question about class methods

[Gabriel Genellina]

En Wed, 14 Mar 2007 18:04:00 -0300, Darren Dale <dd55 at cornell.edu>  
escribió:

> I've run across some code in a class method that I don't understand:
>
>     def example(self, val=0)
>         if val and not self:
>             if self._exp < 0 and self._exp >= -6:

0) "Normal" methods are not class methods, but instance methods. A class  
method is a method who operates on the class itself; its first argument is  
the class (maybe a derived one).

> I am unfamiliar with some concepts here:
>
> 1) Under what circumstances would "if not self" be True?

"if not self" would be "if self evaluates to False as a boolean": 0, 0.0,  
0j, (), [], {}... Other objects usually evaluate always to True, except if  
they define some special methods: __nonzero__ and __len__. Look those on  
the Python Reference Manual.

> 2) If "not self" is True, how can self have attributes?

Perhaps you think of self being None - that should never occur unless you  
called the method in some convoluted way.
The class migh inherit from list, by example, and you want to compute the  
average:

class StatList(list):
     def avg(self):
         if self: return sum(self)/len(self)
         else: raise ValueError("Can't compute average on empty list")

> (This is slightly simplified code from the decimal.Decimal.__str__  
> method,
> line 826 in python-2.4.4)

A Decimal number is False when 0. `if not self` is a faster way to say `if  
self==0`

-- 
Gabriel Genellina