import pytest


@pytest.fixture
def credentials():
    """Fake set of MWS credentials"""
    return {
        "access_key": "AAAAAAAAAAAAAAAAAAAA",
        "secret_key": "AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA",
        "account_id": "AAAAAAAAAAAAAA",
    }

import pytest


@pytest.fixture
def access_key():
    return "AAAAAAAAAAAAAAAAAAAA"


@pytest.fixture
def secret_key():
    return "AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA"


@pytest.fixture
def account_id():
    return "AAAAAAAAAAAAAA"


@pytest.fixture
def timestamp():
    return '2017-08-12T19:40:35Z'


@pytest.fixture
def credentials(access_key, secret_key, account_id):
    """Fake set of MWS credentials"""
    return {
        "access_key": access_key,
        "secret_key": secret_key,
        "account_id": account_id,
    }

Add more pytest fixtures (access_key, secret_key, account_id, timestamp)