Python: Append lambda functions to list

Question

Can anyone do sanity check?

I'm trying to make functions in for-loop. The point I can't understand is summarized in the following code:

f_list = []
for i in range(10):
    f = lambda j : i
    f_list.append(f)

Then,

>>> f_list[0](0)
9                  #I hope this is 0.
>>> f_list[1](0)
9                  #I hope this is 1.

Why is this happen??

Thank you for pointing out the past question/answers. Yes, this is actually a duplicate of them. — ywat, Dec 12 '13 at 06:32

score 10 · Accepted Answer · edited May 23 '17 at 10:27

10

Edit: Almost the same problem is already discussed in Stackoverflow, here.

This is because of the closure property of python. To get what you actually need, you need to do like this

f = lambda j, i = i : i

So, the output of this program becomes like this

f_list = []
for i in range(5):
    f = lambda j, i = i : i
    f_list.append(f)

for i in range(5):    
    print f_list[i](0)

Output

edited May 23 '17 at 10:27

Community

answered Dec 12 '13 at 06:21

thefourtheye

Thank you for the simple answer. I understand the point. – ywat Dec 12 '13 at 06:33

1 Answers1