I think it should contain different lists for each "append" but it is not

Question

after I run this little piece of code, it would always return a list that contains exactly same smaller lists...

import time
from random import randint

t = [0,0]
a = []

def rec():
    a.append(t)

for i in range(10):
    t[0] = randint(10,20)
    t[1] = randint(40,500)
    rec()
    time.sleep(.0001)

print a

I get something like this as output, why?

[[17, 134], [17, 134], [17, 134], [17, 134], [17, 134], [17, 134], [17, 134], [17, 134], [17, 134], [17, 134]]

You could say `a.append(list(t))` which will make a copy of `t` first. — Peter Wood, Aug 21 '15 at 13:33
http://stackoverflow.com/questions/2612802/how-to-clone-or-copy-a-list-in-python — Padraic Cunningham, Aug 21 '15 at 13:36
You append the **same** list everytime, `t` is always the same list, changing the list will change the list wherever that same list exists. Assignment, sending as a parameter to a function or returning just means that you get another name for the same object. — skyking, Aug 21 '15 at 13:51

score 2 · Answer 1 · edited May 23 '17 at 12:14

2

Welcome to the wacky world of mutable vs immutable types!

The short of it is that in Python list's are mutable, meaning references to lists will actually change the original list! You will have to create a new list by doing t = [randint(10,20), randint(40,500)]. That should do the trick for you :)

For more information on mutable vs immutable types refer to this SO Q/A!

I would also recommend not having functions like rec. Try to avoid using globals when you do not need them (they make debugging more difficult than it needs to be). Why not just pass in a parameter? Or just use append inline!

edited May 23 '17 at 12:14

Community

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answered Aug 21 '15 at 13:33

Michael S Priz

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But you need to declare it `global` first, otherwise it will just be a local list and won't be the one appended by `rec`. – Peter Wood Aug 21 '15 at 13:34
But `t` was already declared in the global scope! The OPs code with my suggestion added works for me :) – Michael S Priz Aug 21 '15 at 14:07
thanks! Well because I am using a decorator to limit the speed that rec() runs, so I need to define a function there... – mengqimusic Aug 21 '15 at 15:30

score 0 · Answer 2 · answered Aug 21 '15 at 13:35

0

How about using list comprehensions - [[randint(10,20), randint(40,500)] for i in range(10)]?

answered Aug 21 '15 at 13:35

matino

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I think it should contain different lists for each "append" but it is not

2 Answers2