[EMAIL PROTECTED] wrote: > Hi. > > I've got a question on the differences and how to define static and > class variables. AFAIK, class methods are the ones which receives the > class itself as an argument, while static methods are the one which > runs statically with the defining class. > > Hence, my understanding is that static variables must be bound to the > class defining the variables and shared by children of parent class > where the variable is defined. But, please have a look at this code in > which a guy told me that the variable a is static: > > >>>> class Foo: >>>> > a = 1 > @classmethod > def increment(cls): > cls.a += 1 > print cls.a > In your increment() method, you do this:
cls.a += 1 It does the following thing: #1. read cls.a #2. add one #3. assign this value to cls.a In point #3, you really bind a name to a value. As you probably know, in Python, there are names and objects. The initial value of the name 'a' is 1. It is an immutable object. The "+=" operator usually increments a value of an object. However, because the 'int' type is immutable, the += operator will rather rebind this variable to a newly created value. I believe this is what is happening here. Your question "is variable a static or class variable?" has no real answer. After running the increment() method on a descendant class, e.g. Child1 will rebind the name Child1.a, creating a new name in the namespace of the class. So the variable Foo.a is still there, but you are accessing Child1.a instead. If you want to HANDLE a as a static variable, you can handle it with a static method. That won't bind a new name in the descendant class. (However, you can still rebind it, e.g. "Child.a=42") Now here is a good question: how do you handle a variable as static, from a class (not static) method? Here is an example: >>> class Foo(object): ... a = 1 ... @classmethod ... def increment(cls): ... Foo.a += 1 ... print cls.a ... >>> class Child1(Foo): ... pass ... >>> Child1.increment() 2 >>> Child1.increment() 3 >>> Foo.a 3 >>> Child1.a = 10 >>> Child1.increment() 10 >>> Child1.increment() 10 >>> Child1.increment() 10 >>> Foo.a 6 >>> However, the question is: why would you do this? :-) BTW you should use new style classes whenever it is possible. Old style classes will have gone... Hope this helps, Laszlo -- http://mail.python.org/mailman/listinfo/python-list
