An iterator is an object in Python used to iterate over iterable objects like lists, tuples, strings, etc. It implements two special methods:

• __iter__()
• __next__()

What Is an Iterable?

An iterable is any Python object capable of returning its members one at a time. Common examples:

my_list = [1, 2, 3]
my_str = "hello"

You can convert them into iterators using iter().

Example 1: Creating an Iterator from a List

my_list = [10, 20, 30]
my_iter = iter(my_list)
print(next(my_iter))  # 10
print(next(my_iter))  # 20
print(next(my_iter))  # 30

Output:

10  
20  
30

If you call next() again, it will raise StopIteration.

Custom Iterator Example

You can create your own iterator by defining a class with __iter__() and __next__() methods.

class CountUpTo:
   def __init__(self, max):
       self.max = max
       self.current = 1
   def __iter__(self):
       return self
   def __next__(self):
       if self.current <= self.max:
           val = self.current
           self.current += 1
           return val
       else:
           raise StopIteration
counter = CountUpTo(3)
for num in counter:
   print(num)

Output:

1  
2  
3

Iterator vs Iterable

Using iter() and next()

nums = [1, 2, 3]
it = iter(nums)
print(next(it))  # 1
print(next(it))  # 2
print(next(it))  # 3

Summary

• Iterables can be looped through.
• Iterators are used to get items from iterables one by one using next().
• Iterators remember their state.
• You can build custom iterators using __iter__() and __next__().