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.


`1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21`	`>>> 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:


`1 2 3 4 5 6 7 8 9 10`	`>>> 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`


`1 2 3 4 5`	`>>> 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.


`1 2 3 4`	`>>> 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:


`1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18`	`>>> 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):


`1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30`	`>>> 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") ... 10 13 16 19 22 25 28 31 34 37 40 43 46 49`

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:

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 >>> 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.


`1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23`	>>> 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.