def find_tree_root(tree, key):
    """Find a root in a tree by it's key

    :param dict tree: the pkg dependency tree obtained by calling
                     `construct_tree` function
    :param str key: key of the root node to find
    :returns: a root node if found else None
    :rtype: mixed

    """
    result = [p for p in tree.keys() if p.key == key]
    assert len(result) in [0, 1]
    return None if len(result) == 0 else result[0]
Find a root in a tree by it's key

    :param dict tree: the pkg dependency tree obtained by calling
                     `construct_tree` function
    :param str key: key of the root node to find
    :returns: a root node if found else None
    :rtype: mixed