Python PEP342

Specification Summary

    1. Redefine "yield" to be an expression, rather than a statement.
       The current yield statement would become a yield expression
       whose value is thrown away.  A yield expression's value is
       None whenever the generator is resumed by a normal next() call.

除了作为一个statement将一个函数变成一个generator以外，yield可以被视为是一个表达式，并用于赋值。

比如

>>> def whizbang():
        for i in range(10):
            x = yield i
            print 'got sent:', x


>>> i = whizbang()
>>> next(i)
0
>>> next(i)
got sent: None
1
>>> i.send("hi")
got sent: hi
2

    2. Add a new send() method for generator-iterators, which resumes
       the generator and "sends" a value that becomes the result of the
       current yield-expression.  The send() method returns the next
       value yielded by the generator, or raises StopIteration if the
       generator exits without yielding another value.

见1，一个yield expression如果不通过send传入一个值，那么其值就默认是None

举例：
给出各基数下的最小的素数（比如给出iterations = 3, base = 10，就相当于分别求出大于10^1,10^2,10^3的最小的素数）

import math
def is_prime(number):
    if number > 1:
        if number == 2:
            return True
        elif number % 2 == 0:
            return False
        for current in range(3, int(math.sqrt(number) + 1), 2):
            if number % current == 0:
                return False
        return True
    return False

def print_successive_primes(iterations, base=10):
    prime_generator = get_primes(base)
    prime_generator.send(None)
    for power in range(iterations):
        print(prime_generator.send(base ** power))


def get_primes(number):
    while True:
        if is_prime(number):
            number = yield number
        number += 1

可以看到我们在访问迭代器之前需要首先对迭代器get_primes的实例使用send，传入None，为什么呢？因为：

    Because generator-iterators begin execution at the top of the
    generator's function body, there is no yield expression to receive
    a value when the generator has just been created.  Therefore,
    calling send() with a non-None argument is prohibited when the
    generator iterator has just started, and a TypeError is raised if
    this occurs (presumably due to a logic error of some kind).  Thus,
    before you can communicate with a coroutine you must first call
    next() or send(None) to advance its execution to the first yield
    expression.

send()会访问一个yield expression，而generator实例化之后并没有执行到yield expression，需要执行一个next()或者与其等价的send(None)来访问到yield expression

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其余见https://www.python.org/dev/peps/pep-0342/