default argument in method

[Steven D'Aprano]

On Thu, 30 Dec 2010 11:26:50 -0800, DevPlayer wrote:

> There's some_object.some_method.func_defaults 

Not quite -- method objects don't expose the function attributes 
directly. You need some_object.some_method.im_func to get the function 
object, which then has a func_defaults attribute.


> and
> some_function.func_defaults both are a settable attribute. How to set
> the methods func_defaults? 

(1) You shouldn't mess with func_defaults unless you know what you're 
doing.

(2) If you do know what you are doing, you probably won't want to mess 
with func_defaults.

(3) But if you insist, then you would so the same way you would set any 
other object's attribute.


>>> class C(object):
...     def method(self, x=[]):
...             print x
... 
>>> C().method()
[]
>>> function = inst.method.im_func
>>> function.func_defaults
([],)
>>> function.func_defaults = ("spam",)
>>> inst.method()
spam


(4) Seriously, don't do this.


> You'd have to have code in
> _getattribute__(yourmethod) if not __getattr__(yourmethod)
> 
> def __getattribute__(self, attr):
>     if attr == self.my_method:
>         # something like this, but i'm probably a little off 
>         # you might need to use super or something to prevent
> recursive __getattribute__ calls here
>         self.my_method.func_defaults = self.foo


*cries*

A much better solution would be:

class MyClass:
    def my_method(self, x=None):
        if x is None:
            x = self.foo
        ...

Don't write slow, confusing, complex, convoluted, self-modifying code 
when you can write fast, simple, straight-forward, obvious code. Unless 
you're doing it to win a bet.



-- 
Steven