sentinels.novalue

Singleton class representing a missing value.

In untyped Python, both None and () are often used as sentinel values. When typing tools like MyPy are used, both are shoe-horded into Union types.

NoValue() is a singleton object representing a missing value.

Given variables

x: int | NoValue
y: int | NoValue

Equality between x and y means both values exist and compare as equal. If one or both of theses values are missing, then what is there to compare?

x == y

x∖y	NoValue()	42	57
NoValue()	false	false	false
42	false	true	false
57	false	false	true

x != y

x∖y	NoValue()	42	57
NoValue()	false	false	false
42	false	false	true
57	false	true	false

Of course, we can also compare directly by identity.

if x is NoValue():
    print('nobody home')
    ...

Warning

Threadsafe only if instantiated before going multi-threaded.

Warning

Non-standard (unpythonic?) == and != comparison operators.