Append item to a specified list in a list of lists (Python)
append, list, python
Solution
Python lists are mutable objects, so when you do `[[1]]*20` it creates one list object `[1]` and then places 20 references to it in the toplevel list.
As far as the mutability problem is concerned, this is the same as the following
a = [1,2,3]
b = a
b.append(4)
a # [1,2,3,4]
This happens because `b=a` merely copies the reference to the list instance from `a` to `b`. They are both referring to the same actual list.
In order to create a list of lists, like you tried above, you need to create a unique list for each entry. A list comprehension works nicely:
mainlist = [[1] for x in range(20)]
mainlist[0].append(2)
mainlist # [[1,2],[1],[1],...]
Edit
As an aside, since type names are metaclasses in Python, naming your variables by the type name is a bad idea. The reason is that can cause several issues further down in the code:
a = range(3) # [0,1,2]
type(a) # (type 'list')
isinstance(a, list) # True
Now, create a variable named `list`
list = range(3)
list # [0,1,2]
isinstance(list, list)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: isinstance() arg 2 must be a class, type, or tuple of classes and types
Not to mention, now you cant use the `list()` operator
c = list((1,2,3))
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: 'list' object is not callable
Problem
I'm practicing my progamming skills by solving problems from project euler at the moment, and now I've come across some (in my opinion) strange behavior on Python. When I do: ``` list = [[1]]*20 ``` I get a list of 20 lists containing element 1, as expected. However, when I would like to append a 2 to the third element from this list, I would do that as follows: ``` list[3].append(2) ``` This however changes ALL the elements in the list. Even when I take a detour, like: ``` l = list[3] l.append(2) list[3] = l ``` All my elements get changed. Can anyone please tell me how to do this and get an output like so: ``` [[1], [1], [1], [1, 2], [1] .... [1]] ``` Thanks in advance.