Python iterator
last modified January 29, 2024
Python iterator tutorial shows how to use iterators in Python.
Iterator
Iteration is the process of looping through the items in a collection. Iterable is an object that can be iterated over. Iterator is an object which allows traversing through all the elements of a collection, regardless of its specific implementation. It produces successive values from its associated iterable.
In Python, an iterator is an object which implements the iterator protocol. The iterator protocol consists of two methods. The iter method, which must return the iterator object, and the next method, which returns the next element from a sequence. The iter built-in function is used to obtain an iterator from an iterable.
If all the values from an iterator have been returned already, a subsequent call of the next function raises the StopIteration exception.
Iterators have several advantages:
- cleaner code
- ability to work with infinite sequences
- saving resources
Python has several built-in objects, which implement the iterator protocol. For example lists, tuples, strings, dictionaries or files.
Python iterator simple example
The following simple example uses an iterator from a string object.
simple_it.py
#!/usr/bin/python
text = “falcon”
it = iter(text)
print(next(it)) print(next(it)) print(next(it)) print(next(it)) print(next(it)) print(next(it))
In the code example, we use a built-in iterator on a string. In Python a string is an immutable sequence of characters.
it = iter(text)
The iter function returns an iterator on object.
print(next(it))
The next function returns the next element from the iterable.
$ ./simple_it.py f a l c o n
Python iterator and for loop
The for loop is a common way of working with iterators in Python. The for loop does the following:
- calls iter to obtain an iterator
- calls next repeatedly to get the next item from iterable
- terminates the loop when next raises StopIteration
for e in vals: print(e)
The loop body is executed once for each item next returns. The variable e is set to the given item for each iteration.
for_loop.py
#!/usr/bin/python
text = “an old falcon”
for val in text: print(val, end=" “)
print()
In the example, we go through the text using the for loop. We print each character and put a space between them.
$ ./for_loop.py a n o l d f a l c o n
The list, tuple, and set builtins
When we use the list, tuple, and set built-in functions on an iterable, we force the iterator to return all items. Note that some iterables can be very large.
list_tuple_set.py
#!/usr/bin/python
text = “an old falcon”
data = list(text) print(data)
data2 = tuple(text) print(data2)
data3 = set(text) print(data3)
In the example, we use the list, tuple, and set builtins to produce a list, a tuple, and a set of characters from the given text.
$ ./list_tuple_set.py [‘a’, ’n’, ’ ‘, ‘o’, ’l’, ’d’, ’ ‘, ‘f’, ‘a’, ’l’, ‘c’, ‘o’, ’n’] (‘a’, ’n’, ’ ‘, ‘o’, ’l’, ’d’, ’ ‘, ‘f’, ‘a’, ’l’, ‘c’, ‘o’, ’n’) {‘f’, ’d’, ’n’, ‘o’, ‘a’, ’ ‘, ‘c’, ’l’}
Python iterator read lines
When we use iterators to read data from files, we can get the next line without reading the whole file into memory. This way we save system resources.
words.txt
blue sky cloud winter blow water falcon
This is the words.txt file.
read_data_iterator.py
#!/usr/bin/python
with open(‘words.txt’, ‘r’) as f:
for line in f:
print(line.rstrip())
The open function returns a file object, which is an iterator. We can use it in a for loop. With the usage of an iterator, the code is cleaner.
$ ./read_data_iterator.py blue sky cloud winter blow water falcon
Python custom iterator
To create a custom iterator, it must implement the iterator protocol: the iter and next functions.
inf_seq.py
#!/usr/bin/python
class InfSeq:
def init(self):
self.x = 0
def next(self):
self.x += 1
return self.x ** self.x
def iter(self):
return self
infseq = InfSeq() n = 0
for e in infseq:
print(e) n += 1
if n > 10: break
In the code example, we create a sequence of numbers 1, 4, 27, 256, … . This demonstrates that with iterators, we can work with infinite sequences.
def iter(self):
return self
The for statement calls the iter function on the container object. The function returns an iterator object that defines the method next, which accesses elements one at a time.
def next(self):
self.x += 1
return self.x ** self.x
The next function returns the next element of a sequence.
if n > 10: break
Because we are working with an infinite sequence, we must interrupt the for loop at some point.
$ ./inf_seq.py 1 4 27 256 3125 46656 823543 16777216 387420489 10000000000 285311670611
StopIteration
The iteration process is terminated by raising the StopIteration exception.
random_words.py
#!/usr/bin/python
import random
class RandomWord:
def __init__(self):
self.words = ['sky', 'blue', 'cloud', 'milk', 'cosmos',
'rainbow', 'drop', 'new', 'stop']
def __iter__(self):
return self
def __next__(self):
random_word = random.choice(self.words)
if random_word == 'stop':
raise StopIteration
else:
return random_word
rw = RandomWord()
for word in rw: print(word)
In the code example, we have a custom iterator which produces random words. We raise the StopIteration exception when the stop word is picked.
$ ./random_words.py sky blue new drop milk drop milk drop blue
Source
In this article we have worked with iterators in Python.
Author
My name is Jan Bodnar, and I am a passionate programmer with extensive programming experience. I have been writing programming articles since 2007. To date, I have authored over 1,400 articles and 8 e-books. I possess more than ten years of experience in teaching programming.
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