Say I have function f as follows:
def f(c=None):
return 42 if c is None else c
Then I can't get None out of this function. Now you could think "well, just check for another value like 2128.213 or whatever" but then I can't get that specific value out of the function can I?
That's why I'd like to distinguish, if possible, between f() and f(None) so that I can have
f() -> 42
f(None)-> None
Bear in mind this is a simplified example. In practice it's a class's __init__(...) function with multiple positional arguments which I'd like to handle as c in this example.
The common practice in such cases is to use specific sentinel value you never want to return.
class Sentinel():
pass
_sentinel = Sentinel()
# _sentinel = object() # this is the option too
def f(x=_sentinel):
return 42 if x is _sentinel else x
assert f() == 42
assert f(None) is None
assert f(5) == 5