Python rule of for statement

Question

I just began learning Python. So I am a beginner. I have a question about "for statement." I think I still don't know the rule of it. Please see below.

example:

list1 = []
list2 = []

def forStatement():
    for i in range(3):
        for j in range(5, 7):
            list2.append(j)
        list1.append(list2)
    return list1

The result I am looking for is;

[[5, 6], [5, 6], [5, 6]]

But when I run that code, it turns out like this.

[[5, 6, 5, 6, 5, 6], [5, 6, 5, 6, 5, 6], [5, 6, 5, 6, 5, 6]]

Can anyone help me? How can I get that result? Thank you so much.

Possible duplicate of [List of lists changes reflected across sublists unexpectedly](https://stackoverflow.com/questions/240178/list-of-lists-changes-reflected-across-sublists-unexpectedly) — Kind Stranger, Mar 01 '18 at 03:37

score 3 · Answer 1 · answered Mar 01 '18 at 03:08

3

You are almost correct, the only problem with your code is that you keep adding elements to list2. Instead, you should create a new list every time:

list1 = []

for i in range(3):
    list2 = []
    for j in range(5, 7):
        list2.append(j)
    list1.append(list2)

answered Mar 01 '18 at 03:08

damores

2,251
2
18
31

If my answer helped solve your problem, please consider marking it as the accepted answer, thanks! – damores Mar 01 '18 at 03:26

score 0 · Answer 2 · answered Mar 01 '18 at 03:14

0

You are appending in the loop, where you want to reset list2 each iteration of i

list1 = []

for i in range(3):
    list2 = []
    for j in range(5, 7):
        list2.append(j)
    list1.append(list2)

>>> print list1
[[5, 6], [5, 6], [5, 6]]

answered Mar 01 '18 at 03:14

Jared B.

106
4

score 0 · Answer 3 · answered Mar 01 '18 at 03:35

For your code, list1 is a container object which takes references to an object to which list2 is bound. (Usually, such a object is mentioned as list2 for brevity.) After the execution of the code, list1 finally contains three elements (i.e. references to list2) and list2 obviously contains triple (outer loop executed 3 times) consecutive 5, 6 (every execution of inner loop append 5 and 6 to list2).

The following code should be what you expect:

list1 = []

list2 = None

for i in range(3):
    list2 = [] # To create a new empty list object and let it bound to the variable list2 every outer loop.
    for j in range(5, 7):
        list2.append(j)
    list1.append(list2)

In this code, after the execution, list1 contains three elements that are distinct objects. But they were all bound to the variable list2 in the past.

Python rule of for statement

3 Answers3