python生成器中产量的确切返回点

Question

In a quest to teach myself Python generators, especially their send() method, I wanted to write a toy example that would allow me calculate an "interruptible" Fibonacci sequence.

Eventually I succeeded, but I don't understand exactly what is going on. Consider these two examples.

Successful example:

def genFib(a=0, b=1):
    while True:
        c = yield a+b
        if c:
            a, b = b, c
        else:
            a, b = b, a+b
    return
​
fib_number = genFib()
​
print fib_number.next()
print fib_number.next()
print fib_number.next()
print fib_number.next()
print fib_number.next()
print fib_number.send(100)
print fib_number.next()
print fib_number.next()
​
1
2
3
5
8
105
205
310

Failure that I thought would work:

def genFib(a=0, b=1):
    while True:
        c = yield a+b
        a, b = b, c
    return
​
fib_number = genFib()
​
print fib_number.next()
print fib_number.next()
1
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-59-4513153ea517> in <module>()
      8 
      9 print fib_number.next()
---> 10 print fib_number.next()

<ipython-input-59-4513153ea517> in genFib(a, b)
      1 def genFib(a=0, b=1):
      2     while True:
----> 3         c = yield a+b
      4         a, b = b, c
      5     return

TypeError: unsupported operand type(s) for +: 'int' and 'NoneType'

Why does the second example not work? In particular, the second example seems assign a value of None to c in all but the first executions of the generator. Why? How does it know to assign to c the first time next() is called, but not the next next() time?

My understanding was that generators resume execution (when they are called again) beginning on the line after the yield line, but my example has me thinking this is wrong, at least in some cases. Where exactly does execution resume from? Does it re-execute the yield line but this time with somehow leaving off everything after the yield ? How does c get assigned to None ?

Answer 1

yield is a bit of a strange expression. Execution pauses at the yield after the right-hand side is evaluated, but before the assignment takes place.

The value that is returned in the yield statement is whatever you .send to the generator. Since you are calling .next rather than .send , the value returned from the yield expression ( c ) is None . In the first example, that's fine since c is checked for truthiness and then is not used if it is falsy (eg None ). However, in the second example, c isn't checked so you end up adding None to an integer (which obviously doesn't produce desirable results).