---
title: Unfold list
type: snippet
language: python
tags: [function,list]
cover: painters-desk
dateModified: 2020-11-02T19:28:35+02:00
---

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.

```py
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])]
```

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