Python基础 (yield生成器)

如果在一个函数中使用了yield，那么这个函数实际上生成的是一个生成器函数 ,返回的是一个generator object。生成器是实现迭代的一种方式

特点:

• 其实返回的就是可以的迭代对象
• 和迭代的方法一样，可以使用next()，for循环的方法取值:
• 当一个yeild语句被执行，这个迭代器(函数)的状态像是被冻结(frozen)了一样并且返回next()调用的结果
• 协程

The "yield" statement
*********************

   yield_stmt ::= yield_expression

The "yield" statement is only used when defining a generator function,
and is only used in the body of the generator function. Using a
"yield" statement in a function definition is sufficient to cause that
definition to create a generator function instead of a normal
function.
"yield"语句仅用于当定义一个生成器函数并且作为这个生成器函数的主体
在一个函数内容使用yiled足以创建一个生成器函数，来替代普通的函数.

When a generator function is called, it returns an iterator known as a
generator iterator, or more commonly, a generator.  The body of the
generator function is executed by calling the generator's "next()"
method repeatedly until it raises an exception.
当一个生成器函数被调用，它返回一个迭代器称作生成器函数，或者一般的生成器,
这个生成器函数是通过next()方法反复的执行直到出现了异常

When a "yield" statement is executed, the state of the generator is
frozen and the value of "expression_list" is returned to "next()"'s
caller.  By "frozen" we mean that all local state is retained,
including the current bindings of local variables, the instruction
pointer, and the internal evaluation stack: enough information is
saved so that the next time "next()" is invoked, the function can
proceed exactly as if the "yield" statement were just another external
call.
当一个yeild语句被执行，这个迭代器的状态像是被冻结(frozen)并且返回next()调用的结果,
通过"frozen"意味着所有的局部(local)状态被保存，包括当前绑定的局部变量，指令指针.

As of Python version 2.5, the "yield" statement is now allowed in the
"try" clause of a "try" ...  "finally" construct.  If the generator is
not resumed before it is finalized (by reaching a zero reference count
or by being garbage collected), the generator-iterator's "close()"
method will be called, allowing any pending "finally" clauses to
execute.

For full details of "yield" semantics, refer to the Yield expressions
section.

Note: In Python 2.2, the "yield" statement was only allowed when the
  "generators" feature has been enabled.  This "__future__" import
  statement was used to enable the feature:

     from __future__ import generators

See also: **PEP 0255** - Simple Generators

     The proposal for adding generators and the "yield" statement to
     Python.

  **PEP 0342** - Coroutines via Enhanced Generators
     The proposal that, among other generator enhancements, proposed
     allowing "yield" to appear inside a "try" ... "finally" block.

help(yield)

1、如果函数遇到了return，这个函数就执行完了

def func1():
    return 'one'
    return 'two'
print func1()
one

2、如果将retrun换成yield

• 返回的是一个generator(生成器)对象，可以通过next()方法取值直到抛出异常
• 可以通过for循环迭代

def func1():
    yield 'one'
    yield 'two'
    yield 'three'
reslut = func1()
print reslut
StopIteration
print reslut.next()
one
print reslut.next()
two
print reslut.next()
three
print reslut.next()  #异常
StopIteration

for循环

for item in reslut:
    print item
one
two
three

2、实现类似xrange的功能

def mxrange(arg):
    temp = -1
    while True:
        temp = temp + 1
        if temp >= arg:
            return
        else:
            yield temp
for item in mxrange(10):
    print item,

总结：

• yield用来做生成器函数，可以通过next()、for循环方法取值，只是遇到了yield时函数就像冻结一样，保存当前的变量，返回的是next()的结果
• 和生成器、迭代器一样不用在内存中创建大量的数据，而是需要调用的时候通过迭代返回数据