python - Cycle through list starting at a certain element

Question

Say I have a list:

l = [1, 2, 3, 4]

And I want to cycle through it. Normally, it would do something like this,

1, 2, 3, 4, 1, 2, 3, 4, 1, 2...

I want to be able to start at a certain point in the cycle, not necessarily an index, but perhaps matching an element. Say I wanted to start at whatever element in the list ==4, then the output would be,

4, 1, 2, 3, 4, 1, 2, 3, 4, 1...

How can I accomplish this?

Answer 1

Look at itertools module. It provides all the necessary functionality.

from itertools import cycle, islice, dropwhile

L = [1, 2, 3, 4]

cycled = cycle(L)  # cycle thorugh the list 'L'
skipped = dropwhile(lambda x: x != 4, cycled)  # drop the values until x==4
sliced = islice(skipped, None, 10)  # take the first 10 values

result = list(sliced)  # create a list from iterator
print(result)

Output:

[4, 1, 2, 3, 4, 1, 2, 3, 4, 1]