How to Use Circular List in Python

1. Use itertools.cycle to Use Circular List in Python
2. Use generators to Use Circular List in Python

A regular linked list has a root node where each node has a pointer to the next node and a piece of data except for the last node, which does not have a next node (so holds a Null value) that tells us that’s the last node. However, in a circular linked list, the last node has a pointer back to the first node in the list.

So, circular linked lists are linked lists structured such that all the nodes within the said structure form a circle. This linear and dynamic data structure whose nodes can’t be None; has the last node connected to the first node so every node can lead to the previous or the next, and on and on.

Typically, an iterator object allows us to move from one element to another within a list or dictionary data structure. With a circular list, the connection between each node allows for that especially using built-in functions.

This article discusses implementing and using a circular list in Python, natively or with modules.

Use itertools.cycle to Use Circular List in Python

Python has a built-in module called itertools that enables sequences with iterators and functional looping. Within this module, iterators (methods) help work out efficient systems to loop across different contexts, from short input sequences to combinatoric scenarios.

More important to use, however, is the cycle() iterator which is an infinite iterator that takes a list and iterates its infinitely.

This occurs by creating an iterator that returns its elements and copies each element from the iterable. Afterward, it returns the items from the saved copy after the iterable has finished.

To use the module itertools and the method cycle(), we make use of this import statement:

from itertools import cycle

All we have to do from here is pass a list to the cycle() method, and we have a circular linked list. In addition, we can use a built-in function, next(), to access and return the next element or node from the iterator, which is the circular linked list.

from itertools import cycle

numList = ["one", "two", "three", "four", "five"]

circularList = cycle(numList)

print(next(circularList))
print(next(circularList))

Output:

one
two

So, with the next() method, we can easily advance the iterator one by one instead of using the for loop, which loops forever.

from itertools import cycle

numList = ["one", "two", "three", "four", "five"]

circularList = cycle(numList)

for num in circularList:
    print(num)

Output:

one
two
three
four
five
one
two
three
four

Use generators to Use Circular List in Python

With generators, we can create iterations and functions that return an iterator which we can iterate over. Unlike typical functions where we use return statements, we use the yield statements in generators.

Unlike return statements, the yield statement pauses the function, saves all its environment variables (or state), and carries one after that with subsequent calls. In addition, a generator function can contain a return statement and more than one yield statement.

To create a circular list, we can use the yield statement to save the function’s state and wait on successive calls.

def circular(args):
    while True:
        for element in args:
            yield element


numList = ["one", "two", "three", "four", "five"]

circularList = circular(numList)
print(circularList)

Output:

<generator object circular at 0x0000018AFFD39A10>

With the generator function called circular(), we can pass an iterable argument to it. It loops forever using the while True statement but can pause the function to return the current state (which means the current element or node it’s on) and continue when another call has been made.

This helps us create a circular list.

You can use the next() method to retrieve the next element within the circular list.

def circular(args):
    while True:
        for element in args:
            yield element


numList = ["one", "two", "three", "four", "five"]

circularList = circular(numList)

print(next(circularList))
print(next(circularList))

Output:

one
two

Or loop infinitely too.

def circular(args):
    while True:
        for element in args:
            yield element


numList = ["one", "two", "three", "four", "five"]

circularList = circular(numList)

for i in circularList:
    print(i)

Output:

one
two
three
four
five
one
two