In your first example you are defining instance attributes. In the second, class attributes.
Class attributes are shared between all instances of that class, where as instance attributes are "owned" by that particular instance.
Difference by example
To understand the differences let's use an example.
We'll define a class with instance attributes:
class MyClassOne:
def __init__(self):
self.country = "Spain"
self.city = "Barcelona"
self.things = []
And one with class attributes:
class MyClassTwo:
country = "Spain"
city = "Barcelona"
things = []
And a function that prints out information about one of these objects:
def information(obj):
print "I'm from {0}, ({1}). I own: {2}".format(
obj.city, obj.country, ','.join(obj.things))
Let's create 2 MyClassOne
objects and change one to be Milan, and give Milan "something":
foo1 = MyClassOne()
bar1 = MyClassOne()
foo1.city = "Milan"
foo1.country = "Italy"
foo1.things.append("Something")
When we call information()
on the foo1
and bar1
we get the values you'd expect:
>>> information(foo1)
I'm from Milan, (Italy). I own: Something
>>> information(bar1)
I'm from Barcelona, (Spain). I own:
However, if we were to do exactly the same thing, but using instances of MyClassTwo
you'll see that the class attributes are shared between instances.
foo2 = MyClassTwo()
bar2 = MyClassTwo()
foo2.city = "Milan"
foo2.country = "Italy"
foo2.things.append("Something")
And then call information()
...
>>> information(foo2)
I'm from Milan, (Italy). I own: Something
>>> information(bar2)
I'm from Barcelona, (Spain). I own: Something
So as you can see - things
is being shared between the instances. things
is a reference to a list that each instance has access to. So if you append to things from any instance that same list will be seen by all other instances.
The reason you don't see this behaviour in the string variables is because you are actually assigning a new variable to an instance. In this case that reference is "owned" by the instance and not shared at the class level. To illustrate let's assign a new list to things for bar2
:
bar2.things = []
This results in:
>>> information(foo2)
I'm from Milan, (Italy). I own: Something
>>> information(bar2)
I'm from Barcelona, (Spain). I own: