我对python中变量的范围感到困惑。这是我正在尝试做的一个玩具示例：
a = True
enumerated_set = enumerate(['tic','tac','toe'])
for i,j in enumerated_set:
if a == True:
print j
我得到的结果是：
tic
tac
toe
现在，
print a
返回
`True`
如果我再跑
for i,j in enumerated_set:
if a == True:
print j
我没有输出。
我很困惑......因为 global a = True，为什么在第二个循环期间没有执行打印。
我感谢您的帮助。
编辑：另一个我很困惑的例子
y = 'I like this weather'.split()
for item in y:
for i,j in enumerated_set:
if a == True:
print j
也没有输出....
你的布尔值实际上不是问题。那总是True。
enumerated_set是一个发电机。一旦你循环通过它，它就会筋疲力尽。您需要创建一个新的。
In [9]: enumerated_set = enumerate(['tic','tac','toe'])
In [10]: enumerated_set.next()
Out[10]: (0, 'tic')
In [11]: enumerated_set.next()
Out[11]: (1, 'tac')
In [12]: enumerated_set.next()
Out[12]: (2, 'toe')
In [13]: enumerated_set.next()
---------------------------------------------------------------------------
StopIteration Traceback (most recent call last)
/usr/local/<ipython-input-13-7b0a413e4250> in <module>()
----> 1 enumerated_set.next()
StopIteration:
This is not due to scope, it is due to the nature of enumerate, which creates a generator, not a list. Generators are single-use: they pop off elements in sequence, they don't create a list which can be evaluated again. This saves memory.
If you wanted to iterate over enumerated_set twice, you might do this:
enumerated_set = list(enumerate(['tic','tac','toe']))
It is nothing to do with the a variable. You are using an enumerator object,and in the first loop it is gone to its end. You have to recreate it for the second loop.