variable changes if I change a different variable

Question

Whenever I create an instance of a class, create a variable that's assigned that first instance, and use an attribute of the class on the second variable my first variable changes.

class number:
    def __init__(self, value):
        self.value = value
    def add(self):
        self.value = self.value + 1
a = number(10)
b = a
b.add()
a.value

why does a.value give me 11 when I didn't use a.add()?

Answer 1

Because when you do b = a you're not creating a new object of the class number just passing the reference of the object which a references.

Answer 2

@juanpa.arrivillaga provided good comments to your question. I just want to add how to fix your code to do what you expect it to do:

Method 1:

class number:
    def __init__(self, value):
        self.value = value
    def add(self):
        self.value = self.value + 1
a = number(10)
b = number(a.value) # create a new object with the same value as 'a'
b.add()
a.value

Method 2:

import copy
class number:
    def __init__(self, value):
        self.value = value
    def add(self):
        self.value = self.value + 1
a = number(10)
b = copy.copy(a) # make a shallow copy of the object a 
# b = copy.deepcopy(a) # <-- that is what most people think of a "real" copy
b.add()
a.value