Circular list iterator in Python

I need to iterate over a circular list, possibly many times, each time starting with the last visited item.

The use case is a connection pool. A client asks for connection, an iterator checks if pointed-to connection is available and returns it, otherwise loops until it finds one that is available.

Is there a neat way to do it in Python?

Solution 1:

Use itertools.cycle, that's its exact purpose:

from itertools import cycle

lst = ['a', 'b', 'c']

pool = cycle(lst)

for item in pool:
    print item,

Output:

a b c a b c ...

(Loops forever, obviously)

In order to manually advance the iterator and pull values from it one by one, simply call next(pool):

>>> next(pool)
'a'
>>> next(pool)
'b'

Solution 2:

The correct answer is to use itertools.cycle. But, let's assume that library function doesn't exist. How would you implement it?

Use a generator:

def circular():
    while True:
        for connection in ['a', 'b', 'c']:
            yield connection

Then, you can either use a for statement to iterate infinitely, or you can call next() to get the single next value from the generator iterator:

connections = circular()
next(connections) # 'a'
next(connections) # 'b'
next(connections) # 'c'
next(connections) # 'a'
next(connections) # 'b'
next(connections) # 'c'
next(connections) # 'a'
#....

Solution 3:

Or you can do like this:

conn = ['a', 'b', 'c', 'd', 'e', 'f']
conn_len = len(conn)
index = 0
while True:
    print(conn[index])
    index = (index + 1) % conn_len

prints a b c d e f a b c... forever