Mutable default argument usage

Question

I have a function that returns a ChromeDriver instance and logs browser information only once in a session:

from selenium import webdriver
def init_chromedriver(path, timeout=30, _l=[]):
    driver = webdriver.Chrome(executable_path=path)
    driver.set_page_load_timeout(timeout)
    if not _l:
        _l.append(None)
        logger = logging.getLogger('driver')
        for k, v in driver.capabilities.items():
            logger.log(logging.INFO, f"{k}: {v}")
    return driver

Is it a good practice or should I use another method?

Is this actually your code? Cause some_args isn't used, and I doubt you'd be using webdriver.Chrome(...). If it's not, then please include your actual code. — Peilonrayz
– Peilonrayz ♦, Commented Jan 22, 2018 at 17:15
Peilonrayz, thank you for your comment. I have added my actual code. — x42ov
– x42ov, Commented Jan 23, 2018 at 7:45

Gareth Rees · Accepted Answer · 2018-01-23 14:38:48Z

A global flag would be clear and simple:

_LOGGED_DRIVER = False

def init_chromedriver(path, timeout=30):
    driver = webdriver.Chrome(executable_path=path)
    driver.set_page_load_timeout(timeout)
    global _LOGGED_DRIVER
    if not _LOGGED_DRIVER:
        _LOGGED_DRIVER = True
        logger = logging.getLogger('driver')
        for k, v in driver.capabilities.items():
            logger.log(logging.INFO, f"{k}: {v}")
    return driver

Alternatively, you could replace the function after it has been called for the first time:

def _init_chromedriver(path, timeout=30):
    driver = webdriver.Chrome(executable_path=path)
    driver.set_page_load_timeout(timeout)
    return driver

def init_chromedriver(path, timeout=30):
    driver = _init_chromedriver(path, timeout)
    logger = logging.getLogger('driver')
    for k, v in driver.capabilities.items():
        logger.log(logging.INFO, f"{k}: {v}")
    global init_chromedriver
    init_chromedriver = _init_chromedriver
    return driver

But I think the global flag would be clearer.

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