Count vs len on a Django QuerySet

Question

In Django, given that I have a QuerySet that I am going to iterate over and print the results of, what is the best option for counting the objects? len(qs

Answer 1

For people who prefer test measurements(Postresql):

If we have a simple Person model and 1000 instances of it:

class Person(models.Model):
    name = models.CharField(max_length=100)
    age = models.SmallIntegerField()

    def __str__(self):
        return self.name

In average case it gives:

In [1]: persons = Person.objects.all()

In [2]: %timeit len(persons)                                                                                                                                                          
325 ns ± 3.09 ns per loop (mean ± std. dev. of 7 runs, 1000000 loops each)

In [3]: %timeit persons.count()                                                                                                                                                       
170 ns ± 0.572 ns per loop (mean ± std. dev. of 7 runs, 10000000 loops each)

So how can you see count() almost 2x faster than len() in this particular test case.