從消費者生成器返回單個值

Question

我有一個消費者生成器管道。從這些消費者的最後，我想在某個時間點返回結果。此作品：

class StopIterationWithResult(StopIteration): 
    def __init__(self, result): 
     super(StopIterationWithResult, self).__init__() 
     self.result = result 

# for definition fo consumer decorator see http://www.python.org/dev/peps/pep-0342/ 
@consumer 
def waitfor3(): 
    while True: 
     value = (yield) 
     if value == 3: 
      raise StopIterationWithResult('Hello') 

c = waitfor3() 
for i in range(10): 
    try: 
     print 'calling', i 
     c.send(i) 
    except StopIterationWithResult as r: 
     print 'stopped', r.result 
     break 

有沒有更好的方法？例如，如果由於return陳述引發StopIteration，您能否訪問生成器的返回值？

按照要求通過@alexis，這裏是一個管道的例子：

class StopIterationWithResult(StopIteration): 
    def __init__(self, result): 
     super(StopIterationWithResult, self).__init__() 
     self.result = result 

@consumer 
def add1_filter(consumer): 
    while True: 
     value = (yield) 
     consumer.send(value+1) 

@consumer 
def waitfor3(): 
    while True: 
     value = (yield) 
     print 'got', value 
     if value == 3: 
      raise StopIterationWithResult('Hello') 

c = waitfor3() 
f = add1_filter(c) 
for i in range(10): 
    try: 
     print 'calling', i 
     f.send(i) 
    except StopIterationWithResult as r: 
     print 'stopped', r.result 
     break 

這裏還有與@Martijn Pieters的答案是相同的 - 使得過濾器有點更難看，但：

@consumer 
def add1_filter(consumer): 
    result = None 
    while True: 
     value = (yield result) 
     result = consumer.send(value+1) 

@consumer 
def waitfor3(): 
    while True: 
     value = (yield) 
     print 'got', value 
     if value == 3: 
      yield 'Hello' 
      break 

c = waitfor3() 
f = add1_filter(c) 
r = None 
for i in range(10): 
    try: 
     print 'calling', i 
     r = f.send(i) 
    except StopIteration: 
     print 'stopped', r 
     break 

Answer 1

收益率是雙向的。在右邊的表達式中使用，它將接收並用作語句，它將生成表達式的結果。

只是產生這一結果的值：

def waitfor3(): 
    while True: 
     value = (yield) 
     if value == 3: 
      yield 'Hello' 
      break 

c = waitfor3() 
for i in range(10): 
    try: 
     print 'calling', i 
     result = c.send(i) 
    except StopIteration: 
     print 'stopped', result 
     break 

Answer 2

你並不需要繼承StopIteration，您可以訪問args你傳遞給它的構造函數。見http://docs.python.org/2/library/exceptions.html#exceptions.BaseException：

@consumer 
def add1_filter(consumer): 
    while True: 
     value = (yield) 
     consumer.send(value+1) 

@consumer 
def waitfor3(): 
    while True: 
     value = (yield) 
     print 'got', value 
     if value == 3: 
      # This doesn't work: 
      # return 456 # SyntaxError: 'return' with argument inside generator 
      # But this does: 
      raise StopIteration(123) 

c = waitfor3() 
f = add1_filter(c) 
for i in range(10): 
    try: 
     print 'calling', i 
     f.send(i) 
    except StopIteration as r: 
     print 'stopped (StopIteration)', r, type(r.args[0]) 
     break