list_display with a function

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hi

I have a model that defines a function:

class Test(models.Model):
name = models.CharField(max_length=200)

data_expira = models.DateField()

def expired(self):
return self.data_expira < datetime.date.today()

expired.short_description = 'Expirou?'

I want to put it in list_display for this model:
list_display = ["name", "expired"]

BUT
django admin displays it as a text, "True" or "False"...

can I tell django that it is a boolean field?