Mutability, copying lists but not sharing?

[cnb]

Is it not possible to have mutability without this? I know I can use
sorted and list(reversed) instead of .sort and .reverse but if I want
to copy a list and then change that list without changing the first
one?
And there isn't a .copy function so I have to "new = [] for element in
list: new.append(element)" ?
(I guess mutability is there for performance? Because I prefer a =
sorted(a) conceptually.)

>>a = [1,2,3]
b = a
b.append(4)
b

[1, 2, 3, 4]

>>a

[1, 2, 3, 4]

>>c = "hello"
d = c
d += " sir!"
c

'hello'

>>d

'hello sir!'

>>>

and what is the difference between extend and + on lists?

[Alan G Isaac]

cnb wrote:

And there isn't a .copy function so I have to "new = []
for element in list: new.append(element)"?

You can do
new = list(old)
which I like for being explicit, or just
new = old[:]

and what is the difference between extend and + on lists?

>>a = range(3)
b = a + range(3)
b

[0, 1, 2, 0, 1, 2]

>>a

[0, 1, 2]

>>a.extend(range(3))
a

[0, 1, 2, 0, 1, 2]

hth,
Alan Isaac