# Where a node's values come from

Function nodes receive values as named arguments. Each comes from a message
port, a node setting or a Python default.

## Ports: what the graph carries

A port binds to a message name and reads its current value. Declare inputs
`reading` and `setpoint`, and output `heat`:

```python
def process(reading, setpoint):
    return {"heat": reading < setpoint}
```

Matching message names connect nodes. A port can have a different name from
its message: bind port `reading` to `house.temperature`, for example.

## Settings: constants of this node

Use settings for values that stay fixed while the flow runs: units, retries,
thresholds. Add them in the Function node's **Settings** section; they are
stored with the flow and passed beside the ports:

```python
def process(reading, unit="C"):
    return {"shown": reading if unit == "C" else reading * 1.8 + 32}
```

With a `unit` setting, its configured value is passed to the function. Without
one, the Python default applies. A setting cannot share a name with a port.

Changing a setting edits the flow draft, so publish it to apply the change.
For a value that should change at runtime, use a message. To share a constant
between nodes, publish it from one node and let the others consume it.

## Flow inputs: what arrives from outside

Dashboard controls, the API and batch runs can supply messages without a
producer node. Declare them as **Inputs** in the flow settings:

```json
{"inputs": [{"spec": {"name": "setpoint", "dtype": "float"}, "initial": 21.0}]}
```

The initial value seeds an unset live message. Later publications take over;
changing the starting value does not overwrite a value already in state.
Use **Publish this value now** in the input row to send it immediately.

The canvas draws flow inputs as labelled endpoints. Without a producer or
declared input, a consuming node reports `unconnected_input`.

A batch flow's inputs are run parameters. Its **Result** section selects the
messages returned when the run finishes. See [Runs](runs.md).

## See also

- [Flows, nodes and messages](flows.md)
- [Keeping state in a flow](state.md)
- [Writing node code](../code/nodes.md)