one point in a flow to the next. These are the rules a custom node follows; the file format itself is in Writing custom nodes.

The message

A message is a dict: msg = {'payload': ..., 'topic': ..., <anything else>}.

payload is the node's value. It is what the output port's declared type checks against and the one key a generic downstream node reads — debug prints only payload. Put the value you want wired and checked here.
topic is a routing string. Set it to a real protocol topic when the node has one, otherwise ''. Don't put the node's own event or state in topic; use a named field instead.
anything else rides along under its own name, and only a node written to read that name sees it.

Put one value in payload and leave the rest as named fields. The DHT22 example sends the temperature in payload and the humidity in msg['humidity'].

Ports

Port types are int, number, bool, string, bytes and any, the same as built-in nodes, with the same wire checks. A declared output type must match what the node puts in payload. Use any when unsure. A custom node has one input and one output: a source has the output, a sink the input.

Kind

Every node is one of three kinds:

source — makes its own messages, on the intervalMs schedule.
transform — one message in, one out. Return the message, or None to stop it.
sink — consumes a message and produces nothing.

Errors

A runtime error in your Python is reported from the board, attributed to your node. A transform that returns None stops that message; anything it passes on continues. The full list is under Errors in Writing custom nodes.

Limits

A custom node has the following limits:

A source's emit runs on a timer, not on an external event — there are no event-driven custom sources.
Two instances of the same custom node get independent setup, so don't put two on the same physical resource, such as the same pin.
Your Python runs with the same trust as the function node. Read a package before you put it in the folder.