Class Variables Behave Differently For List And Int?

The class shared variables are shared with all the instances of the classes as far as I know. But I am having trouble getting my head around this. class c(): a=[1] b=1

Solution 1:

x.a.append(1)

changes the class attribute c.a, a list, by calling its append method, which modifies the list in-place.

x.b += 1

is actually a shorthand for

x.b = x.b + 1

because integers in Python are immutable, so they don't have an __iadd__ (in-place add) method. The result of this assignment is to set an attribute b on the instance x, with value 2 (the result of evaluating the right-hand side of the assignment). This new instance attribute shadows the class attribute.

To see the difference between an in-place operation and an assignment, try

x.a += [1]

and

x.a = x.a + [1]

These will have different behavior.

EDIT The same functionality can be obtained for integers by boxing them:

classHasABoxedInt(object):
    boxed_int = [0]    # int boxed in a singleton list

a = HasABoxedInt()
a.boxed_int[0] += 1
b = HasABoxedInt()
print(b.boxed_int[0])  # prints 1, not zero

or

classBoxedInt(object):def__init__(self, value):
        self.value = value
    def__iadd__(self, i):
        self.value += i

Solution 2:

larsmans' answer is excellent, but it might provide additional insight if we look at id of x.b before and after the assignment.

classc():
    a=[1]
    b=1def__init__(self):
        pass

x=c()

print"initial a : {} at {}".format(x.a, id(x.a))
print"initial b : {} at {}".format(x.b, id(x.b))

x.a.append(1)
x.b+=1# x.b = x.b + 1, created a new object# we created an instance variable x.b and it# is shadowing the class variable b.  print"after change a : {} at {}".format(x.a, id(x.a))
print"after change b : {} at {}".format(x.b, id(x.b))

y=c()

# We can already see from the class object that # b has not changed valueprint"in class c b : {} at {}".format(c.b, id(c.b))

print"in instance y a : {} at {}".format(y.a, id(y.a))
print"in instance y b : {} at {}".format(y.b, id(y.b))

Result:

initiala : [1]at50359040initialb : 1at40974280afterchangea : [1, 1]at50359040afterchangeb : 2at40974256 # Showsidofinstancevariablex.b; henceitis
                               # differentinclasscb : 1at40974280ininstanceya : [1, 1]at50359040ininstanceyb : 1at40974280

If you want to use an int as a class variable, this should work:

classMyClass(object):
    b=1def increase_b(self, n):
        MyClass.b += n

Result:

>>>mc_1 = MyClass()>>>mc_1.b
1
>>>mc_1.increase_b(5)>>>mc_1.b
6
>>>mc_2 = MyClass()>>>mc_2.b
6
>>>mc_2.increase_b(10)>>>MyClass.b
16
>>>mc_2.b
16
>>>mc_1.b
16