Pass state in instead of sharing globals

How to do it

Keep settings in one object built at startup and passed to the code that needs it. Make shared values constant. Replace singletons with one instance created at startup and injected. A module-level cache of pure results is allowed, such as memoised formatters or ETags of fixed files. It only saves work, so it never changes an answer.

Example

# Bad: any function can change it, and tests leak into each other
current_currency = "EUR"

def price(amount):
    return f"{amount} {current_currency}"

# Good: the value is passed in
def price(amount: Decimal, currency: Currency) -> str:
    return f"{amount} {currency.code}"

// Bad
let currentCurrency = 'EUR';
const price = (amount: number) => `${amount} ${currentCurrency}`;

// Good
const price = (amount: number, currency: Currency) => `${amount} ${currency.code}`;

How to check

Tests run in random order and still pass, for example with Jest --randomize, pytest-randomly or PHPUnit --order-by=random. Pylint global-statement flags writes to globals. In JavaScript, a grep fails let or var at the start of a line, which marks a module-level variable.