On Wed, Mar 01, 2006 at 09:25:36PM +0100, Christoph Zwerschke wrote: > Usually, you initialize class variables like that: > > class A: > sum = 45 > > But what is the proper way to initialize class variables if they are the > result of some computation or processing as in the following silly > example (representative for more: > > class A: > sum = 0 > for i in range(10): > sum += i > > The problem is that this makes any auxiliary variables (like "i" in this > silly example) also class variables, which is not desired. > > Of course, I could call a function external to the class > > def calc_sum(n): > ... > > class A: > sum = calc_sum(10) > > But I wonder whether it is possible to put all this init code into one > class initialization method, something like that:
Yes, it is called a meta class. > class A: > > @classmethod > def init_class(self): > sum = 0 > for i in range(10): > sum += i > self.sum = sum > > init_class() > > However, this does not work What we normally think of as an instance is an instance of a class. Classes are actually instances of metaclasses. What you are looking for is __init__, but not the __init__ defined after 'class A...' you want the __init__ that is called when you type 'class A....' Python 2.4.1 (#2, Mar 30 2005, 21:51:10) [GCC 3.3.5 (Debian 1:3.3.5-8ubuntu2)] on linux2 Type "help", "copyright", "credits" or "license" for more information. >>> class MyMeta(type): ... def __init__(cls, *ignored): ... cls.sum = 0 ... for (i) in range(10): ... cls.sum += i ... >>> class A(object): ... __metaclass__ = MyMeta ... >>> print A.sum 45 >>> print A.i Traceback (most recent call last): File "<stdin>", line 1, in ? AttributeError: type object 'A' has no attribute 'i' >>> For something as simple as this example it is easier and cleaner to move the loop into a function and do the one-liner assignment. Because the metaclass can do _anything_ to the class the reader is obliged to go read its code. The simple assignment from a function obviously has no side effects. Hope that helps, -jackdied -- http://mail.python.org/mailman/listinfo/python-list
