yield和生成器， 通过斐波那契数列学习（2.5）

实现斐波那契数列的集中方法

1. 返回一个数

   def fib(max):
   n, a, b = 0, 0, 1
   while n < max:
       print(b)
       a, b = b, a+b
       n += 1
   
   
   fib(5)
   

2. 返回列表

   def fib(max):
   res = []
   n, a, b = 0, 0, 1
   while n < max:
       res.append(b)
       a, b = b, a+b
       n += 1
   return res
   fib(5)
   

3. 使用可迭代对象

   from collections import Iterator
   class Fib(Iterator):
       def __init__(self, max):
           self.max = max
           self.index = 0
           self.a = 0
           self.b = 1
       def __next__(self):
           if self.index < self.max:
               res = self.b
               self.a, self.b = self.b , self.b + self.a
               self.index += 1
               return res
           else:
               raise StopIteration #  StopIteration()
       def __iter__(self):
           return self
       
   for x in Fib(5):
       print(x)
   

4. 使用yield 生成器

   def fib4(max):
   n, a, b = 0, 0, 1
   while n < max:
       yield b
       a, b = b, a+b
       n += 1
       
   f = fib4(5)
   
   for i in range(6):
       print('%s:  %s' % (i,next(f)))
   

5. yield防止读大文件出现内存不够的问题

   def read_by_chunk(filename):
   BLOCK_SIZE = 1024
   with open(filename, 'rb') as f:
       while True:
           block = f.read(BLOCK_SIZE)
           if block:
               yield block
           else:
               return
   

6. 使用pandas读文件，类似5的操作

   import pandas as pd
   reader = pd.read_table('AA.csv', chunksize=1024)
   for chunk in reader:
       print(chunk)
   

7. 使用pandas读文件，运行时才制定块大小

   import pandas as pd
   

reader = pd.read_csv('AA.csv', sep='|', iterator=True)
while True:
try:
chunk = reader.get_chunk(5)
print(chunk)
except:
break
```

8. yield生成斐波那契数列的迭代对象

   class Fib:
       def __init__(self, start, end):
           self.start = start
           self.end = end
       def __iter__(self):
           index, a, b = 0, 0, 1
           while index < self.start:
               a, b = b, a+b
               index += 1
           while index <= self.end:
               yield b
               a, b = b, a+b
               index += 1
               
   for i in Fib(1,10):
       print(i)
   

9. yield生成质数的迭代对象

   	class PrimeNum:
       def __init__(self, start, end):
           self.start = start
           self.end = end
       def is_prime(self, num):
           if num < 2:
               return False
           for i in range(2,num):
               if num%i == 0:
                   return False
           return True
       def __iter__(self):
           for i in range(self.start, self.end):
               if self.is_prime(i):
                   yield i
                   
   for i in PrimeNum(1, 20):
       print(i)
   

注： 参考了runoob