Creating Unique Non-None Placeholders in Python

Let’s say we want to create a function with an optional parameter that distinguishes between None as a value and the genuine absence of a value. A solution is to create a unique object with object() and check the identity of the argument with the is operator.

Since the object() function returns a new featureless object every time it is called, we can use it to create unique dummy values – or sentinel values.

Example:

MISSING = object()

def func(arg=MISSING):
    if arg is MISSING:
        print("No argument provided")
    else:
        print(f"Argument provided: {arg}")

With this pattern, we can pass None as an argument and the function will correctly identify it as a value.

References

• An article by Trey Hunner on creating unique non-None sentinel values in Python
• Python documentation on the object() function
• Wikipedia: Sentinel value