Don't understand why this quicksort code works

[nilamo]

Because the data structure is a list, which is mutable. When you assign something to a variable (x = 42), the old value of the variable still exists, but there's a new binding to the new value. But with lists, the actual variable was never reassigned, the only thing that changed was the contents of the list.

Using your example...

>>> def f(x):
...   x[0] = x[0] * 2
...   return "unrelated string"
...
>>> x = [5, 4, 3, 2, 1, 0]
>>> f(x)
'unrelated string'
>>> x
[10, 4, 3, 2, 1, 0]
>>> # but if we were to re-assign x, instead of using an element...
...
>>> def f(x):
...   x = ['green', 'eggs', 'and', 'spam']
...   return 0
...
>>> x
[10, 4, 3, 2, 1, 0]
>>> f(x)
0
>>> x
[10, 4, 3, 2, 1, 0]