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def check_tokens():
    """Checks the availability of environment variables."""
    ENV_VARS = {
        'PRACTICUM_TOKEN': PRACTICUM_TOKEN,
        'TELEGRAM_TOKEN': TELEGRAM_TOKEN,
        'TELEGRAM_CHAT_ID': TELEGRAM_CHAT_ID,
    }
    missing_vars = [var for var, value in ENV_VARS.items() if not value]
    if missing_vars:
        logging.critical(
            f'Required environment variables are missing: '
            f'{", ".join(missing_vars)}'
        )
    else:
        return True