我如何比较时区和日期时间的实例

Question

i created a todo model in django with a method of clear old todo that is supposed to delete todos that were published more than 24 hours ago, i can't seem to be able to compare datetime and timezone instances in my if condition

class Todo(models.Model):
    description = models.CharField(max_length=200)
    Todo_date = models.DateTimeField('Todo Date')
    pub_date = models.DateTimeField('Date Published')

    def __str__(self):
        return self.description

    def create_todo(self, description, Todo_date, pub_date):
        todo = Todo(description=description,
                    Todo_date=Todo_date, pub_date=pub_date)
        todo.save()
        return todo

    def delete_todo(self, description):
        todo = Todo.objects.get(description=description)
        todo.delete()
        return "Todo removed"

    def clear_old_todo(self):
        todos = Todo.objects.all()
        time_limit = datetime.timedelta(hours=24)
        for todo in todos:
            if (timezone.now()-todo.pub_date) > (timezone.now()-time_limit):
                todo.delete()
                return "old todo cleared"

>>> Todo.clear_old_todo("self")
Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "E:\projects\1stDjangoApp\ToDoList\ToDo\models.py", line 36, in clear_old_todo
    if (timezone.now()-todo.pub_date) > (timezone.now()-time_limit):
TypeError: '>' not supported between instances of 'datetime.timedelta' and 'datetime.datetime'

Answer 1

Subtracting a datetime from a datetime gives you a timedelta , the difference between the two times. Subtracting a timedelta from a datetime gives you a datetime , a new timestamp different from the first by the amount of the timedelta .

In timezone.now()-todo.pub_date , you're subtracting two datetime .
In timezone.now()-time_limit , you're subtract a timedelta from a datetime .

You either want to do timezone.now() - todo.pub_date to produce a timedelta and check if that timedelta is > / < some specific value (ie compare two timedelta s), or you want to do timezone.now() - time_limit to produce a datetime in the past and check whether that is > / < your todo.pub_date datetime .