Understanding python class attributes

Question

I am very new to programming and started learning python. Might look very stupid question, so please pardon my ignorance. Consider the following snippet of code :

class Test1:
    bar = 10    
    def display(self,foo):
        self.foo=foo
        print "foo : ",self.foo #80
    def display1(self):
        print "bar: ", self.bar #10
        print "again foo: ", self.foo #80

if __name__ == '__main__':
    test1 = Test1()
    test1.display(80)
    test1.display1()
    print test1.bar #10
    print test1.foo #80

I want to understand what is the difference between using foo and bar (wrt to where we have defined them) as in scope wise they are equally accessible at all places compared to each other and only difference is that one is inside function and other is inside Class but they both are still "instance" variable. So which is good practice?

Also, if I slightly modify display function as below :

    def display(self,foo):
        self.foo=foo
        foo = foo
        print "self.foo : ",self.foo 
        print "foo : ",foo 

Can someone please explain how python sees this, as in what difference/significance this self keyword is bringing in between two foo.

Answer 1

bar is a class attribute and foo is an instance attribute. The main difference is that bar will be available to all class instances while foo will be available to an instance only if you call display on that instance

>>> ins1 = Test1()

ins1.bar works fine because it is a class attribute and is shared by all instances.

>>> ins1.bar
10

But you can't access foo directly here as it is not defined yet:

>>> ins1.foo
Traceback (most recent call last):
  File "<ipython-input-62-9495b4da308f>", line 1, in <module>
    ins1.foo
AttributeError: Test1 instance has no attribute 'foo'

>>> ins1.display(12)
foo :  12
>>> ins1.foo
12

If you want to initialize some instance attributes when the instance is created then place them inside the __init__ method.

class A(object):
    bar =  10
    def __init__(self, foo):
        self.foo = foo   #this gets initialized when the instance is created
    def func(self, x):
        self.spam = x    #this will be available only when you call func() on the instance
...         
>>> a = A(10)
>>> a.bar
10
>>> a.foo
10
>>> a.spam
Traceback (most recent call last):
  File "<ipython-input-85-3b4ed07da1b4>", line 1, in <module>
    a.spam
AttributeError: 'A' object has no attribute 'spam'

>>> a.func(2)
>>> a.spam
2