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生成器
生成器很容易实现，但理解起来有些困难。
生成器用于创建迭代器，但采用了不同的方法。生成器是一些简单的函数，以一种特殊的方式一次返回一个可迭代的项目集合。
当使用 for 语句开始对一组项目进行迭代时，生成器就会运行。一旦生成器的函数代码达到 "yield" 语句，生成器就会将执行权交还给 for 循环，从集合中返回一个新值。生成器函数可以根据需要生成任意多个值（可能是无限的），并在每次轮到它时产生一个。
这是一个简单的生成器函数示例，它返回 7 个随机整数：
```python import random
def lottery(): # 返回 6 个介于 1 到 40 之间的数字 for i in range(6): yield random.randint(1, 40)
# 返回第 7 个介于 1 到 15 之间的数字
yield random.randint(1, 15)
for random_number in lottery(): print("And the next number is... %d!" % (random_number)) ```
这个函数自行决定如何生成随机数，并逐一执行 yield 语句，在此期间暂停以将执行权交还给主 for 循环。
Exercise
编写一个生成器函数，返回斐波那契数列。它们是使用以下公式计算的：数列的前两个数字总是等于 1，每返回一个连续的数字都是最后两个数字的和。
提示：你可以在生成器函数中只使用两个变量吗？记住，可以同时进行赋值。代码
python
a = 1
b = 2
a, b = b, a
print(a, b)
将同时交换 a 和 b 的值。
# fill in this function
def fib():
pass #this is a null statement which does nothing when executed, useful as a placeholder.
# testing code
import types
if type(fib()) == types.GeneratorType:
print("Good, The fib function is a generator.")
counter = 0
for n in fib():
print(n)
counter += 1
if counter == 10:
break
# fill in this function
def fib():
a, b = 1, 1
while 1:
yield a
a, b = b, a + b
# testing code
import types
if type(fib()) == types.GeneratorType:
print("Good, The fib function is a generator.")
counter = 0
for n in fib():
print(n)
counter += 1
if counter == 10:
break
test_output_contains("Good, The fib function is a generator.")
success_msg('Good work!')
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