Package

The folder structure should now look like this;

├── blackjack
│   ├── __init__.py
│   └── common.py
├── setup.py
└── tests
    ├── __init__.py
    └── test_blackjack.py

The __init__.py files should be empty.

blackjack/common.py

def card_score(cards):
    if not isinstance(cards, str):
        raise ValueError("The input for `card_score` needs to be a string.")
    if len(cards) < 2:
        raise ValueError("The `card_score` function requires at least 2 cards.")
    numbers = [c for c in cards if c in "23456789"]
    faces = [c for c in cards if c in "JQK"]
    n_aces = sum([1 for c in cards if c == "A"])
    score = sum([int(i) for i in numbers]) + len(faces) * 10
    while n_aces > 0:
        score += 1 if score + 11 > 21 else 11
        n_aces -= 1
    return score if score <= 21 else 0

tests/test_blackjack.py

import pytest

from blackjack.common import card_score

@pytest.mark.parametrize("cards,score", [('JK', 20), ('KKK', 0), ('AA', 12), ('AK', 21)])
def test_simple_usecase(cards, score):
    assert card_score(cards) == score

To be able to work on the package in developer mode you'll need to run;

python setup.py develop

And from here you can safely run pytest again.

pytest