fix: Reject connections whose current value the target refuses

[philippjfr]

Description

I made connection validation reject an untyped output whose current value the target input would refuse, and made a connection whose first propagated value is refused roll back instead of staying wired. An output registered through provides without a type (as the built-in Select does) passed the static type check against any input, so the drag indicator showed a typed input such as param.Integer as valid; after the drop the first value failed in the target component, the error was only reported through on_error, and the edge stayed on the canvas and in the graph with the node state holding the rejected value.

• InputPort now keeps the component's own parameter when ports are introspected from a Viewer class, and InputPort.check_value() runs that parameter's validation without assigning anything.
• DataflowGraph.validate_connection checks the source's current value against the target input after the static checks, so drag validation marks the handle invalid with the parameter's reason, for example "Input 'start_year' rejects the current value of 'selected': Integer parameter 'app.start_year' must be an integer, not <class 'str'>."
• DataflowGraph.add_edge now returns "Connection rejected: ..." when the initial propagation raises, removing the watcher and edge and restoring the target's previous value. This covers targets without an introspected parameter, and FlowDash already removes a rejected edge from the canvas.

Behavior change: a connection whose first value is rejected used to be kept and reported through on_error; it is now refused. Values that change after a connection is made are still reported through on_error, since their compatibility cannot be known in advance.

import panel as pn
import param
from panel.viewable import Viewer

from panel_flowdash import register
from panel_flowdash.editor import FlowDash

pn.extension()


@register(page=False, component=True, provides=[{"key": "picked"}])
class Picker(Viewer):
    value = param.Parameter(default="A")

    @param.output(param.Parameter)
    @param.depends("value")
    def picked(self):
        return self.value

    def __panel__(self):
        return str(self.value)


class YearRange(Viewer):
    start_year = param.Integer(default=2000)

    def __panel__(self):
        return str(self.start_year)


editor = FlowDash({"Demo/picker": Picker, "Demo/years": YearRange})
picker = editor.add_component("Demo/picker", position=(0, 0))
years = editor.add_component("Demo/years", position=(400, 0))
print(editor.connect(picker, "picked", years, "start_year"))
editor.servable()

Before, connect returned True, logged a runtime wiring error, and left the edge in place; dragging picked onto start_year showed the handle as valid. After, connect returns the rejection reason, and the drag marks start_year invalid with that reason as its tooltip.

I added test_initial_propagation_failure_rejects_connection (replacing test_error_on_initial_propagation, which asserted the old behavior), test_current_value_checked_against_input_parameter, test_unset_output_is_not_value_checked, test_untyped_output_with_rejected_value_is_refused, test_untyped_output_connects_once_its_value_fits, and the Playwright test test_editor_drag_validation_rejects_untyped_value_of_wrong_type. All but the last compatibility test fail on main. pixi run test (334 passed, 2 skipped), the UI suite (3 passed), and pre-commit pass.

AI Disclosure

Tool & Model: Kilo Code + Claude Opus 5.5
Usage: Diagnosed the report from the complex dataflow example, implemented the fix and tests, and drafted this description; I reviewed the diff and ran the listed checks.

• I have tested all AI-generated content in my PR.
• I take responsibility for all AI-generated content in my PR.

Checklist

• Tests added and are passing