Global

How to set up a global variable with a null value and then set it later.

Why null?

Note that a null variable is actually desirable here, at least for the Selenium case.

Setting this up is possible at the top of the module:

from selenium import webdriver

driver = webdriver.Firefox()

But it launches the browser immediately. Which is not desirable when importing code or doing tests.

So we rather want to set it to be null and set it later.

from selenium import webdriver

driver = None


def setup_driver():
    global driver
    driver = webdriver.Firefox()

Though, this presents challenges with types, which is what this page is about.

No value

This is the only approach I found that works.

Initialize without a value. This forces the typechecking to see it as initialized with a type and never has a None value.

VARIABLE: TYPE

I had errors initially from the linter about the variable not set in the functions, but that went away.

e.g.

# pylint: disable=global-statement,global-variable-not-assigned
from typing import Optional

from selenium import webdriver
from selenium.webdriver.firefox.webdriver import WebDriver

driver: WebDriver


def close() -> None:
    global driver
    driver.quit()
    
    
def setup_driver() -> None:
    global driver
    driver = webdriver.Firefox()
    
setup_driver()
close()

Optional

Set the value to be either None (the starting value) or a type (which it is set to later).

VARIABLE: Optional[TYPE] = None

Or with implied None initial value:

VARIABLE: Optiona[TYPE]

Or in Python 3.10 syntax.

VARIABLE: TYPE | None

Here is an example:

from typing import Optional

from selenium import webdriver
from selenium.webdriver.firefox.webdriver import WebDriver


driver: Optional[WebDriver] = None


def close() -> None:
    global driver
    driver.quit()


def setup_driver() -> None:
    global driver
    driver = webdriver.Firefox()

Though I still get this error from Mypy:

Item "None" of "Optional[WebDriver]" has no attribute "quit"

Note on setting as None

Note that you cannot set a value to be None and also another type.

driver: WebDriver = None
# error: Incompatible types in assignment (expression has type "None", variable has type "WebDriver")