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61 lines
2.1 KiB
Markdown
61 lines
2.1 KiB
Markdown
# Where a node's values come from
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Function nodes receive values as named arguments. Each comes from a message
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port, a node setting or a Python default.
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## Ports: what the graph carries
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A port binds to a message name and reads its current value. Declare inputs
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`reading` and `setpoint`, and output `heat`:
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```python
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def process(reading, setpoint):
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return {"heat": reading < setpoint}
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```
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Matching message names connect nodes. A port can have a different name from
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its message: bind port `reading` to `house.temperature`, for example.
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## Settings: constants of this node
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Use settings for values that stay fixed while the flow runs: units, retries,
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thresholds. Add them in the Function node's **Settings** section; they are
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stored with the flow and passed beside the ports:
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```python
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def process(reading, unit="C"):
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return {"shown": reading if unit == "C" else reading * 1.8 + 32}
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```
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With a `unit` setting, its configured value is passed to the function. Without
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one, the Python default applies. A setting cannot share a name with a port.
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Changing a setting edits the flow draft, so publish it to apply the change.
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For a value that should change at runtime, use a message. To share a constant
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between nodes, publish it from one node and let the others consume it.
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## Flow inputs: what arrives from outside
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Dashboard controls, the API and batch runs can supply messages without a
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producer node. Declare them as **Inputs** in the flow settings:
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```json
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{"inputs": [{"spec": {"name": "setpoint", "dtype": "float"}, "initial": 21.0}]}
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```
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The initial value seeds an unset live message. Later publications take over;
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changing the starting value does not overwrite a value already in state.
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Use **Publish this value now** in the input row to send it immediately.
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The canvas draws flow inputs as labelled endpoints. Without a producer or
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declared input, a consuming node reports `unconnected_input`.
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A batch flow's inputs are run parameters. Its **Result** section selects the
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messages returned when the run finishes. See [Runs](runs.md).
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## See also
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- [Flows, nodes and messages](flows.md)
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- [Keeping state in a flow](state.md)
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- [Writing node code](../code/nodes.md)
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