How can i create a permutation with a limit on specific characters efficently?

Question

To give you an idea of what i'm talking about. This is the code i have now:

chrs = 'ABCDEF1234567890'
with open('two.txt', 'w') as two:
    for xs in itertools.product(chrs, repeat=10):
        h = sum(x.isalpha() for x in xs)
        if h == 2: two.write(''.join(xs) + '\n')

Unfortunately this literally takes days as it creates every possible combination, most of which do not match.

EDIT: To clarify, i need a way of finding all possible combinations of the variable chrs where there are only two letters ('ABCDEF'). Without creating an entire list of all combinations and checking each one. I need to create just the ones that have two letters in them.

To go even further:

'AA12345678' = Good
'1234A123A4' = Good
'AAA1234567' = Bad
'A123456789' = Bad

I really don't know how much more i can explain just 5 lines of python code.

Answer 1

This is based on stars and bars rather than combinations and permutations.

Start with a standard stars and bars algorithm, a refinement of my previous work:

def stars_and_bars(star='*', bar='|', star_count=8, bar_count=2):
    if star_count == 0:
        yield (bar,) * bar_count
        return
    if bar_count == 0:
        yield (star,) * star_count
        return
    for left in range(star_count + 1):
        right = star_count - left
        assert right >= 0
        assert left + right == star_count
        for partial in stars_and_bars(star, bar, left, bar_count - 1):
            yield partial + (bar,) + (star,) * right

Verify that it works:

>>> sandb = stars_and_bars(star_count=3, bar_count=2)
>>> for result in sandb:
...     print(''.join(result))
... 
||***
|*|**
*||**
|**|*
*|*|*
**||*
|***|
*|**|
**|*|
***||

As you can see, this produces each possible arrangement of stars and bars. With 8 stars and two bars, we can use this to find all possible arrangements of numbers and letters. We'll then pass that to itertools.product():

def combos():
    letters = 'ABCDEF'
    numbers = '1234567890'
    argument_combos = stars_and_bars(numbers, letters)
    for args in argument_combos:
        yield from itertools.product(*args)

The result will be exactly what OP asked for.