How do I append an item to a list within a list in python?

Question

OK, check out the following codes first:

Demo1 = [[], []]
Demo2 = [[]] * 2
Demo1[0].append(1)
Demo2[0].append(1)
print "Demo1: ", Demo1
print "Demo2: ", Demo2

And here's the output:

Demo1: [[1], []]
Demo2: [[1], [1]]

I need to create a list whose items are all list as well just like Demo1 and Demo2, of course I used Demo2 in my script and it kept getting into trouble until I found the reason which is what you can see from above codes. So why is this happening? Most of the cases I would use Demo2 to create such list as its length differs each time, but how do I append an item to separate lists within the list without getting into such mess?

• drhanlau: I repeat the print "Demo2: ", Demo2 a couple of times and I found that online first line doesn't work, subsequent line is working, weird huh
• Kirill: Should be in the next weekly newsletter :)

Answer 1

For you first question: It is happening because in Demo2 case your list contains two copies of the same object. See for example below where I print the memory locations of those elements, noting that they differ for Demo1 but match for Demo2.

>>> Demo1 = [[], []]
>>> Demo2 = [[]] * 2
>>> print id(Demo1[0]), id(Demo1[1])
33980568 34018800
>>> print id(Demo2[0]), id(Demo2[1])
34169920 34169920

---

For your second question: you could use a list comprehension like [[] for i in xrange(n)], in order to be creating a new list n times rather than duplicating the same list n times.

Example:

>>> Demo2 = [[] for i in xrange(2)]
>>> Demo2
[[], []]
>>> Demo2[0].append(1)
>>> Demo2
[[1], []]

• Shane: Thanks mate, problem solved. Although still don't understand why it's designed to be like this. To me, it does not make sense creating duplicate stuff using Demo2 = [[]] * 2.

Answer 2

Demo2 is a list containing two references to the same list.

Demo2 = [[] for x in range(2)]