What is the result of a yield expression in Python?

Question

I know that yield turns a function into a generator, but what is the return value of the yield expression itself? For example:

def whizbang(): 
    for i in range(10): 
        x = yield i

What is the value of variable x as this function executes?

I've read the Python documentation: http://docs.python.org/reference/simple_stmts.html#grammar-token-yield_stmt and there seems to be no mention of the value of the yield expression itself.

Answer 1

You can also send values to generators. If no value is sent then x is None, otherwise x takes on the sent value. Here is some info: http://docs.python.org/whatsnew/2.5.html#pep-342-new-generator-features

>>> 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