Python:是否可以将生成器和递归函数混合使用?

def add(n):
    yield n
    for m in add(n+1):
        yield m

使用递归生成器,很容易构建复杂的回溯器:

def resolve(db, goals, cut_parent=0):
    try:
        head, tail = goals[0], goals[1:]
    except IndexError:
        yield {}
        return
    try:
        predicate = (
            deepcopy(clause)
                for clause in db[head.name]
                    if len(clause) == len(head)
        )
    except KeyError:
        return
    trail = []
    for clause in predicate:
        try:
            unify(head, clause, trail)
            for each in resolve(db, clause.body, cut_parent + 1):
                for each in resolve(db, tail, cut_parent):
                    yield head.subst
        except UnificationFailed:
            continue
        except Cut, cut:
            if cut.parent == cut_parent:
                raise
            break
        finally:
            restore(trail)
    else:
        if is_cut(head):
            raise Cut(cut_parent)

...

for substitutions in resolve(db, query):
    print substitutions

这是一个由递归生成器实现的Prolog引擎。db是表示事实和规则的Prolog数据库的dict。unify()是一个unification函数,它为当前目标创建所有替换,并将更改附加到trail中,以便以后可以撤消。restore()执行撤消操作,is \u cut()测试当前目标是否为“!”,这样我们就可以修剪树枝了。

• Python Generator Tricks

n、 n+1,n+2,。。。。

def add(x):
    while True:
        yield x
        x+=1

for index in add(5):
    if not index<100: break ## do equivalent of range(5,100)
    print(index)

这不是递归的,但我认为这里不需要递归样式。

基于other answers链接的递归版本,其中生成器调用生成器,但不是递归的:

from __future__ import generators

def range_from(n):
    yield n
    for i in range_from(n+1):
        yield i

for i in range_from(5):
    if not i<100: break ## until 100 (not including)
    print i