What happens when I instantiate class in Python?

Question

Could you clarify some ideas behind Python classes and class instances?

Consider this:

class A():
    name = 'A'

a = A()

a.name = 'B' # point 1 (instance of class A is used here)

print a.name
print A.name

prints:

B
A

if instead in point 1 I use class name, output is different:

A.name = 'B' # point 1 (updated, class A itself is used here)

prints:

B
B

Even if classes in Python were some kind of prototype for class instances, I'd expect already created instances to remain intact, i.e. output like this:

A
B

Can you explain what is actually going on?

Answer 1

First of all, the right way in Python to create fields of an instance (rather than class fields) is using the __init__ method. I trust that you know that already.

Python does not limit you in assigning values to non-declared fields of an object. For example, consider the following code:

class Empty: pass
e = Empty()
e.f = 5
print e.f # shows 5

So what's going in your code is:

1. You create the class A with a static field name assigned with A.
2. You create an instance of A, a.
3. You create a new field for the object a (but not for other instances of A) and assign B to it
4. You print the value of a.name, which is unique to the object a.
5. You print the value of the static field A.name, which belongs to the class

Answer 2

You also should look at these SO threads for further explanations:
http://stackoverflow.com/questions/68645/python-static-variable
http://stackoverflow.com/questions/707380/in-python-how-can-i-access-static-class-variables-within-class-methods

And an official tutorial: http://docs.python.org/tutorial/classes.html#SECTION0011320000000000000000

Keep in mind that the assignment "=" operator in python behaves differently than C++ or Java:
http://docs.python.org/reference/simple_stmts.html#assignment-statements

Answer 3

Perhaps this example may make things more help clarify. Recall that Python names are not storage (as variables are in other languages) but references to storage. You can find what a name refers to with id(name). The identity operator x is y tells whether two names point at the same object.

>>> class A(object):
...     name = 'A'
... 
>>> x = A()
>>> A.name is x.name
True
>>> x.name = 'fred'  # x.name was bound to a new object (A.name wasn't)
>>> A.name is x.name
False
>>> x = A()          # start over
>>> A.name is x.name
True                 # so far so good
>>> A.name = 'fred'
>>> A.name is x.name 
True                 # this is somewhat counter-intuitive