accessor/mutator functions

[Andrew Dalke]

On Mon, 28 Feb 2005 15:50:22 -0500, Dan Sommers wrote:
> The reason their code is so inflexible is that they've filled their
> classes with boiler plate get/set methods.
> 
> Why do users of classes need such access anyway?  If my class performs
> useful functions and returns useful results, no user of my class should
> care about its attributes.  If I "have to" allow access to my attributes
> in order that my users be happy, then I did something else wrong when I
> designed the class and its public interface in the first place.

Consider an interface to a temperature controller.  It will
have several properties:

  - current temperature (read only)
  - target temperature (read/write)
  - various tuning parameters that affect the rate of change,
     stability, etc.; eg, lookup details on a PID controller

These are properties of the temperature controller object.
In Python these properties are traditionally mapped to attributes.
Under languages that don't have a __getattr__/"property" mechanism
for separating interface from implementation these are implemented
via get/set accessor methods.

It is very often useful to change the temperature setting of
a controller over time.  Eg, a chemical protocol might say
"warm from 50 to 80 over a period of 30 minutes, 1 degree
per minute".

In Python that might be written as:

  temp_controller = TemperatureController("/dev/com3")
  temp_controller.target = 50
  print "Bringing reactor to", temp_controller.target
  while 1:
    print "Current temperature is", temp_controller.current
    if abs(temp_controller.current - temp_controller.target) < 0.1:
      break
    time.sleep(30)

  raw_input("Insert sample and press the <enter> key: ")

  for temp in range(51, 81):
    print "Raising reactor temperature to", temp
    temp_controller.target = temp
    time.sleep(60)
    if abs(temp_controller.current - temp) > 0.1:
      print "Variance too high!", temp_controller.current

  print "DONE!"


What's wrong with the use of attributes in this case and how
would you write your interface?

				Andrew
				dalke at dalkescientific.com