从生成器中强制列出

Question

>>>def change(x): 
...    x.append(len(x))
...    return x
>>>a=[]
>>>b=(change(a) for i in range(3))
>>>next(b)
[0]
>>>next(b)
[0,1]
>>>next(b)
[0,1,2]
>>>next(b)
Traceback ... StopIteration
>>>a=[]
>>>b=(change(a) for i in range(3))
>>>list(b) #expecting [[0],[0,1],[0,1,2]]
[[0,1,2],[0,1,2],[0,1,2]] 

So I was just testing my understanding of generators and messing around with the command prompt and now I'm unsure if I actually understand how generators work.

Answer 1

The problem is that all calls to change(a) return the same object (in this case, the object is the value of a ), but this object is mutable and changes its value. An example of the same problem without using generators:

a = []
b = []
for i in range(3):
   a.append(len(a))
   b.append(a)
print b

If you want to avoid it, you need to make a copy of your object (for example, make change return x[:] instead of x ).