Get next element from a list in Python

March 04, 2012

Sometimes you will need to keep some items in the list and get the next item. It can be done easily using a variable as the current position tracker. Check the code below:

 current_position = 0    
 my_li = [1, 2, 3, 4, 5]    
 for i in range(0, 8):    
     value = my_li[current_position % (len(my_li))]    
     current_position += 1        
     print value    
 for i in range(0, 5):    
     value = my_li[current_position % (len(my_li))]    
     current_position += 1        
     print value

You can also do this using an iterator. Try this python code:

 def get_next_element(my_itr):
     try:
         return my_itr.next()
     except StopIteration:
         return None

 my_li = [1, 2, 3, 4, 5]
 #convert the list to an iterator
 my_itr = iter(my_li)
 for i in range(0, 10):
     print get_next_element(my_itr)

If you run the code you will see the following output:



1

2

3

4

5

None

None

None

None

None

This is because the iterator is not circular. It can be fixed like this:

 def get_next_element(my_itr):    
     try:            
         return my_itr.next()    
     except StopIteration:    
         return None    

 my_li = [1, 2, 3, 4, 5]    
 #convert the list to an iterator    
 my_itr = iter(my_li)    
 for i in range(0, 8):    
     value = get_next_element(my_itr)    
     if value is None:    
         my_itr = iter(my_li)    
         value = get_next_element(my_itr)    
     print value

But I was wondering whether there is any straight-forward method to make the iterator circular.

Comments

Josh English said…

The easiest way is to use the itertools module's cycle function.

for x itertools.cycle([1,2,3]):
print x

1
2
3
1
2
3
1
2
3

This will go on forever, so there needs to be some way to break the loop.

March 5, 2012 at 11:58 AM