yield

return 的作用

在一個 generator function 中，如果沒有 return，則默認執行至函數完畢，如果在執行過程中 return，則直接拋出 StopIteration 終止迭代。

def func():
    a = 0
    b = 1
    while True:
        if b >= 5:
            return
        yield b 
        a, b = b, a + b
f = func()
while True:
    print(next(f))
# 結果：
# 1
# 1
# 2
# 3
# ---------------------------------------------------------------------------
# StopIteration                             Traceback (most recent call last)
# <ipython-input-4-3a2f06d59b2d> in <module>
#       9 f = func()
#      10 while True:
# ---> 11     print(next(f))
# 
# StopIteration: 

所以，想要使for迭代終止，可以在generator function中使用兩種方式，一種是while+條件，第二種就是使用return語句。

 return方式：

def func():
    a = 0
    b = 1
    while True:
        # 當b等於5時終止
        if b >= 5:
            return
        yield b 
        a, b = b, a + b
f = func()
for i in f:
    print(i)

1
1
2
3

 while+條件：

def func(max):
    n = 0
    a = 0
    b = 1
    while n < max:
        yield b 
        a, b = b, a + b
        n += 1
f = func(3)
for i in f:
    print(i)

1
1
2

用yield讀大文件：

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