super() in Python 3

[Rhodri James]

Hi there!  A lot of the answers to your questions are at least implied 
in the Fine Manual 
(https://docs.python.org/3/library/functions.html#super), but it's not 
very clear and written more for precision than comprehension.  Here's my 
attempt at explaining :-)

On 16/07/2019 11:08, אורי wrote:
> Hi,
> 
> 1. When we use super() in Python 3, we don't pass it the first argument
> (self). Why?

Actually the first argument to super() isn't self, it's the class that 
we want the superclass of.  The *second* argument is self.  In the 
normal course of using super() inside a class method, these arguments 
will almost always be the class itself and the instance the method was 
called on.  Since that's almost always the case, the compiler offers us 
a short-cut: omit the "cls" and "self" and the compiler will fill them 
in for us.  That way we don't have to repeat ourselves and risk 
mis-typing something.

> What happens if the first argument is not self?

The first argument of what?  I'm not sure what you're getting at here.

> I think it would make more sense to use something like
> self.super().__init__(*args, **kwargs) or something like this.

That would mean either forbidding classes to have a method named "super" 
or accepting that users' classes could completely screw up inheritance 
for their subclasses.

> 2. I want to override a function called build_suite in an inherited class.
> The function receives an argument "test_labels" which I want to change (I
> define it if it's not defined), but I don't do anything with the argument
> "extra_tests". Is it possible to include "extra_tests" in *args, **kwargs

Yes.

> and how?

Don't list it in your parameters :-)

def build_suite(self, test_labels=None, *args, **kwargs):
     ...
     return super().build_suite(test_labels=test_labels, *args, **kwargs)

> I think maybe they will release another version in the future
> without "extra_tests", or with additional arguments, and I don't want to
> have to change my code then.

With a shim layer like this, your chances of getting away with making no 
changes to your code when an API you use changes are rather small.

-- 
Rhodri James *-* Kynesim Ltd