Python Object Oriented Design; Return, Set Instance Variable Or Both

Question

Ok I have some code that boils down to a pattern like this

class Foo(object):
def get_something1(self):
    # in the actual code it asks a web server but here I replaced that with "foo_something1" instead of the code from a web server
    self.something1 = "foo_something1"

def get_something2(self):
    # needs the result of get_something1; right now I have it get it by instance variable like so
    self.something2 = self.something1 + "_something_2"

My question is, should I do the above (methods getting what they need by instance variables) or by taking arguments like so; also should I return something1 and something2 in the get_* methods?

class Foo(object):
def get_something1(self):
    # in the actual code it asks a web server but here I replaced that with "foo_something1" instead of the code from a web server
    self.something1 = "foo_something1"

def get_something2(self, something_1):
    # needs the result of get_something1; right now I have it get it by instance variable like so
    self.something2 = something1 + "_something_2"

Now in my code the methods return deferreds (twisted); Right now when the deferreds fire it sets the instance variables and returns None; In the actual code I have another function that checks to see if something1 or something2 is expired; I update them which brings up a problem because I update it like so...

d = self.get_something1()
##d.addCallback(self.get_something2) # errors because it gets the argument None (so I do below)
d.addCallback(lambda ignored: self.get_something2())# problem the deferred for get_something2 is inaccessible

So right now the update code is either ugly, error filled, or both. So I have a feeling that I am doing something either a) un pythonic or b) un twisted or c) some other bad design.

---

Thanks for the answers, but those won't work because the functions return deferreds and not the actual value (other wise it would have to block).

Answer 1

Don't use get_* methods. In Python the better way is to use properties:

class Foo(object):
    @property
    def something1(self):
        # in the actual code it asks a web server but here I replaced that with "foo_something1" instead of the code from a web server
        return "foo_something1"
    @property
    def something2(self):
        # needs the result of get_something1; right now I have it get it by instance variable like so
        return self.something1 + "_something_2"

Note that when something1 is a property, self.something1 (without parentheses!) calls the corresponding function Foo.something1.

Once you have working code, you can see how something2 is being used. If you have both

    @property
    def something2(self):
        return self.something1 + "_something_2"

    @property
    def something3(self):
        return self.otherthing1 + "_something_2"

Then you might want to refactor the code to use

    def something2(self,prefix):
        return prefix+"_something_2"

We don't know enough about your situation to tell you which is better. But the answer may become obvious to you once you see the use patterns of your working code.