Difference between Python's Generators and Iterators

Question

What is the difference between iterators and generators? Some examples for when you would use each case would be helpful.

Answer 1

I am writing specifically for Python newbies in a very simple way, though deep down Python does so many things.

Let’s start with the very basic:

Consider a list,

l = [1,2,3]

Let’s write an equivalent function:

def f():
    return [1,2,3]

o/p of print(l): [1,2,3] & o/p of print(f()) : [1,2,3]

Let’s make list l iterable: In python list is always iterable that means you can apply iterator whenever you want.

Let’s apply iterator on list:

iter_l = iter(l) # iterator applied explicitly

Let’s make a function iterable, i.e. write an equivalent generator function. In python as soon as you introduce the keyword yield; it becomes a generator function and iterator will be applied implicitly.

Note: Every generator is always iterable with implicit iterator applied and here implicit iterator is the crux So the generator function will be:

def f():
  yield 1 
  yield 2
  yield 3

iter_f = f() # which is iter(f) as iterator is already applied implicitly

So if you have observed, as soon as you made function f a generator, it is already iter(f)

Now,

l is the list, after applying iterator method "iter" it becomes, iter(l)

f is already iter(f), after applying iterator method "iter" it becomes, iter(iter(f)), which is again iter(f)

It's kinda you are casting int to int(x) which is already int and it will remain int(x).

For example o/p of :

print(type(iter(iter(l))))

is

Never forget this is Python and not C or C++

Hence the conclusion from above explanation is:

list l ~= iter(l)

generator function f == iter(f)