classmethod lambda Python

Question

I'm having a hard time understanding this piece of code. I understand that we are using some mock instead of the API endpoint to save time on the tests.

What I don't understand is the classmethod(lambda cls: self.preapproval) structure. What is the point of using a lambda cls if inside the code I don't use at all the cls.

I hope I'm clear enough, I'd be very happy if someone could shed some light on this one..

Thanks a lot.

@patch("paypaladaptive.api.endpoints.UrlRequest",
       MockUrlRequestPreapproval)
def test_preapproval(self):
    # I don't understand this code, it is very confusing. Why do I need to use a lambda structure if in the code itself I don't use cls (self.preapproval)
    MockUrlRequestPreapproval.preapproval = (
        classmethod(lambda cls: self.preapproval))
    self.assertTrue(self.preapproval.process())
    self.preapproval = Preapproval.objects.get(pk=self.preapproval.pk)
    self.assertNotEqual(self.preapproval.preapproval_key, "")
    self.assertEqual(self.preapproval.status, "created")

parchment · Accepted Answer · 2014-10-17 14:06:40Z

2

classmethod's first argument needs to be a function that takes one argument or more. Calling any of this will cause an error:

classmethod(self.preapproval) # Not a function
classmethod(lambda: self.preapproval) # Needs one argument

This works, but it's too verbose:

def get_preapproval(cls):
    return self.preapproval

classmethod(get_preapproval)

That's why that code uses lambda.

Probably it can be improved a little bit:

classmethod(lambda _: self.preapproval)

This makes it obvious that the argument is not needed.

edited Oct 17, 2014 at 14:06

answered Oct 17, 2014 at 13:54

parchment

4,0121 gold badge22 silver badges33 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

parchment Over a year ago

@GP89 Ah, right, it can take >= 1, because the class needs to be passed. Thanks!

jhagege Over a year ago

That makes it MUCH clearer. But why do we need a classmethod in the firstplace therefore. Couldn't we put just the lambda? Or even better, why not do just: MockUrlRequestPreapproval.preapproval = self.preapproval

parchment Over a year ago

I think it's because the rest of the code expects a classmethod. Doing anything else might change the behavior of the class. @cyberjoac

Kasravnd · Accepted Answer · 2014-10-17 14:01:19Z

0

cls is just a argument name for lambda ! and you don't use it at all ! lambda cls: self.preapproval) yields a function objec for classmethod ! , As lambda yields a function object you need to use it for pass self.preapproval because of classmethods first argument needs to be a function object !

edited Oct 17, 2014 at 14:01

answered Oct 17, 2014 at 13:47

Kasravnd

108k19 gold badges167 silver badges195 bronze badges

2 Comments

jhagege Over a year ago

How would you write this code to be more readable? Why should it be a classmethod and use a lambda in the first place?

Kasravnd Over a year ago

as lambda yields a function object you need to use it for pass self.preapproval because of classmethods first argument needs to be a function object !

Collectives™ on Stack Overflow

classmethod lambda Python

2 Answers 2

3 Comments

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

3 Comments

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related