Iterators in Python
Last Updated :
26 Apr, 2023
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
class Test:
def __init__(self, limit):
self.limit = limit
def __iter__(self):
self.x = 10
return self
def __next__(self):
x = self.x
if x > self.limit:
raise StopIteration
self.x = x + 1;
return x
for i in Test(15):
print(i)
for 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
print("List Iteration")
l = ["geeks", "for", "geeks"]
for i in l:
print(i)
print("\nTuple Iteration")
t = ("geeks", "for", "geeks")
for i in t:
print(i)
print("\nString Iteration")
s = "Geeks"
for i in s :
print(i)
print("\nDictionary Iteration")
d = dict()
d['xyz'] = 123
d['abc'] = 345
for i in d :
print("%s %d" %(i, d[i]))
|
Output:
List Iteration
geeks
for
geeks
Tuple Iteration
geeks
for
geeks
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')
tup_iter = iter(tup)
print("Inside loop:")
for index, item in enumerate(tup_iter):
print(item)
if index == 2:
break
print("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:")
for item in iterable:
print(item, end=",")
print("\nIterable Loop 2:")
for item in iterable:
print(item, end=",")
print("\nIterating on an iterator:")
for item in iterator_obj:
print(item, end=",")
print("\nIterator: Outside loop")
print(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
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