pytest-dev/pytest · error · TypeError

config is not a valid argument for from_parent

Error message

config is not a valid argument for from_parent

What it means

Node.from_parent intentionally rejects a `config` keyword. config and session are always inherited from the parent in from_parent, so passing them would create inconsistency. The guard forces correct usage of the public constructor.

Source

Thrown at src/_pytest/nodes.py:222

        #: own use.
        self.stash: Stash = Stash()
        # Deprecated alias. Was never public. Can be removed in a few releases.
        self._store = self.stash

    @classmethod
    def from_parent(cls, parent: Node, **kw) -> Self:
        """Public constructor for Nodes.

        This indirection got introduced in order to enable removing
        the fragile logic from the node constructors.

        Subclasses can use ``super().from_parent(...)`` when overriding the
        construction.

        :param parent: The parent node of this Node.
        """
        if "config" in kw:
            raise TypeError("config is not a valid argument for from_parent")
        if "session" in kw:
            raise TypeError("session is not a valid argument for from_parent")
        return cls._create(parent=parent, **kw)

    @property
    def ihook(self) -> pluggy.HookRelay:
        """Path-sensitive hook proxy used to call pytest hooks."""
        return self.session.gethookproxy(self.path)

    def __repr__(self) -> str:
        return "<{} {}>".format(self.__class__.__name__, getattr(self, "name", None))

    def warn(self, warning: Warning) -> None:
        """Issue a warning for this Node.

        Warnings will be displayed after the test session, unless explicitly suppressed.

        :param Warning warning:

View on GitHub (pinned to 98b357f69e)

Solutions

1. Remove the config kwarg from the from_parent call; config is derived from the parent
2. If you truly need a different config, call the constructor (_create / __init__) directly instead of from_parent

Example fix

// before
item = MyItem.from_parent(parent, name=n, config=cfg)
// after
item = MyItem.from_parent(parent, name=n)

def safe_from_parent(cls, parent, **kw):
    kw.pop('config', None)  # never forward config
    return cls.from_parent(parent, **kw)

def kwargs_have_no_config(kw) -> bool:
    return 'config' not in kw

When it happens

Trigger: Calling MyNode.from_parent(parent, config=cfg, ...) or passing config via **kw to from_parent. Plugin code migrated from the old constructor pattern often leaks config through.

Common situations: Older pytest plugins/tutorials that passed config= to constructors; copy-paste from __init__ into a from_parent override.

Related errors

• session is not a valid argument for from_parent
• config or parent must be provided
• session or parent must be provided
• nodeid or parent must be provided
• runtest must be implemented by Item subclass

AI-assisted analysis of pytest-dev/pytest@98b357f69e (2026-08-04). Data as JSON: /data/errors/91cab732c8fd9b15.json. Report an issue: GitHub.