Understanding Python Iterators

In Python, iterators are objects that allow traversal through a sequence of values, one element at a time. They serve as a standardized mechanism to access elements without exposing the underlying structure.

What Is an Iterator?

An iterator is defined by the implementation of two essential methods: __iter__() and __next__(). These methods constitute the iterator protocol.

Iterator vs Iterable

Objects such as lists, tuples, sets, dictionaries, and strings are all iterables. This means they contain a collection of items and can return an iterator using the iter() function.

Example: Tuple Iterator

mytuple = ("apple", "banana", "cherry")
myit = iter(mytuple)

print(next(myit))
print(next(myit))
print(next(myit))

Example: String Iterator

Strings, being sequences of characters, are also iterable:

mystr = "banana"
myit = iter(mystr)

print(next(myit))
print(next(myit))
print(next(myit))
print(next(myit))
print(next(myit))
print(next(myit))

Iterating Using a For Loop

Python simplifies iteration using the for loop, which implicitly creates an iterator and calls next() internally.

Example: Tuple Iteration

mytuple = ("apple", "banana", "cherry")

for x in mytuple:
  print(x)

Example: String Iteration

mystr = "banana"

for x in mystr:
  print(x)

Creating a Custom Iterator

To define a custom iterator, you need to create a class that implements both __iter__() and __next__(). Below is a simple example that returns consecutive integers starting from 1.

Example: Number Iterator

class MyNumbers:
  def __iter__(self):
    self.a = 1
    return self

  def __next__(self):
    x = self.a
    self.a += 1
    return x

myclass = MyNumbers()
myiter = iter(myclass)

print(next(myiter))
print(next(myiter))
print(next(myiter))
print(next(myiter))
print(next(myiter))

Using StopIteration to End Iteration

Without a termination condition, the iterator could run indefinitely. To control this behavior, raise a StopIteration exception once the desired limit is reached.

Example: Stop After 20 Iterations

class MyNumbers:
  def __iter__(self):
    self.a = 1
    return self

  def __next__(self):
    if self.a <= 20:
      x = self.a
      self.a += 1
      return x
    else:
      raise StopIteration

myclass = MyNumbers()
myiter = iter(myclass)

for x in myiter:
  print(x)

Iterators are a powerful feature in Python for sequential data access and are widely used in loops and data processing. For further learning and Python resources, explore more tutorials at Devyra.