Python’s next() function returns the next item of an iterator.
Example
Let us see a few examples to see how the next() method in Python works.
Python3
l = [1, 2, 3]
l_iter = iter(l)
print(next(l_iter))
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Note:
The .next() method was a method for iterating over a sequence in Python 2. It has been replaced in Python 3 with the next() function, which is called using the built-in next() function rather than a method of the sequence object.
Python next() Method Syntax
The next() method in Python has the following syntax:
Syntax : next(iter, stopdef)
Parameters :
- iter : The iterator over which iteration is to be performed.
- stopdef : Default value to be printed if we reach end of iterator.
Return : Returns next element from the list, if not present prints the default value. If default value is not present, raises the StopIteration error.
Python next() Method Examples
Iterating a List using the next() Function
Here we will see the next() in a Python loop. next(l_iter, “end”) will return “end” instead of raising the StopIteration error when iteration is complete.
Python3
l = [1, 2, 3]
l_iter = iter(l)
while True:
item = next(l_iter, "end")
if item == "end":
break
print(item)
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Get the next item from the iterator
In this example, we take a Python list and use the next() function on it. When for the first time the next() function is called, it returns the first element from the iterator list. When the second time the next() function is called, it returns the second element of the list.
Python3
list1 = [1, 2, 3, 4, 5]
l_iter = iter(list1)
print("First item in List:", next(l_iter))
print("Second item in List:", next(l_iter))
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Output
First item in List: 1
Second item in List: 2
Passing default value to next()
Here we have passed “No more element” in the 2nd parameter of the next() function so that this default value is returned instead of raising the StopIteration error when the iterator is exhausted.
Python3
list1 = [1]
list_iter = iter(list1)
print(next(list_iter))
print(next(list_iter, "No more element"))
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Python next() StopIteration
In this example, when the next function is called beyond the size of the list, that is for the third time, it raised a ‘StopIteration” exception which indicates that there are no more items in the list to be iterated.
Python3
l_iter = iter([1, 2])
print("Next Item:", next(l_iter))
print("Next Item:", next(l_iter))
print("Next Item:", next(l_iter))
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Output:
Next Item: 1
Next Item: 2
---------------------------------------------------------------------------
StopIteration Traceback (most recent call last)
Input In [69], in <cell line: 6>()
4 print("Next Item:", next(l_iter))
5 # this line should raise StopIteration exception
----> 6 print("Next Item:", next(l_iter))
StopIteration:
While calling out of the range of the iterator then it raises the Stopiteration error, to avoid this error we will use the default value as an argument.
Performance Analysis
This example demonstrates two approaches to iterating a list in Python. One is using the next method and the other is by using a for loop and comparing them with each other to see which method performs better and in less time.
Python3
import time
l = [1, 2, 3, 4, 5]
l_iter = iter(l)
print("[Using next()]The contents of list are:")
start_next = time.time_ns()
while (1):
val = next(l_iter, 'end')
if val == 'end':
break
else:
print(val, end=" ")
print(f"\nTime taken when using next()\
is : {(time.time_ns() - start_next) / 10**6:.02f}ms")
print("\n[Using For-Loop] The contents of list are:")
start_for = time.time_ns()
for i in l:
print(i, end=" ")
print(f"\nTime taken when using for loop is\
: {(time.time_ns() - start_for) / 10**6:.02f}ms")
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Output
[Using next()]The contents of list are:
1 2 3 4 5
Time taken when using next()is : 0.02ms
[Using For-Loop] The contents of list are:
1 2 3 4 5
Time taken when using for loop is: 0.01ms
Conclusion: Python For loop is a better choice when printing the contents of the list than next().
Applications: next() is the Python built-in function for iterating the components of a container of an iterator type. Its usage is when the size of the container is not known, or we need to give a prompt when the iterator has exhausted (completed).