列表理解有副作用吗？

Question

我想用 python 写一个树的先序递归。我可以用for语句，但是如果我用list comprehension，我希望通过它的副作用达到递归的效果，为什么不行？

这个方法有效：

def preorder(t):
    ret = [label(t)]
    for branch in branches(t):
    ret.extend(preorder(branch))
    return ret

但这并没有给我正确的结果。

def preorder(t):
    ret = [label(t)]
    a = (ret.extend(preorder(b)) for b in branches(t))
    return ret

用第二种方法：

T = tree(1, [tree(2), tree(3, [tree(4), tree(5)]), tree(6, [tree(7)])])
preorder(T)

回报只有

[1]

树ADT细节

def tree(label, branches=[]):
    """Construct a tree with the given label value and a list of branches."""
    for branch in branches:
        assert is_tree(branch), 'branches must be trees'
    return [label] + list(branches)


def label(tree):
    """Return the label value of a tree."""
    return tree[0]


def branches(tree):
    """Return the list of branches of the given tree."""
    return tree[1:]