On 30 August 2012 15:11, Marco Nawijn <naw...@gmail.com> wrote:
>
>
> Learned my lesson today. Don't assume you know something. Test it first
> ;). I have done quite some programming in Python, but did not know that
> class attributes are still local to the instances. It is also a little
> surprising I must say. I always considered them like static variables in
> C++ (not that I am an expert in C++).
>
Class attributes are analogous to static variables in C++ provided you only
ever assign to them as an attribute of the class.
>>> class A(object):
... static = 5
...
>>> a = A()
>>> a.static
5
>>> A.static
5
>>> b = A()
>>> b.static
5
>>> A.static = 10
>>> a.static
10
>>> b.static
10
An instance attribute with the same name as a class attribute hides the
class attribute for that instance only.
>>> b.static = -1
>>> a.static
10
>>> b.static
-1
>>> del b.static
>>> b.static
10
This is analogous to having a local variable in a function that hides a
module level variable with the same name:
x = 10
def f1():
x = 4
print(x)
def f2():
print(x)
f2() # 10
f1() # 4
f2() # still 10
If you want f1 to modify the value of x seen by f2 then you should
explicitly declare x as global in f1.
Likewise if you want to modify an attribute for all instances of a class
you should explicitly assign to the class attribute rather than an instance
attribute.
Oscar
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