Don't use caps, numbers and letters; those are all constants available from the string module.

Don't assign j since it isn't used; name the iteration variable _ instead.

Replace your length / index / slice with a random.choices.

Don't call a variable list, since 1. it shadows an existing type called list, and 2. it isn't very descriptive.

Rather than your manual, unrolled string appending, just use ''.join().

A strictly equivalent implementation could be

import random
from string import ascii_letters, ascii_uppercase, digits

for _ in range(4):
    fill_caps = random.choice(ascii_uppercase)
    fill_number = random.choice(digits)
    fill_letter = random.choice(ascii_letters)
    choices = (fill_letter, fill_caps, fill_number)
    word = ''.join(random.choices(choices, k=6))
    print(word)

but your algorithm has some odd properties that, according to your comments, you did not intend. The output word will have the choice of only one lower-case letter, one upper-case letter and one digit. The simpler and less surprising thing to do is generate a word from any of those characters:

import random
from string import ascii_letters, ascii_uppercase, digits

choices = ascii_letters + ascii_uppercase + digits

for _ in range(4):
    word = ''.join(random.choices(choices, k=6))
    print(word)

Don't use caps, numbers and letters; those are all constants available from the string module.

Don't assign j since it isn't used; name the iteration variable _ instead.

Replace your length / index / slice with a random.choices.

Don't call a variable list, since 1. it shadows an existing type called list, and 2. it isn't very descriptive.

Rather than your manual, unrolled string appending, just use ''.join().

A strictly equivalent implementation could be

import random
from string import ascii_lowercase, ascii_uppercase, digits

for _ in range(4):
    fill_caps = random.choice(ascii_uppercase)
    fill_number = random.choice(digits)
    fill_letter = random.choice(ascii_lowercase)
    choices = (fill_letter, fill_caps, fill_number)
    word = ''.join(random.choices(choices, k=6))
    print(word)

but your algorithm has some odd properties that, according to your comments, you did not intend. The output word will have the choice of only one lower-case letter, one upper-case letter and one digit. The simpler and less surprising thing to do is generate a word from any of those characters:

import random
from string import ascii_lowercase, ascii_uppercase, digits

choices = ascii_lowercase + ascii_uppercase + digits

for _ in range(4):
    word = ''.join(random.choices(choices, k=6))
    print(word)

Don't use caps, numbers and letters; those are all constants available from the string module.

Don't assign j since it isn't used; name the iteration variable _ instead.

Replace your length / index / slice with a random.choice.

Don't call a variable list, since 1. it shadows an existing type called list, and 2. it isn't very descriptive.

Rather than your manual, unrolled string appending, just use ''.join().

A strictly equivalent implementation could be

import random
from string import ascii_letters, ascii_uppercase, digits

for _ in range(4):
    fill_caps = random.choice(ascii_uppercase)
    fill_number = random.choice(digits)
    fill_letter = random.choice(ascii_letters)
    choices = (fill_letter, fill_caps, fill_number)
    word = ''.join(
        random.choice(choices) for _ in range(6)
    )
    print(word)

but your algorithm has some odd properties and it's not clear that you intended them. The output word will have the choice of only one lower-case letter, one upper-case letter and one digit. The simpler and less surprising thing to do is generate a word from any of those characters:

import random
from string import ascii_letters, ascii_uppercase, digits

choices = ascii_letters + ascii_uppercase + digits

for _ in range(4):
    word = ''.join(
        random.choice(choices) for _ in range(6)
    )
    print(word)

Don't use caps, numbers and letters; those are all constants available from the string module.

Don't assign j since it isn't used; name the iteration variable _ instead.

Replace your length / index / slice with a random.choices.

Don't call a variable list, since 1. it shadows an existing type called list, and 2. it isn't very descriptive.

Rather than your manual, unrolled string appending, just use ''.join().

A strictly equivalent implementation could be

import random
from string import ascii_letters, ascii_uppercase, digits

for _ in range(4):
    fill_caps = random.choice(ascii_uppercase)
    fill_number = random.choice(digits)
    fill_letter = random.choice(ascii_letters)
    choices = (fill_letter, fill_caps, fill_number)
    word = ''.join(random.choices(choices, k=6))
    print(word)

but your algorithm has some odd properties that, according to your comments, you did not intend. The output word will have the choice of only one lower-case letter, one upper-case letter and one digit. The simpler and less surprising thing to do is generate a word from any of those characters:

import random
from string import ascii_letters, ascii_uppercase, digits

choices = ascii_letters + ascii_uppercase + digits

for _ in range(4):
    word = ''.join(random.choices(choices, k=6))
    print(word)