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

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