>>> tab = [['x']*3]*3
>>> tab
[['x', 'x', 'x'], ['x', 'x', 'x'], ['x', 'x', 'x']]
>>> tab[1][0] = 5
>>> tab
[[5, 'x', 'x'], [5, 'x', 'x'], [5, 'x', 'x']]
>>> 

why not only the element tab[1][0] is changed ?

Because elements of the tab list are the same list. And since list is a mutable type, modifying list object will be visible to via all the reference / names that point to that object. There are various ways to work around this behaviour, typically a list comprehension is used.

P.S. Please don't add spurious components to the issue, it adds people to the nosy list.

To add to what SilentGhost just said; the '*' operator for lists doesn't create new lists, but simply creates new references to the same list (i.e. it doesn't re-evaluate the expression).

To create separate lists, you can do a list comprehension such as:

[['x'] * 3 for _ in range(3)]

More information is available on the FAQ: https://docs.python.org/3/faq/programming.html#why-did-changing-list-y-also-change-list-x