Instance attributes vs method arguments

[M.-A. Lemburg]

On 2008-11-25 08:27, John O'Hagan wrote:
> 
> Is it better to do this:
> 
> class Class_a():
> 	def __init__(self, args):
> 		self.a = args.a		
> 		self.b = args.b
> 		self.c = args.c
> 		self.d = args.d
> 	def method_ab(self):
> 		return self.a + self.b
> 	def method_cd(self):		
> 		return self.c + self.d
> 
> or this:
> 
> class Class_b():
> 	def method_ab(self, args):
> 		a = args.a
> 		b = args.b
> 		return a + b
> 	def method_cd(self, args)	
> 		c = args.c
> 		d = args.d
> 		return c + d
> 
> ?

That depends entirely on what you intend to do with objects
of Class_a and Class_b. In the first case, you are persisting
the argument attributes in the object, in the second case,
you are merely working on them - just like you would in a
function.

> Assuming we don't need access to the args from outside the class,
> is there anything to be gained (or lost) by not initialising attributes that 
> won't be used unless particular methods are called? 

It is always good practice to provide default values for
instance variables in the class definition, both to enhance
readability and to allow adding documentation regarding
the variables, e.g.

class Class_a:

   # Foo bar
   a = None

   # Foo baz
   b = None

   ...

-- 
Marc-Andre Lemburg
eGenix.com

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