首先看兩段代碼及其輸出
def gener(num):
while True:
print("0:before yield num is: %d" % num)
num = yield
print("1:after yield num is %d" % num)
print("exc end")
g = gener(1)
g.send(None)
print("goto loop")
for i in range(5):
print("---loop---")
print(">>>>send back:" + str(g.send(i)))
# 輸出為 >>
0:before yield num is: 1
goto loop
---loop---
1:after yield num is 0
0:before yield num is: 0
>>>>send back:None
---loop---
1:after yield num is 1
0:before yield num is: 1
>>>>send back:None
def gener(num):
while True:
print("0:before yield num is: %d" % num)
num = yield num**2 # 代碼唯一有變化的地方
print("1:after yield num is %d" % num)
print("exc end")
g = gener(1)
g.send(None)
print("goto loop")
for i in range(5):
print("---loop---")
print(">>>>send back:" + str(g.send(i)))
# 輸出為 >>
0:before yield num is: 1
goto loop
---loop---
1:after yield num is 0
0:before yield num is: 0
>>>>send back:0
---loop---
1:after yield num is 1
0:before yield num is: 1
>>>>send back:1
分析
generator函數調用后，第一次send(None)，generator執行到yield之前，之后每次調用send(), 從yield所在行開始執行，運行到片段結尾或再次遇到yield， generator從yield中接受參數，如果yield后有表達式，則返回表達式的值，類似return關鍵字的功能。yield關鍵字保存了generator每次的執行狀態。
