Circular list iterator in Python

Question

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?

Answer 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)

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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' 

Answer 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' #....