Python Iterators

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Python Iterators

An iterator is an object that lets you traverse through a sequence of values one element at a time. Instead of holding all values in memory at once, an iterator produces values on demand.

In Python, an iterator follows the iterator protocol, which means it implements two special methods:

    • __iter__() → returns the iterator object

    • __next__() → returns the next value from the sequence

Iterator vs Iterable

    • Iterable: An object you can loop over (list, tuple, string, set, dictionary, etc.)

    • Iterator: The object that actually keeps track of iteration and returns values one by one

You can get an iterator from any iterable using the built-in iter() function.

Getting an Iterator from an Iterable

Example: Iterator from a List

fruits = ["apple", "orange", "mango"] iterator = iter(fruits) print(next(iterator)) print(next(iterator)) print(next(iterator))

Output:

apple
orange
mango

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Example: Iterator from a String

Strings are also iterable, character by character.

text = "python" char_iter = iter(text) print(next(char_iter)) print(next(char_iter)) print(next(char_iter))

Output:

p
y
t

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Looping Through an Iterator

You usually don’t call next() manually. A for loop does this automatically behind the scenes.

Example: Looping Through a Set

colors = {"red", "blue", "green"} for color in colors: print(color)

Output:

red
green
blue

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Example: Looping Through a String

word = "code" for letter in word: print(letter)

Output:

c
o
d
e

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Note: Internally, the for loop:

    • Creates an iterator using iter()

    • Calls next() repeatedly

    • Stops when StopIteration is raised

Creating Your Own Iterator

To create a custom iterator, define a class that implements:

    • __iter__()

    • __next__()

Example: Custom Iterator (Even Numbers)

This iterator generates even numbers starting from 2.

class EvenNumbers: def __iter__(self): self.num = 2 return self def __next__(self): value = self.num self.num += 2 return value even_iter = iter(EvenNumbers()) print(next(even_iter)) print(next(even_iter)) print(next(even_iter)) print(next(even_iter))

Output:

2
4
6
8

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Preventing Infinite Iteration – StopIteration

Without a stopping condition, an iterator can run forever.

To stop iteration, raise the StopIteration exception inside __next__().

Example: Iterator with a Stop Condition

This iterator generates square numbers up to a limit.

class SquareLimit: def __iter__(self): self.n = 1 return self def __next__(self): if self.n <= 5: result = self.n ** 2 self.n += 1 return result else: raise StopIteration squares = SquareLimit() for value in squares: print(value)

Output:

1
4
9
16
25

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