Question

I implemented a (simple) python generator. Now, I want to make another one (from it), which will iterate through all values, but last one.

def gen(x):                   # Generate the interval [x, 10]
    if x <= 10:
        yield x
        for v in gen(x + 1):
            yield v

What would be the best way to accomplish that? Is it possible to alter the original generator using a decorator?

Answer 1

Using a temporary variable as a simple “queue”, you can forward elements from any iterator and keep the last one at the end:

def islice_to_last(it):
    prev_value = next(it)
    for value in it:
        yield prev_value
        prev_value = value

or more generally (for generators up to x last elements):

from collections import deque
import itertools as it

def islice_without_end(it, x):
    d = deque(it.islice(it, x))
    for value in it:
        yield d.popleft()
        d.append(value)