singletons.nada

Pythonic FP - Collection of singleton classes

final class pythonic_fp.singletons.nada.Nada

Singleton class representing & propagating failure.

• singleton _nada: nada = Nada() represents a non-existent value

• returns itself for arbitrary method calls

• returns itself if called as a Callable with arbitrary arguments

• interpreted as an empty container by standard Python functions

• warning: non-standard equality semantics

  • comparison compares true only when 2 non-missing values compare true

  • thus a == b means two non-missing values compare as equal

Usage:

• import Nada and then

  • either use Nada() directly

  • or define _nada: Final[Nada] = Nada() don’t export it

• start propagating failure by setting a propagating value to Nada()

  • works best when working with expression

  • failure may fail to propagate

    • for a function/method with just side effects

    • engineer Nada() to fail to trigger side effects

• test for failure by comparing a result to Nada() itself using

  • is and is not

• propagate failure through a calculation using

  • == and !=

  • the Nada() value never equals itself

  • and never equals anything else

nada_get(alt: Any = Sentinel('Nada')) → Any

Get an alternate value, defaults to Nada().