Python从小白到大白-Generator 漫谈 (续1)

按, 上篇介绍了generator相关的概念. 这篇来探究一下generator相关的操作方法. 以下都是使用Python 2.7.10.

根据官方文档 generator 通过 yield 来实现 next() 方法. > A function which returns a generator iterator. It looks like a normal > function except that it contains yield expressions for producing a > series of values usable in a for-loop or that can be retrieved one at > a time with the next() function.

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>>> def fib():
...     a, b = 0, 1
...     while True:
...             yield b
...             a, b = b, a + b
...
>>> fib
<function fib at 0x10d930c80>
>>> f = fib()
>>> f
<generator object fib at 0x10d92faa0>
>>> next(f)
1
>>> next(f)
1
>>> next(f)
2
>>> next(f)
3
>>> next(f)
5

以上, 我们可以看到yield用法. 每次执行到yield, 都会记住执行状态的局部变量以及表达式。再下次恢复时, 即next(), 它会从记住的状态继续执行.

generator的另一种调用方法是通过send(value)来实现的。我们来看一个例子:

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>>> def gen():
...     while True:
...             value = yield
...             print(value)
...
>>> g = gen()
>>> g.send("ek")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: can't send non-None value to a just-started generator

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>>> next(g)
>>> g.send("ek")
ek
>>> next(g)
None

以上, 展示了如何讲value传入yield当前表达式. 具体来说, 使用send(value)时, generator停在yield的语句. 传入的值被复制到value, 然后print函数打印value, 经过循环遇到yield时, 暂停. 需要注意的是在没有执行next()前, generator状态并没有停在yield状态，也就无法传入值.

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>>> g = gen()
>>> g.send(None)
>>> g.send("ek")
ek

以上, 除了next(), 可以让generator到达yield, send(None)作用是一样的.

让我们乘热打铁, 看一个稍微复杂的例子:

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>>> def gen(value = None):
...     while True:
...             value = (yield value)
...             print("The value is", value)
...             if value:
...                     value += 1
...
>>> g = gen(1)
>>> next(g)
1
>>> g.send(2)
('The value is', 2)
3
>>> g.send(10)
('The value is', 10)
11
>>> next(g)
('The value is', None)

在这个例子里, value = (yield value) 这个形式看起来很复杂. 但两个value 的含义并不相同. 具体来看, 执行next()时, generator执行到yield value表达式, 保存上下文环境暂停返回当前值1. 再执行send(value)时, 从value = yield开始, 打印传入的值 2, 再次遇到yield value暂停返回当前值 3 (上个循环时加上 1). send(10)是一样的. 在看最后一个next(), 这里需要记住的是调用next()表达式的值时, yield的值总是为None. 因此, 在这里返回None.

接下来, 我们来实现一下Python 内建函数 range() 实现方法 (复刻版, 非真实实现):

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>>> def my_range(start, stop, step = 1):
...     if stop <= start:
...         raise RuntimeError("Start must be smaller than stop")
...     i = start
...     while i < stop:
...         yield i
...         i += step
...
>>> try:
...     for k in my_range(10, 50, 3):
...         print(k)
... except RuntimeError as ex:
...     print(ex)
... except:
...     print("Unknown error occurred")
...
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最后, generator 的另外两个方法分别是 throw(type[, value[, traceback]]) 和 close(). 前者用于抛出 type 异常, 后者用于关闭generator. 我们直接看下Python文档的例子:

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>>> def echo(value=None):
...     print("Execution starts when 'next()' is called for the first time.")
...     try:
...         while True:
...             try:
...                 value = (yield value)
...             except Exception as e:
...                 value = e
...     finally:
...         print("Don't forget to clean up when 'close()' is called.")
...
>>> generator = echo(1)
>>> print(next(generator))
Execution starts when 'next()' is called for the first time.
1
>>> print(next(generator))
None
>>> print(generator.send(2))
2
>>> generator.throw(TypeError, "spam")
TypeError('spam',)
>>> generator.close()
Don't forget to clean up when 'close()' is called.

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