Question

I am not a complete beginner but fairly new to Python. Whilst working on a project today I just had an idea and wondered regarding the usage of “self“; about which I’ve been reading for the past some while and I still can not figure out if it’s always necessary or not. My question solely concerns instances of classes and instance parameters/variables. This question is not about Class Variables which affect all instances.

Example:

class C:
    def __init__(self, parent = None):
        super(C, self).__init__(parent)
        self.some_temp_var = AnotherClass()
        self.important_property = self.some_temp_var.bring_the_jewels()

ins_c = C()
print ins_c.important_property 

In the above code, I use self before both variable declarations (some_temp_var and important_property).

I will need to access **important_property** later from the outside (where instance’s been created) and maybe even modify it.

But I will not need access to the instance of AnotherClass() and/or variable pointing to it (some_temp_var). I just need an instance of it class once, and I need to execute its method bring_the_jewels only once to populate value of important_property.

Should I still use self before declaring that variable as well?

self.some_temp_var = ....
self.important_property = ....

or can it be:

some_temp_var = ....
self.important_property = ....

Thank you for your help.

Ps. I did my research in a length way. Due to lack of my English and/or CS knowledge, I may not have found a currently existing duplicate however I did search and I did search a lot. Before calling this question “duplicate” or “not constructive” please read it throughly. This is a question with clear answer and it’s very important, and complicated matter. Thank you for your understanding.

Answer 1

If you dont use self.some_temp_var, then you are defining a local variable which will be garbage collected after the constructor.

So yes you need self if you want to define an instance attribute that should persist with the instance. Whether you need it to be public or private is a matter of naming. Prefix it with an _ for protected or __ for private.

If some_temp_var is just a temp for the constructor and you never need it again then no, you dont need self. It is about persistence and member attributes. You do not need self for the pure fact of working with a variable within a single method.

Consider this… What if some_temp_var were some huge data structure that you build up to arrive at the real result value? You dont want to use self to store a temp variable because it wont free memory until you delete it or the instance. You want it to be cleaned up when it is no longer needed.