An iterator in Python is an object that is used to iterate over iterable objects like lists, tuples, dicts, and sets. The Python iterators object is initialized using the iter() method. It uses the next() method for iteration.
- __iter__(): The iter() method is called for the initialization of an iterator. This returns an iterator object
- __next__(): The next method returns the next value for the iterable. When we use a for loop to traverse any iterable object, internally it uses the iter() method to get an iterator object, which further uses the next() method to iterate over. This method raises a StopIteration to signal the end of the iteration.
Python iter() Example
Python3
string = "GFG"ch_iterator = iter(string)print(next(ch_iterator))print(next(ch_iterator))print(next(ch_iterator)) |
Output :
G F G
Creating and looping over an iterator using iter() and next()
Below is a simple Python Iterator that creates an iterator type that iterates from 10 to a given limit. For example, if the limit is 15, then it prints 10 11 12 13 14 15. And if the limit is 5, then it prints nothing.
Python3
# A simple Python program to demonstrate# working of iterators using an example type# that iterates from 10 to given value# An iterable user defined typeclass Test: # Constructor def __init__(self, limit): self.limit = limit # Creates iterator object # Called when iteration is initialized def __iter__(self): self.x = 10 return self # To move to next element. In Python 3, # we should replace next with __next__ def __next__(self): # Store current value ofx x = self.x # Stop iteration if limit is reached if x > self.limit: raise StopIteration # Else increment and return old value self.x = x + 1; return x# Prints numbers from 10 to 15for i in Test(15): print(i)# Prints nothingfor i in Test(5): print(i) |
Output:
10 11 12 13 14 15
Iterating over built-in iterable using iter() method
In the following iterations, the iteration state and iterator variable is managed internally (we can’t see it) using an iterator object to traverse over the built-in iterable like list, tuple, dict, etc.
Python3
# Sample built-in iterators# Iterating over a listprint("List Iteration")l = ["Lazyroar", "for", "Lazyroar"]for i in l: print(i) # Iterating over a tuple (immutable)print("\nTuple Iteration")t = ("Lazyroar", "for", "Lazyroar")for i in t: print(i) # Iterating over a Stringprint("\nString Iteration") s = "Geeks"for i in s : print(i) # Iterating over dictionaryprint("\nDictionary Iteration") d = dict()d['xyz'] = 123d['abc'] = 345for i in d : print("%s %d" %(i, d[i])) |
Output:
List Iteration Lazyroar for Lazyroar Tuple Iteration Lazyroar for Lazyroar String Iteration G e e k s Dictionary Iteration xyz 123 abc 345
Iterable vs Iterator
Python iterable and Python iterator are different. The main difference between them is, iterable in Python cannot save the state of the iteration, whereas in iterators the state of the current iteration gets saved.
Note: Every iterator is also an iterable, but not every iterable is an iterator in Python.
Read more – Difference between iterable and iterator.
Iterating on an Iterable
Iterating on each item of the iterable.
Python3
tup = ('a', 'b', 'c', 'd', 'e')for item in tup: print(item) |
Output:
a b c d e
Iterating on an iterator
Python3
tup = ('a', 'b', 'c', 'd', 'e')# creating an iterator from the tupletup_iter = iter(tup)print("Inside loop:")# iterating on each item of the iterator objectfor index, item in enumerate(tup_iter): print(item) # break outside loop after iterating on 3 elements if index == 2: break# we can print the remaining items to be iterated using next()# thus, the state was savedprint("Outside loop:")print(next(tup_iter))print(next(tup_iter)) |
Output:
Inside loop: a b c Outside loop: d e
Getting StopIteration Error while using iterator
Iterable in Python can be iterated over multiple times, but iterators raise StopIteration Error when all items are already iterated.
Here, we are trying to get the next element from the iterator after the completion of the for-loop. Since the iterator is already exhausted, it raises a StopIteration Exception. Whereas, using an iterable, we can iterate on multiple times using for loop or can get items using indexing.
Python3
iterable = (1, 2, 3, 4)iterator_obj = iter(iterable)print("Iterable loop 1:")# iterating on iterablefor item in iterable: print(item, end=",")print("\nIterable Loop 2:")for item in iterable: print(item, end=",")print("\nIterating on an iterator:")# iterating on an iterator object multiple timesfor item in iterator_obj: print(item, end=",")print("\nIterator: Outside loop")# this line will raise StopIteration Exception# since all items are iterated in the previous for-loopprint(next(iterator_obj)) |
Output:
Iterable loop 1:
1,2,3,4,
Iterable Loop 2:
1,2,3,4,
Iterating on an iterator:
1,2,3,4,
Iterator: Outside loop
Traceback (most recent call last):
File "scratch_1.py", line 21, in <module>
print(next(iterator_obj))
StopIteration
