Parameters in functions: take care!

I found a nice page with something really strange for me, I'll have to keep this in my mind because it's totally different from IDL.
The page: http://hetland.org/writing/instant-python.html
The "problem", as I rewrote it:
Let's define 2 functions:
def test(x):
    x=2
def test2(x):
    x[0]=2
Now, we'll call test and test2 with different variables:
In [3]: y=1
In [4]: test(y)
In [5]: y
Out[5]: 1
y is not changed. BUT:
In [7]: y=[1,2,3]
In [8]: test2(y)
In [9]: y
Out[9]: [2, 2, 3]
Now y[0] is changed!!! And even worst:
In [10]: y=[[4,5,6],[14,15,16],[24,25,26]]
In [11]: test2(y)
In [12]: y
Out[12]: [2, [14, 15, 16], [24, 25, 26]]
And the best for the end:

In [13]: y=[[4,5,6],[14,15,16],[24,25,26]]

In [15]: test2(y[0])
In [16]: y
Out[16]: [[2, 5, 6], [14, 15, 16], [24, 25, 26]]
So part of the table can be changed, but a single variable not: exactly the opposite of IDL...