python yield from 语法

python yield from 语法

yield语法比较简单, 教程也很多 , yield from的中文讲解很少 , python官网是这样解释的

PEP 380 adds the yield from expression, allowing a generator to delegate part of its operations to another generator. This allows a section of code containing yield to be factored out and placed in another generator. Additionally, the subgenerator is allowed to return with a value, and the value is made available to the delegating generator.

官网链接

大意是说 yield from 表达式允许一个生成器代理另一个生成器, 这样就允许生成器被替换为另一个生成器, 子生成器允许返回值

def g1(x):
     yield  range(x)
def g2(x):
     yield  from range(x)
it1 = g1(5)
it2 = g2(5)
print( [ x for x in it1] )
#out [range(0, 5)]
print( [ x for x in it2] )
#out [0, 1, 2, 3, 4]

可以看到 , yield返回一个生成器 , 这个生成器就是range自身 , yield from 也返回一个生成器, 这个生成器是由range代理的

yield from 在递归结构中常用 , 例如

class Node:
    def __init__(self, value):
        self._value = value
        self._children = []
    def __repr__(self):
        return 'Node({!r})'.format(self._value)
    def add_child(self, node):
        self._children.append(node)
    def __iter__(self):
        return iter(self._children)
    def depth_first(self):
        yield self
        for c in self:
            yield from c.depth_first()

如果不加上from , depth_first 只能返回根节点的值