Django datetime error; fromisoformat: argument must be str

I am trying to make a website where I have a very basic Subscription system. I have different plans (30, 60, 90 and 365 days) and I would like to save, upon the user payment, the duration of the subscription plan.
But I am receiving this weird response, probably is something wrong in my model but I couldn’t fix it. Here is some code.

models.py

from datetime import datetime as dt, date

DURATION_CHOICES = (
(30, 30),
(60, 60),
(180, 180),
(365, 365),
)

class Subscription(models.Model):
    user = models.OneToOneField(CustomUser, on_delete=models.CASCADE)
    issue_date = models.DateTimeField(auto_now_add=True)
    duration = models.IntegerField(choices=DURATION_CHOICES)
    expiration_date = models.DateTimeField()


def __str__(self):
    return f'{self.user} - {str(self.expiration_date)}'

def get_expiration(self):
    issue_date = date(self.issue_date)
    expiration_date = issue_date + dt.timedelta(days=self.duration)
    return expiration_date


def save(self, *args, **kwargs):
    self.expiration_date = self.get_expiration
    super(Subscription, self).save(*args, **kwargs)

views.py

def new_subscription(request, days):
    user = request.user
    subscription = Subscription(user=user, duration=days)
    subscription.save()
    messages.success(request, "Subscription successfully obtained")
    return HttpResponseRedirect(reverse('main:index'))

I know there is probably something wrong in my save method or get_expiration method but I couldn’t find it.

>Solution :

In your save() method in first line you have this:

self.expiration_date = self.get_expiration

However it must changed to this:

self.expiration_date = self.get_expiration()

Explanation:

In your save() method, you were assigning something like <function a at 0x100d1c1f0> which is a reference to memory, and it means that you are assigning a function to self.expiration_date, while you need to assign a value to it.