def METHOD_NAME(self, key):
    """Find a part in the assembly by its key.
    Parameters
    ----------
    key : int | str, optional
        The identifier of the part in the assembly.
    Returns
    -------
    :class:`~compas.datastructures.Part` | None
        The identified part,
        or None if the part can't be found.
    """
    if key not in self.graph.node:
        return None
    return self.graph.node_attribute(key, "part")