详解Python3中yield生成器的用法

def count(n): 
  while n > 0: 
    yield n  #生成值：n 
    n -= 1 

c = count(5) 
c.__next__() #python 3.4.3要使用c.__next__()不能使用c.next()
>>> 5 
c.__next__() 
>>>4 

def count(n): 
  print ( "cunting" )
  while n > 0: 
    yield n  #生成值：n 
    n -= 1 

def count(n): 
  print ("cunting" ) 
  while n > 0: 
    print ('before yield') 
    yield n  #生成值：n 
    n -= 1 
    print ('after yield' )

for i in count(5): 
  print (i), 
 

import time  
def tail(f):  
  f.seek(0,2)#移动到文件EOF 
  while True:  
    line = f.readline() #读取文件中新的文本行 
    if not line:  
      time.sleep(0.1)  
      continue  
    yield line  
  
def grep(lines,searchtext):  
  for line in lines:  
    if searchtext in line:  
      yield line 
 
flog = tail(open('warn.log'))  
pylines = grep(flog,'python')  
for line in pylines:  
  print ( line, ) 
#当此程序运行时，若warn.log文件中末尾有新增一行，且该一行包含python,该行就会被打印出来 
#若打开warn.log时，末尾已经有了一行包含python，该行不会被打印，因为上面是f.seek(0,2)移动到了文件EOF处 
#故，上面程序实现了tail -f warn.log | grep 'python'的功能，动态实时检测warn.log中是否新增现了 
#新的行，且该行包含python  

def fibonacci(): 
  a=b=1 
  yield a 
  yield b 
  while True: 
    a,b = b,a+b 
    yield b 

for num in fibonacci(): 
  if num > 100: 
    break 
  print (num), 

def read_file(path): 
  size = 1024 
  with open(path,'r') as f: 
    while True: 
      block = f.read(SIZE) 
      if block: 
        yield block 
      else: 
        return 

>>> def test_return(): 
...   yield 4 
...   return 0 
... 
 File "<stdin>", line 3 
SyntaxError: 'return' with argument inside generator 

>>> def mygenerator(): 
...   print 'start...' 
...   yield 5 
...  
>>> mygenerator()      //在此处调用,并没有打印出start...说明存在yield的函数没有被运行,即暂停 
<generator object mygenerator at 0xb762502c> 
>>> mygenerator().next()   //调用next()即可让函数运行. 
start... 
5 
>>>  

>>> def fun2(): 
...   print 'first' 
...   yield 5 
...   print 'second' 
...   yield 23 
...   print 'end...' 
...  
>>> g1 = fun2() 
>>> g1.next()       //第一次运行,暂停在yield 5        
first 
5 
>>> g1.next()       //第二次运行,暂停在yield 23 
second 
23 
>>> g1.next()       //第三次运行,由于之后没有yield,再次next()就会抛出错误 
end... 
Traceback (most recent call last): 
 File "<stdin>", line 1, in <module> 
StopIteration 
>>> 

>>> def fun(): 
...   print 'start...' 
...   m = yield 5 
...   print m 
...   print 'middle...' 
...   d = yield 12 
...   print d 
...   print 'end...' 
...  
>>> m = fun()       //创建一个对象 
>>> m.next()        //会使函数执行到下一个yield前 
start... 
5 
>>> m.send('message')   //利用send()传递值 
message          //send()传递进来的  
middle... 
12 
>>> m.next() 
None            //可见next()返回值为空 
end... 
Traceback (most recent call last): 
 File "<stdin>", line 1, in <module> 
StopIteration