How do i format strings in a generator?

Question

I want to format some values that are in a generator. When I try to do that I get an error:

TypeError: unsupported format string passed to generator.__format__

I tried different types of string formatting and nothing worked.

This is my code:

def gen_secs():
    for i in range (60):
        yield i

def gen_minutes():
    for i in range (60):
        yield i

def gen_hours():
    for i in range (24):
        yield i

def gen_time(hours,minutes,seconds):
    yield (f'{hours:02}:{minutes:02}:{seconds:02}')

seconds = gen_secs()
minutes = gen_minutes()
hours = gen_hours()

gt = gen_time(hours,minutes,seconds)
while True:
    print(next(gt))

I want to get back all the different combinations of times in a day.

00:00:00
00:00:01
00:00:02
00:00:03
00:00:04
00:00:05
00:00:06 

and so on...

Answer 1

You have a few problems here.

1. One is that your main generator gen_time yields one time so at best, eliminating all other issues, you will get only one time.

2. you pass to your gen_time the generator objects as arguments. Then you are formatting the string on the generator objects themselves and not their elements and hence your error.

3. You use a while True loop on the generator which will throw a StopIteration and break your program. I'm not sure that's what you want.

---

To solve this, taking in count point (1), you want some kind of loop wrapping your yield in the main generator. So, you could alter your main generator, gen_time to loop on the other generators. Like so:

def gen_time():
    for hours in gen_hours():
        for minutes in gen_minutes():
            for seconds in gen_secs():
                yield (f'{hours:02}:{minutes:02}:{seconds:02}')

You could also write that more compact using the yield from construct:

def gen_time():
    yield from (f'{hours:02}:{minutes:02}:{seconds:02}' for hours in gen_hours() for minutes in gen_minutes() for seconds in gen_secs())

Now considering point (3) you can change your main code to:

for gt in gen_time():
    print(gt)