Module singletons

Pythonic FP - Collection of singleton classes

final class pythonic_fp.fptools.singletons.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=Sentinel('Nada'))

Get an alternate value, defaults to Nada().

Parameters:

alt (Any)

Return type:

Any

class pythonic_fp.fptools.singletons.NoValue

Singleton class representing a missing value.

• similar to None but

  • while None represents “returned no values”

  • NoValue() represents the absence of a value

• Usage

  • import NoValue from pythonic-fp.fptools.singletons and then

    • either use NoValue() directly

    • or define _noValue: Final[NoValue] = NoValue() don’t export it

  • compare using is and is not

    • not == or !=

    • None means returned no values, so None == None makes sense

    • if one or both values are missing, then what is there to compare?

final class pythonic_fp.fptools.singletons.Sentinel(sentinel_name)

Singleton classes representing a sentinel values.

• intended for library code, not to be exported/shared between modules

  • otherwise some of its intended typing guarantees may be lost

• useful substitute for None as a hidden sentinel value

  • allows None to be stored in data structures

  • allows end users to choose to use None or () as sentinel values

  • always equals itself (unlike NoValue)

• usage

  • import Sentinel and then either

    • define _my_sentinel: Final[Sentinel] = Sentinel('my_sentinel')

    • or use Sentinel('my_sentinel') directly

  • compare using either

    • is and is not or == and !=

    • the Sentinel() value always equals itself

    • and never equals anything else, especially other sentinel values

Parameters:

sentinel_name (str)