idiom for initialising variables

[Erik Max Francis]

Rob Hall wrote:

> I'm just wondering on the correct python idiom for initialising
> variables.
> I see some code like this:
> 
> class myClass:
>     self.a = 1
>     def __init__(self):
>         ...more stuff here

This is illegal.  You probably meant:

	class MyClass:
	    a = 1
	    def __init__(self): ...

> class myClass
>     def __init__(self):
>         self.a = 1
>         ...more stuff here
> 
> Which is the correct idiom?  or is there a subtle difference I am
> missing?

In the former corrected version, the variable a is a class variable;
that is, it's shared by all instances.  In the latter, it's an instance
variable, and is specific only to one instance.

>>> class C:
...  s = 1
...  def __init__(self, x):
...   self.x = x
... 
>>> c1 = C(2)
>>> c2 = C(3)
>>> c1.x
2
>>> c2.x
3
>>> c1.s
1
>>> c2.s
1
>>> C.s = 10 # note: changing the class variable, not an instance one
>>> c1.s
10
>>> c2.s
10

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