human
I wrote the below code

    def fix_single(tree):
        if isinstance(tree, PX.Phylogeny):
            return tree
        if isinstance(tree, PX.Clade):
            return tree.to_phylogeny()
        if isinstance(tree, PX.BaseTree.Tree):
            return PX.Phylogeny.from_tree(tree)
        if isinstance(tree, PX.BaseTree.Clade):
            return PX.Phylogeny.from_tree(PX.BaseTree.Tree(root=tree))
        else:
            raise ValueError('iterable    must    contain    Tree    or    Clade    types')
    if isinstance(obj, PX.Phyloxml):
        pass
    elif isinstance(obj, (PX.BaseTree.Tree, PX.BaseTree.Clade)):
        obj = fix_single(obj).to_phyloxml()
    elif hasattr(obj, '__iter__'):
        obj = PX.Phyloxml({}, phylogenies=(fix_single(t) for t in obj))
    else:
        raise ValueError('First    argument    must    be    a    Phyloxml,    Phylogeny,    Tree,    or    iterable    of    Trees    or    Phylogenies.')
    return Writer(obj).write(file, encoding=encoding, indent=indent)

What's a good function header?
 ... Here's a good function header?
assistant
#Here's a good function header.

def write(obj, file, encoding=DEFAULT_ENCODING, indent=True):