Python Iterable and Iterator

Iterable

Iterable is something that can be looped over. For example a list.

If Something is iterable then it needs to have __iter__() method(Which is also called dunder or magic or special method).

We can check __iter__() method using dir(list)

nums = [1, 2, 3]

for num in nums:
    print(num)

print(dir(nums))

In above example, in background, for loop is calling __iter__() method on an object(list) and returning an iterator that we can loop over.

Iterator

An iterator is an object with a state so that it remembers where it is, during iteration, it also knows how to get next value using __next__() method.

A list is iterable but its not an iterator, but if we run dunder iter method(__iter__()) on a list, it returns an iterator.

nums = [1, 2, 3]

print(next(nums))  # It will give error as its not an iterator

# Make it iterator

i_nums = iter(nums)  # iter function calls nums.__iter__() in background
print(i_nums)  # Now its an iterator
print(dir(i_nums))  # Now this has __next__() method in it as it is an iterator

Iterators are also iterables but the difference is __iter__() method returns the same object for iterator.

As iterators remember the state, it will print next value after each time we call next(). It throws StopIteration Exception when it runs out of values.

for num in i_nums:
    print(num)
1
2
3

# Or 

print(next(i_nums)) # 1
print(next(i_nums)) # 2

In background the for loop doing Something like this

while True:
    try:
        item = next(i_nums)
        print(item)
    except StopIteration:
        break

Let's crreate a class that behaves like builtin range() function.

class MyRange:
    def __init__(self, start, end):
        self.value = start
        self.end = end

    def __iter__(self):  # For something to be iterable, it needs to have __iter__() method
        return self

    def __next__(self):  # For something to be iterator, it needs to have __next__() method
        if self.value >= self.end:
            raise StopIteration
        current = self.value
        self.value += 1
        return current


nums = MyRange(1, 10)
for num in nums:
    print(num)

# Or

print(next(nums))   # next() method calls __next__() in background
print(m.__next__())
print(m.__next__())
print(m.__next__())

Genrators

Generators are also iterators but __iter__ and __next__ methods are created automatically for them.

def my_range(start, end):
    current = start
    while current < end:
        yield current
        current += 1

nums = my_range(1, 10)

for num in nums:
    print(num)

# Or
print(next(nums))
print(next(nums))
print(next(nums))
print(next(nums))

Creating our own iterators, iterable

class Sentence:

    def __init__(self, sentence):
        self.words = sentence.split()
        self.index = 0

    def __iter__(self):
        return self

    def __next__(self):
        print(f'Index is {self.index}')
        if self.index >= len(self.words):
            raise StopIteration
        current = self.index
        self.index += 1
        return self.words[current]


my_sentance = Sentence('This is a test')
for word in my_sentance:
    print(word)

# Or

print(next(my_sentance))
print(next(my_sentance))
print(next(my_sentance))
print(next(my_sentance))

Now create generator which does the same this

def sentence(words):
    for word in words.split():
        yield word


my_sentance = sentence('This is a test')
for word in my_sentance:
    print(word)

# Or

print(next(my_sentance))
print(next(my_sentance))
print(next(my_sentance))
print(next(my_sentance))