Adding to lists in Python 2.7

Question

>>> A = [1,2,3,4]
>>> D = A
>>> D
[1, 2, 3, 4]
>>> D = D + [5]
>>> A
[1, 2, 3, 4]
>>> C = A
>>> C += [5]
>>> A
[1, 2, 3, 4, 5]

Why does C += [5] modifies A but D = D + [5] doesn't?

Is there any difference between = and += in python or any other language in that sense?

Answer 1

Actually yes there is. When you use += you're still referencing to the same object, however with + you're creating a new object, and with = you reassign the reference to that newly created object. This is especially important when dealing with function arguments. Thanks to @Amadan and @Peter Wood for clarifying that.