>>> 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?
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.
= doesn't create the new object. = modifies the name to reference another object.
= that creates a new object, it's the +. D = D + [5] invokes D = D.__add__([5]); C += [5] invokes C.__iadd__([5]). __add__ creates a new object, __iadd__ does not; = just assigns the reference.
a=[1,2,3] and we pass it to the function def f(l=a): l=[1,2,3,4]. We aren't using any + operators here, however the global a isn't altered. It's the = operator that you should mostly care about, because it creates a new reference, like @Peter Wood said.= operator that reassings the reference but +, so I posted a simple example where + isn't used however the reference is reassigned (due to = usage). What's unclear about that?
D = D + [5]you create a new list object with the same name. When you useD += [5]you alter the existing list.