What is the difference between queryset and list?

Question

Django

ads = Ads.objects.all()
for ad in ads[:10] 
  pass

ads = ads[:10]
for ad in ads:
  pass

The behavior of these two fors is different, slices work in a QuerySet somehow differently, but how? What's the Difference?

Answer 1

Of course, it's different - after all, this is not a list.
Let's see how it works:
https://github.com/django/django/blob/master/djang...

def __getitem__(self, k):
        if not isinstance(k, (slice,) + six.integer_types):
            raise TypeError
        assert ((not isinstance(k, slice) and (k >= 0)) or
                (isinstance(k, slice) and (k.start is None or k.start >= 0) and
                 (k.stop is None or k.stop >= 0))), \
            "Negative indexing is not supported."

        if self._result_cache is not None:
            return self._result_cache[k]

        if isinstance(k, slice):
            qs = self._clone()
            if k.start is not None:
                start = int(k.start)
            else:
                start = None
            if k.stop is not None:
                stop = int(k.stop)
            else:
                stop = None
            qs.query.set_limits(start, stop)
            return list(qs)[::k.step] if k.step else qs

        qs = self._clone()
        qs.query.set_limits(k, k + 1)
        return list(qs)[0]

In my opinion, everything is very clear here:
- you can transfer either a number or a slice
- while the number or the initial and final values ​​​​of the slice must be at least zero
- if a slice is specified, then a copy of the current queryset is made and low_mark and low_mark are set for the query object of the copy /or high_mark, which in the future will turn into OFFSET and LIMIT of the query to the database.
- then two options are possible:
* if the step is not set, the created copy of the queryset is returned
* if the step is set, then we turn the queryset into a list and apply the step to it
In this case, if the step was not set, then no query to the database was not fulfilled.