Python 3 generator expression

Question

def gen_letters(s,l):
    def __gen(s,l):
    if l > 1:
        for c in 'abcdefghijklmnopqrstuvwxyz ':
            __gen(s+c,l-1)
    else:
        print(1)
        for c in 'abcdefghijklmnopqrstuvwxyz ':
            yield s+c
    return __gen(s,l)

I am trying to generate all combinations of letters in the alphabet of length "l". The intended use for this functions is:

combos = gen_letters('', 10)

The second argument being the length of the combinations to return. For instance, if I wanted to generate all possible 3 char length strings from the alphabet and a space, I should get 19683 combinations. When trying to use "next(combos)" or "for combo in combos:" (even on the first next()) I get this error:

Traceback (most recent call last):
File "<stdin>", line 1, in <module>
StopIteration

Help is much appreciated :)

Well `l > 1` is true, so you execute that branch, and there is nothing yielded from that branch. I think you meant to write `yield from __gen(...)` in your recursive call. — kaya3, Nov 16 '21 at 10:14
Does this answer your question? [Can generators be recursive?](https://stackoverflow.com/questions/38254304/can-generators-be-recursive) — kaya3, Nov 16 '21 at 10:14

score 0 · Accepted Answer · answered Nov 16 '21 at 10:42

No need for nested functions and your base case can be even simpler:

def gen_letters(l):
    if not l:
        yield ""
    else:
        for c in 'abcdefghijklmnopqrstuvwxyz ':
            for combo in gen_letters(l-1):
                yield c + combo


    
[*gen_letters(2)]
# ['aa', 'ab', ..., ' x', ' y', ' z', '  ']

Or, closer to your original:

def gen_letters(s,l):
    if l:
        for c in 'abcdefghijklmnopqrstuvwxyz ':
            yield from gen_letters(s+c,l-1)
    else:
        yield s

[*gen_letters('', 2)]
# ['aa', 'ab', ..., ' x', ' y', ' z', '  ']

Notice the yield from line. This is where you did the correct recursive call, but did not do anything with the result.

It is generally more efficient to use `yield from` than to write a loop with `yield` in it. — kaya3, Nov 16 '21 at 11:09
True, tho this is rarely a bottleneck. And there is great merit in cleaner code and a simpler signature — user2390182, Nov 16 '21 at 11:15

Python 3 generator expression

1 Answers1