unfold python Code Snippet in 2025

Builds a list, using an iterator function and an initial seed value.

• The iterator function accepts one argument (seed) and must always return a list with two elements ([value, nextSeed]) or False to terminate.
• Use a generator function, fn_generator, that uses a while loop to call the iterator function and yield the value until it returns False.
• Use a list comprehension to return the list that is produced by the generator, using the iterator function.

def unfold(fn, seed):
  def fn_generator(val):
    while True: 
      val = fn(val[1])
      if val == False: break
      yield val[0]
  return [i for i in fn_generator([None, seed])]

f = lambda n: False if n > 50 else [-n, n + 10]
unfold(f, 10) # [-10, -20, -30, -40, -50]