Problems with self as default parameter

[markus at kepler.ibp.de]

Joonas Paalasmaa <joonas at olen.to> writes:

> How can I use class members as default values.
> The code above raises an error.
> 
> class Test:
>     def __init__(self,s):
>         self.default = s
>     def printer(self, string = self.default):
>         print string

[...]

> NameError: name 'self' is not defined

That's it: the default values are evaluated only once, at the time
when the function is defined. And then the value of self.default
(which belongs to the instance, not to the class) is not defined. A
common idiom to overcome this problem is:

def printer(self, string = None):
	if string is None: string =  self.default
	print string


- Markus