生成器

生成器的例子

任何包含yield的语句的函数称之为生成器．函数会返回一个迭代器．

简单例子

#!/usr/bin/env python
def flatten(nested):
	for sublist in nested:
		for ele in sublist:
			yield ele
nested = [[1,2],[3,4],[5]]
print list(flatten(nested))

递归生成器

def flatten_recursion(nested):
	try:
		for sublist in nested:
			for ele in flatten_recursion(sublist):
				yield ele
	except TypeError:
		yield nested

print list(flatten_recursion([[[1],2],3,4,[5,[6,7]],8]))

处理字符串：（检查一个字符串：将传入的对象和一个字符串拼接，看看是否产生TypeError）

def flattern_string(nested):
	try:
		try:
			nested + ''
		except TypeError:
			pass
		else:
			raise TypeError:
		for sublist in nested:
			for ele in flattern_string(sublist):
				yield ele
	except TypeError:
		yield nested
print list(flattern_string(['foo',['bar']]))

生成器推导式于跟列表推导式类似，只不过将［］改为()

eg:

(i for i in range(8))

模拟生成器

１．在函数开始处添加　result = []
2 .　yield　some_expression 替换为 result.append(some_expression)
3. return result

def flattern_string(nested):
        reuslt = []
	try:
		try:
			nested + ''
		except TypeError:
			pass
		else:
			raise TypeError:
		for sublist in nested:
			for ele in flattern_string(sublist):
				result.append( ele)
	except TypeError:
		result.append(nested)
        return result
print list(flattern_string(['foo',['bar']]))

8皇后问题

#!/usr/bin/env python
import random
# state[0]=3表示第一行皇后放在第四列
def queens(num = 8, state=()):
	result = []
	for pos in range(num):
		if not conflict(state, pos):
			if len(state) == num -1:
				yield (pos,)
			else:
				for result in queens(num, state+(pos,)):
					yield  result +(pos,) 


def conflict(state, nextX):
	nextY = len(state)
	for i in range(nextY):
            ＃下一个皇后和前个皇后的水平距离为０或者等于垂直距离
		if abs(state[i] - nextX) in (0, nextY - i):
			return True
	return False

def prettyprint(solution):
	def line(pos, length = len(solution)):
		return '.'*(pos) + "X" + '.'*(length -1 -pos)
	for pos in solution:
		print line(pos)

prettyprint(random.choice(list(queens(8))))