Question

I’m new to python, and it really confused me.

I want to write something like

class line :
    points = []

    def add(self, point) :
        self.points.append(point)

line1 = line()
line2 = line()
line1.add("Some point")
print line2.points

Output: ['Some point']

And the result is like they refers to the same list.
But I do want a object member not a class member.

And I tried, if points is int, or some other simple type it works fine.

Besides, I know if I write

def __init__(self) :
    self.points = []

it will also work, but I don’t get why they are pointing to same list by default.

ps. I know writing a init will work.

class line :
    name = "abc"

    def changename(self, name) :
        self.name = name

line1 = line()
line2 = line()
line1.changename(123)
print line2.name

But the code above output "abc"

So I don’t understand why same kind of declaration act different by type.

Answer 1

Because in this case

class line :
    points = []

points is a class member. It belongs to the class, and since line1 and line2 are of the same class, points is the same list.

So you are right in using

def __init__(self) :
    self.points = []

instead to create an instance member which is tied to the instance, not the class.

To answer your edit

If you write:

def changename(self, name) :
    self.name = name

you override your class member and create a new instance member using self.name = name.

To get back to your first example, it would be like writing:

def add(self, point) :
    self.points = self.points + [point]