[Tutor] self.name vs. passing a name

[Dave Angel]

Che M wrote:
> This is another very basic structural question, related to one I asked last week, and again is not necessarily germane only to Python.
>
> Let's say you have a sequence of two calculations or manipulations you need to do, each one done as a function called by an overall calculate_something() function.  The "answer" to each function is then used in the next function.  I can think of two ways to make that answer available for use in the next function:  1) pass it in, or 2) declare it as self.answer and then it is available to the whole class instance.
>
> What are the dis/advantages to these two different ways?  Here are examples, with only the overall calculate_something() function shown:
>
> 1.Pass the variable to the second function.
>
> def calculate_something(self):
>     answer = self.do_first_calculation()    #1st function returns answer
>     self.do_second_calculation(answer)    #2nd is passed answer and uses it.
>
> 2. Create the variable in the class instance scope and use that in the second function.
>
> def calculate_something(self):
>
>     self.do_first_calculation()                 #1st function creates self.answer
>
>     self.do_second_calculation()             #2nd uses self.answer
>
> Both of these approaches can work, but I would like to better understand when it is best to do one or the other.  Obviously if I know I will need to make self.answer available for use by other functions, I would want to choose (2).  But what other considerations should I, well, consider?
>
> Thanks,
> Che
>
>
>   
You call these functions, but since you have the "self" argument, I'll 
assume it's a method instead.

use #2 only if the "answer" is relevant for further operations, AND  (is 
time-consuming to calculate compared to the memory consumed by storing 
it, OR  if calculating it has side effects)

prefer #1 because:
      it doesn't take up space in the object
      it goes away when the particular method ends, not when the object 
is destroyed (rule -- the sooner the better)
      it doesn't clutter the namespace of the class