In this python tutorial, you will master the elegant concept of iterators in python. You will also build your own iterator in python with the help of __iter__() and __next__() methods with super handy examples.
Before going to the topic let's have a quick recap on what is iteration and what are iterables in python which we have already discussed in our tutorial Python Loop. Iteration is the process of repeating a set of codes for multiple instances or until they meet the specific condition (Eg: for loop and while loop). Iterables refer to an object that can be repeated or iterated over(Eg: String, List, Tuples, etc).
An iterator in a python programming language is an object which is used to iterate over iterables in an iteration. An iterator stores the state of the iteration and hence it returns one item at a time. In python, Iterators are intrinsically implemented for loops, comprehension, generators, etc.
Technically, an iterator object must employ the iterator protocol which contains two functions __iter__() and __next__(). The iter() function converts the iterables into iterators while the next() function calls the next item in the iterable.
Str_list = ['Yellow','Orange','Red'] it_obj = iter(Str_list) print(it) print(next(it_obj)) print(next(it_obj)) print(next(it_obj)) print(next(it_obj))
Yellow Orange Red Traceback (most recent call last): File "iter_ex.py", line 17, in print(next(it)) StopIteration
In the above code snippet, you can see the creation of an iterator and its implementation.
Str_list, which contains three string elements ‘
Orange’, and ‘
it_obj, with the help of the
iter()function which here converts the list to iterators.
print(it)assuming the outcome will be the iterator value i.e, the first element(yellow)in the list. Instead, it prints the object of the iterator.
print(next(it_obj))prints the first element in the list i.e. Yellow.
print(next(it_obj))prints the next element in the list i.e. Orange.
print(next(it_obj))prints the next element in the list i.e. Red.
final print(next(it_obj))ended up in an exception StopIteration as the list has reached its end and has no more value to return.
From this, we understand that through the use of
next() function we manually iterate over items in an iterable.
To make it more convenient we have another elegant way of iteration which is nothing other than for loop iteration. The same example can be implemented automatically with the use of for loop as given below.
Str_list = ['Yellow','Orange','Red'] for i in Str_list: print(i)
Yellow Orange Red
Before explaining how FOR loop works internally in Python, just look at the below code fragment and analyze it.
Str_list = ['Yellow','Orange','Red'] it_ob = iter(Str_list) while True: try: i = next(it_ob) print(i) except StopIteration: break
Yellow Orange Red
This is how the for loop is internally implemented. The for loop intrinsically creates an iterator object which then iterates the iterables like list using next() function inside a while loop. When the iterable reaches the end and tries to iterate again, the program will halt and raise an exception.
We have seen the working of iterators on built-in constructs. Now let’s try to create our own iterator i.e. the user-defined version. One thing we need to keep in mind is that while creating an iterator we must implement the iterator protocol. To be specific we must use iter() and next()
So we let’s create an iterator to produce an outcome with the square of even numbers. The output will take the form.
class Pow: def __iter__(self): self.a = 0 return self def __next__(self): x = self.a**2 self.a = self.a + 2 return x even = Pow() it_ob = iter(even) print(next(it_ob)) print(next(it_ob)) print(next(it_ob)) print(next(it_ob)) print(next(it_ob))
0 4 16 36 64
When you observe the above code you can notice that no termination steps were included in the code which implies the iterator will iterate infinitely. To understand it clearly let's use ‘for loop’ to iterate our class
for i in Pow(): print(i)
0 4 16 36 64 100 . . .
Those iterators which iterate infinitely are referred to as infinite iterators. While coding this is not a fair practice and hence we must be careful to include the termination statement in the program.
So now we are aware that we need to halt the iteration to prevent the never-ending iteration. To accomplish that we use a StopIteration statement inside the next () function as shown in the below code snippet.
class Pow: def __init__(self, max =0): self.max = max def __iter__(self): self.a = 0 return self def __next__(self): if self.a<=self.max: x = self.a**2 self.a = self.a + 2 return x else: raise StopIteration even = Pow(5) it_ob = iter(even) print(next(it_ob)) print(next(it_ob)) print(next(it_ob)) print(next(it_ob)) print(next(it_ob))
0 4 16 Traceback (most recent call last): File "iter_ex.py", line 47, in
print(next(it_ob)) File "iter_ex.py", line 39, in __next__ raise StopIteration StopIteration
The same code implemented with for loop is as follows:
for i in Pow(8): print(i)
0 4 16 36 64
The key benefit of utilizing iterators in a program is that they save resources which signifies no special memory is needed for getting all the elements except a single memory. Theoretically, it says infinite elements can be stored in finite memory.