What is 'sys.is_finalizing' and when it's useful?

The sys module have a function called is_finalizing. Running help(sys.is_finalizing) will result in

>>> help(sys.is_finalizing)
Help on built-in function is_finalizing in module sys:

is_finalizing()
    Return True if Python is exiting.

What is mean by Python is exiting? The documentation says this,

sys.is_finalizing() Return True if the Python interpreter is shutting down, False otherwise

I am quite not understand what Python interpreter is shutting down means. Does it mean during the exit function call, If yes what's the point of sys.is_finalizing() as it always return False?. I am trying to understand in which scenario sys.is_finalizing return True and in which situation this API is useful?

Solution

It's not something most users would ever need, but is_finalizing() will be True while the interpreter is shutting down, when it's destroying objects, releasing memory, etc. Knowing that the interpreter is shutting down when an object's __del__() method is called might let the object decide to take certain actions, or not.

As a quick display in an interactive session:

>>> import sys
>>> class X:
...     def __del__(self):
...         print(f"__del__: is_finalizing = {sys.is_finalizing()}")
...
>>> x = X()
>>> del x
__del__: is_finalizing = False
>>>
>>> x = X()
>>> sys.is_finalizing()
False
>>> exit()
__del__: is_finalizing = True

Note that, per the Python Data Model, there's no guarantee __del__() will be called:

It is not guaranteed that __del__() methods are called for objects that still exist when the interpreter exits.

(For completeness, note that the example happens to show the object getting deleted immediately as a result of the name being removed with del x, but that behavior is not guaranteed either.)