Header of process and node ports follow each other both ways: a panel edit rewrites only the def line of edited code, a code save reads the header back into input ports and settings (draft and response); Space types in the node editor again

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WgBfkm4rCmtd1T4vHWjEPx

backend/fluksio/flow/controller.py

@@ -1837,6 +1837,93 @@ def _signature_gap(
     )
 
 
+def follow_header(node: NodeDef, code: str) -> NodeDef:
+    """The node with its input ports and settings read off ``def process(...)``.
+
+    The inverse of the canvas's scaffold, which writes the input ports first,
+    in their order, and the settings after them. A parameter already declared
+    keeps its kind. A new one is a setting when it has a default, which becomes
+    its value, or when it follows a setting, and an input port otherwise. One
+    name swapped for one other is a rename, so the port keeps its message and
+    the setting its value. A declared name the header no longer has is dropped.
+
+    A header this cannot read for certain leaves the node as it is: a syntax
+    error, no top-level ``def process``, a decorator, ``*args``, ``**kwargs``,
+    a bare ``*`` or ``/``.
+    """
+    try:
+        tree = ast.parse(code)
+    except SyntaxError:
+        return node
+    defs = [
+        entry
+        for entry in tree.body
+        if isinstance(entry, ast.FunctionDef) and entry.name == "process"
+    ]
+    if not defs:
+        return node
+    args = defs[-1].args
+    if (
+        defs[-1].decorator_list
+        or args.vararg
+        or args.kwarg
+        or args.posonlyargs
+        or args.kwonlyargs
+    ):
+        return node
+    # Defaults belong to the last parameters, so they line up from the end.
+    defaults: list[ast.expr | None] = [None] * (
+        len(args.args) - len(args.defaults)
+    ) + list(args.defaults)
+    header = [
+        (arg.arg, default)
+        for arg, default in zip(args.args, defaults, strict=True)
+        if arg.arg not in RESERVED_SETTINGS
+    ]
+
+    def written(name: str) -> bool:
+        return name.isascii() and name.isidentifier()
+
+    ports = {spec.port: spec for spec in node.requires if written(spec.port)}
+    settings = {
+        key: value
+        for key, value in node.params.items()
+        if key not in RESERVED_SETTINGS and written(key)
+    }
+    names = [name for name, _ in header]
+    gone = [name for name in [*ports, *settings] if name not in names]
+    new = [name for name in names if name not in ports and name not in settings]
+    renamed = {new[0]: gone[0]} if len(gone) == len(new) == 1 else {}
+
+    requires: list[MessageSpec] = []
+    params = {key: value for key, value in node.params.items() if key not in settings}
+    after_setting = False
+    for name, default in header:
+        was = renamed.get(name, name)
+        if was in ports:
+            requires.append(ports[was].model_copy(update={"port": name}))
+        elif was in settings or default is not None or after_setting:
+            params[name] = settings[was] if was in settings else _literal(default)
+            after_setting = True
+        else:
+            requires.append(MessageSpec(port=name))
+    # Ports the header cannot name, such as a row not named yet, stay.
+    requires += [spec for spec in node.requires if not written(spec.port)]
+    return node.model_copy(update={"requires": requires, "params": params})
+
+
+def _literal(default: ast.expr | None) -> Any:
+    """A parameter's default as a setting's value, or empty when it is not JSON."""
+    if default is None:
+        return ""
+    try:
+        value = ast.literal_eval(default)
+        json.dumps(value)
+    except (ValueError, TypeError, SyntaxError, RecursionError):
+        return ""
+    return value
+
+
 def with_settings(
     function: Callable[..., Any], params: dict[str, Any]
 ) -> Callable[..., Any]: